V4L2 API Specification

Video for Linux Two API Specification

Revision 0.24

Michael H Schimek

            
          

Bill Dirks

Hans Verkuil

Martin Rubli

Copyright © 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008 Bill Dirks, Michael H. Schimek, Hans Verkuil, MartinRubli

This document is copyrighted © 1999-2008 by BillDirks, Michael H. Schimek, Hans Verkuil and Martin Rubli.

Permission is granted to copy, distribute and/or modifythis document under the terms of the GNU Free Documentation License,Version 1.1 or any later version published by the Free SoftwareFoundation; with no Invariant Sections, with no Front-Cover Texts, andwith no Back-Cover Texts. A copy of the license is included in theappendix entitled "GNU Free Documentation License".

Programming examples can be used and distributed withoutrestrictions.

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List of Figures

1-1.  Image Cropping, Insertion and Scaling

3-1.  Field Order, Top Field First Transmitted

3-2.  Field Order, Bottom Field First Transmitted

4-1.  Line synchronization

4-2.  ITU-R 525 line numbering (M/NTSC and M/PAL)

4-3.  ITU-R 625 line numbering

List of Examples

1-1.  Information about the current video input

1-2.  Switching to the first video input

1-3.  Information about the current audio input

1-4.  Switching to the first audio input

1-5.  Information about the current video standard

1-6.  Listing the video standards supported by the currentinput

1-7.  Selecting a new video standard

1-8.  Enumerating all controls

1-9.  Changing controls

1-10.  Resetting the cropping parameters

1-11.  Simple downscaling

1-12.  Selecting an output area

1-13.  Current scaling factor and pixel aspect

2-1.  ITU-R Rec. BT.601 color conversion

2-1.  V4L2_PIX_FMT_BGR24 4 × 4 pixelimage

2-1.  V4L2_PIX_FMT_SBGGR8 4 × 4pixel image

2-1.  V4L2_PIX_FMT_SBGGR16 4 × 4pixel image

2-1.  V4L2_PIX_FMT_GREY 4 × 4pixel image

2-1.  V4L2_PIX_FMT_Y16 4 × 4pixel image

2-1.  V4L2_PIX_FMT_YUYV 4 × 4pixel image

2-1.  V4L2_PIX_FMT_UYVY 4 × 4pixel image

2-1.  V4L2_PIX_FMT_Y41P 8 × 4pixel image

2-1.  V4L2_PIX_FMT_YVU420 4 × 4pixel image

2-1.  V4L2_PIX_FMT_YVU410 4 × 4pixel image

2-1.  V4L2_PIX_FMT_YUV422P 4 × 4pixel image

2-1.  V4L2_PIX_FMT_YUV411P 4 × 4pixel image

2-1.  V4L2_PIX_FMT_NV12 4 × 4pixel image

3-1.  Mapping buffers

3-2.  Initiating streaming I/O with user pointers

4-1.  Finding a framebuffer device for OSD

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Introduction

Video For Linux Two is the second version of the Video ForLinux API, a kernel interface for analog radio and video capture andoutput drivers.

Early drivers used ad-hoc interfaces. These were replaced inLinux 2.2 by Alan Cox' V4L API, based on the interface of the bttvdriver. In 1999 Bill Dirks started the development of V4L2 to fix someshortcomings of V4L and to support a wider range of devices. The APIwas revised again in 2002 prior to its inclusion in Linux 2.5/2.6, andwork continues on improvements and additions while maintainingcompatibility with existing drivers and applications. In 2006/2007efforts began on FreeBSD drivers with a V4L2 interface.

This book documents the V4L2 API. Intended audience aredriver and application writers.

If you have questions or ideas regarding the API, pleasewrite to the Video4Linux mailing list:https://listman.redhat.com/mailman/listinfo/video4linux-list. For inquiries aboutthe V4L2 specification contact the [email protected].

The latest version of this document and the DocBook SGMLsources are hosted at http://v4l2spec.bytesex.org,andhttp://linuxtv.org/downloads/video4linux/API/V4L2_API.

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Chapter 1. Common API Elements

Programming a V4L2 device consists of thesesteps:

• Opening the device

• Changing device properties, selecting a video and audioinput, video standard, picture brightness a. o.

• Negotiating a data format

• Negotiating an input/output method

• The actual input/output loop

• Closing the device

In practice most steps are optional and can be executed out oforder. It depends on the V4L2 device type, you can read about thedetails inChapter 4. In this chapter we will discussthe basic concepts applicable to all devices.

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1.1. Opening and Closing Devices

1.1.1. Device Naming

V4L2 drivers are implemented as kernel modules, loadedmanually by the system administrator or automatically when a device isfirst opened. The driver modules plug into the "videodev" kernelmodule. It provides helper functions and a common applicationinterface specified in this document.

Each driver thus loaded registers one or more device nodeswith major number 81 and a minor number between 0 and 255. Assigningminor numbers to V4L2 devices is entirely up to the system administrator,this is primarily intended to solve conflicts between devices.[1] The module options to select minor numbers are namedafter the device special file with a "_nr" suffix. For example "video_nr"for/dev/video video capture devices. The number isan offset to the base minor number associated with the device type.[2] When the driver supports multiple devices of the sametype more than one minor number can be assigned, separated by commas:

> insmod mydriver.o video_nr=0,1 radio_nr=0,1

In /etc/modules.conf this may bewritten as:

alias char-major-81-0 mydriver
alias char-major-81-1 mydriver
alias char-major-81-64 mydriver              
options mydriver video_nr=0,1 radio_nr=0,1   
          

When an application attempts to open a devicespecial file with major number 81 and minor number 0, 1, or 64, load"mydriver" (and the "videodev" module it depends upon).

Register the first two video capture devices withminor number 0 and 1 (base number is 0), the first two radio devicewith minor number 64 and 65 (base 64).

When no minor number is given as moduleoption the driver supplies a default.  Chapter 4recommends the base minor numbers to be used for the various devicetypes. Obviously minor numbers must be unique. When the number isalready in use the offending device will not beregistered.

By convention system administrators create variouscharacter device special files with these major and minor numbers inthe/dev directory. The names recomended for thedifferent V4L2 device types are listed inChapter 4.

The creation of character special files (withmknod) is a privileged operation anddevices cannot be opened by major and minor number. That meansapplications cannotreliable scan for loaded orinstalled drivers. The user must enter a device name, or theapplication can try the conventional device names.

Under the device filesystem (devfs) the minor numberoptions are ignored. V4L2 drivers (or by proxy the "videodev" module)automatically create the required device files in the/dev/v4l directory using the conventional devicenames above.

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1.1.2. Related Devices

Devices can support several related functions. For examplevideo capturing, video overlay and VBI capturing are related becausethese functions share, amongst other, the same video input and tunerfrequency. V4L and earlier versions of V4L2 used the same device nameand minor number for video capturing and overlay, but different onesfor VBI. Experience showed this approach has several problems[3], and to make things worse the V4L videodev moduleused to prohibit multiple opens of a device.

As a remedy the present version of the V4L2 API relaxed theconcept of device types with specific names and minor numbers. Forcompatibility with old applications drivers must still register differentminor numbers to assign a default function to the device. But if relatedfunctions are supported by the driver they must be available under allregistered minor numbers. The desired function can be selected afteropening the device as described inChapter 4.

Imagine a driver supporting video capturing, videooverlay, raw VBI capturing, and FM radio reception. It registers threedevices with minor number 0, 64 and 224 (this numbering scheme isinherited from the V4L API). Regardless if/dev/video (81, 0) or/dev/vbi (81, 224) is opened the application canselect any one of the video capturing, overlay or VBI capturingfunctions. Without programming (e. g. reading from the devicewithdd orcat)/dev/video captures video images, while/dev/vbi captures raw VBI data./dev/radio (81, 64) is invariable a radio device,unrelated to the video functions. Being unrelated does not imply thedevices can be used at the same time, however. Theopen()function may very well return anEBUSY error code.

Besides video input or output the hardware may alsosupport audio sampling or playback. If so, these functions areimplemented as OSS or ALSA PCM devices and eventually OSS or ALSAaudio mixer. The V4L2 API makes no provisions yet to find theserelated devices. If you have an idea please write to the Video4Linuxmailing list: https://listman.redhat.com/mailman/listinfo/video4linux-list.

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1.1.3. Multiple Opens

In general, V4L2 devices can be opened more than once.When this is supported by the driver, users can for example start a"panel" application to change controls like brightness or audiovolume, while another application captures video and audio. In other words, panelapplications are comparable to an OSS or ALSA audio mixer application.When a device supports multiple functions like capturing and overlaysimultaneously, multiple opens allow concurrentuse of the device by forked processes or specialized applications.

Multiple opens are optional, although drivers shouldpermit at least concurrent accesses without data exchange, i. e. panelapplications. This impliesopen()can return anEBUSY error code when thedevice is already in use, as well asioctl() functions initiatingdata exchange (namely theVIDIOC_S_FMT ioctl), and theread()andwrite() functions.

Mere opening a V4L2 device does not grant exclusiveaccess.[4] Initiating data exchange however assigns the rightto read or write the requested type of data, and to change relatedproperties, to this file descriptor. Applications can requestadditional access privileges using the priority mechanism described inSection 1.3.

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1.1.4. Shared Data Streams

V4L2 drivers should not support multiple applicationsreading or writing the same data stream on a device by copyingbuffers, time multiplexing or similar means. This is better handled bya proxy application in user space. When the driver supports streamsharing anyway it must be implemented transparently. The V4L2 API doesnot specify how conflicts are solved.

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1.1.5. Functions

To open and close V4L2 devices applications use theopen() andclose() function, respectively. Devices areprogrammed using theioctl() function as explained in thefollowing sections.

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1.2. Querying Capabilities

Because V4L2 covers a wide variety of devices not allaspects of the API are equally applicable to all types of devices.Furthermore devices of the same type have different capabilities andthis specification permits the omission of a few complicated and lessimportant parts of the API.

The VIDIOC_QUERYCAP ioctl is available to check if the kerneldevice is compatible with this specification, and to query thefunctions andI/Omethodssupported by the device. Other features can be queriedby calling the respective ioctl, for exampleVIDIOC_ENUMINPUTto learn about the number, types and names of video connectors on thedevice. Although abstraction is a major objective of this API, theioctl also allows driver specific applications to reliable identifythe driver.

All V4L2 drivers must supportVIDIOC_QUERYCAP. Applications should always callthis ioctl after opening the device.

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1.3. Application Priority

When multiple applications share a device it may bedesirable to assign them different priorities. Contrary to thetraditional "rm -rf /" school of thought a video recording applicationcould for example block other applications from changing videocontrols or switching the current TV channel. Another objective is topermit low priority applications working in background, which can bepreempted by user controlled applications and automatically regaincontrol of the device at a later time.

Since these features cannot be implemented entirely in userspace V4L2 defines theVIDIOC_G_PRIORITY andVIDIOC_S_PRIORITYioctls to request and query the access priority associate with a filedescriptor. Opening a device assigns a medium priority, compatiblewith earlier versions of V4L2 and drivers not supporting these ioctls.Applications requiring a different priority will usually callVIDIOC_S_PRIORITY after verifying the device withtheVIDIOC_QUERYCAP ioctl.

Ioctls changing driver properties, such as VIDIOC_S_INPUT,return an EBUSY error code after another application obtained higher priority.An event mechanism to notify applications about asynchronous propertychanges has been proposed but not added yet.

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1.4. Video Inputs and Outputs

Video inputs and outputs are physical connectors of adevice. These can be for example RF connectors (antenna/cable), CVBSa.k.a. Composite Video, S-Video or RGB connectors. Only video and VBIcapture devices have inputs, output devices have outputs, at least oneeach. Radio devices have no video inputs or outputs.

To learn about the number and attributes of theavailable inputs and outputs applications can enumerate them with theVIDIOC_ENUMINPUTandVIDIOC_ENUMOUTPUT ioctl, respectively. Thestruct v4l2_input returned by theVIDIOC_ENUMINPUTioctl also contains signal status information applicable when thecurrent video input is queried.

The VIDIOC_G_INPUT and VIDIOC_G_OUTPUT ioctl return theindex of the current video input or output. To select a differentinput or output applications call theVIDIOC_S_INPUT andVIDIOC_S_OUTPUT ioctl. Drivers must implement all the input ioctlswhen the device has one or more inputs, all the output ioctls when thedevice has one or more outputs.

Example 1-1. Information about the current video input

struct v4l2_input input;
int index;

if (-1 == ioctl (fd, VIDIOC_G_INPUT, &index)) {
        perror ("VIDIOC_G_INPUT");
        exit (EXIT_FAILURE);
}

memset (&input, 0, sizeof (input));
input.index = index;

if (-1 == ioctl (fd, VIDIOC_ENUMINPUT, &input)) {
        perror ("VIDIOC_ENUMINPUT");
        exit (EXIT_FAILURE);
}

printf ("Current input: %s\n", input.name);
      

Example 1-2. Switching to the first video input

int index;

index = 0;

if (-1 == ioctl (fd, VIDIOC_S_INPUT, &index)) {
        perror ("VIDIOC_S_INPUT");
        exit (EXIT_FAILURE);
}
      

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1.5. Audio Inputs and Outputs

Audio inputs and outputs are physical connectors of adevice. Video capture devices have inputs, output devices haveoutputs, zero or more each. Radio devices have no audio inputs oroutputs. They have exactly one tuner which in factis an audio source, but this API associatestuners with video inputs or outputs only, and radio devices havenone of these.[5] A connector on a TV card to loop back the receivedaudio signal to a sound card is not considered an audio output.

Audio and video inputs and outputs are associated. Selectinga video source also selects an audio source. This is most evident whenthe video and audio source is a tuner. Further audio connectors cancombine with more than one video input or output. Assumed twocomposite video inputs and two audio inputs exist, there may be up tofour valid combinations. The relation of video and audio connectorsis defined in theaudioset field of therespective struct v4l2_input or struct v4l2_output, where each bit representsthe index number, starting at zero, of one audio input or output.

To learn about the number and attributes of theavailable inputs and outputs applications can enumerate them with theVIDIOC_ENUMAUDIOandVIDIOC_ENUMAUDOUT ioctl, respectively. Thestruct v4l2_audio returned by theVIDIOC_ENUMAUDIO ioctlalso contains signal status information applicable when the currentaudio input is queried.

The VIDIOC_G_AUDIO and VIDIOC_G_AUDOUT ioctl reportthe current audio input and output, respectively. Note that, unlikeVIDIOC_G_INPUTandVIDIOC_G_OUTPUT these ioctls return a structureasVIDIOC_ENUMAUDIO andVIDIOC_ENUMAUDOUT do, not just an index.

To select an audio input and change its propertiesapplications call the VIDIOC_S_AUDIO ioctl. To select an audiooutput (which presently has no changeable properties) applicationscall theVIDIOC_S_AUDOUT ioctl.

Drivers must implement all input ioctls when the devicehas one or more inputs, all output ioctls when the device has oneor more outputs. When the device has any audio inputs or outputs thedriver must set theV4L2_CAP_AUDIO flag in thestruct v4l2_capability returned by theVIDIOC_QUERYCAP ioctl.

Example 1-3. Information about the current audio input

struct v4l2_audio audio;

memset (&audio, 0, sizeof (audio));

if (-1 == ioctl (fd, VIDIOC_G_AUDIO, &audio)) {
        perror ("VIDIOC_G_AUDIO");
        exit (EXIT_FAILURE);
}

printf ("Current input: %s\n", audio.name);
      

Example 1-4. Switching to the first audio input

struct v4l2_audio audio;

memset (&audio, 0, sizeof (audio)); /* clear audio.mode, audio.reserved */

audio.index = 0;

if (-1 == ioctl (fd, VIDIOC_S_AUDIO, &audio)) {
        perror ("VIDIOC_S_AUDIO");
        exit (EXIT_FAILURE);
}
      

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1.6. Tuners and Modulators

1.6.1. Tuners

Video input devices can have one or more tunersdemodulating a RF signal. Each tuner is associated with one or morevideo inputs, depending on the number of RF connectors on the tuner.Thetype field of the respectivestruct v4l2_input returned by theVIDIOC_ENUMINPUT ioctl is set toV4L2_INPUT_TYPE_TUNER and itstuner field contains the index number ofthe tuner.

Radio devices have exactly one tuner with index zero, novideo inputs.

To query and change tuner properties applications use theVIDIOC_G_TUNER andVIDIOC_S_TUNER ioctl, respectively. Thestruct v4l2_tuner returned byVIDIOC_G_TUNER alsocontains signal status information applicable when the tuner of thecurrent video input, or a radio tuner is queried. Note thatVIDIOC_S_TUNER does not switch the current tuner,when there is more than one at all. The tuner is solely determined bythe current video input. Drivers must support both ioctls and set theV4L2_CAP_TUNER flag in the struct v4l2_capabilityreturned by theVIDIOC_QUERYCAP ioctl when the device has one ormore tuners.

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1.6.2. Modulators

Video output devices can have one or more modulators, uh,modulating a video signal for radiation or connection to the antennainput of a TV set or video recorder. Each modulator is associated withone or more video outputs, depending on the number of RF connectors onthe modulator. The type field of therespective struct v4l2_output returned by theVIDIOC_ENUMOUTPUT ioctl isset toV4L2_OUTPUT_TYPE_MODULATOR and itsmodulator field contains the index numberof the modulator. This specification does not define radio outputdevices.

To query and change modulator properties applications usethe VIDIOC_G_MODULATOR and VIDIOC_S_MODULATOR ioctl. Note thatVIDIOC_S_MODULATOR does not switch the currentmodulator, when there is more than one at all. The modulator is solelydetermined by the current video output. Drivers must support bothioctls and set the V4L2_CAP_TUNER (sic) flag inthe struct v4l2_capability returned by theVIDIOC_QUERYCAP ioctl when thedevice has one or more modulators.

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1.6.3. Radio Frequency

To get and set the tuner or modulator radio frequencyapplications use the VIDIOC_G_FREQUENCY and VIDIOC_S_FREQUENCYioctl which both take a pointer to a struct v4l2_frequency. These ioctlsare used for TV and radio devices alike. Drivers must support bothioctls when the tuner or modulator ioctls are supported, orwhen the device is a radio device.

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1.6.4. Satellite Receivers

To be discussed. See also proposals by Peter Schlaf, [email protected] on 23 Oct 2002,subject: "Re: [V4L] Re: v4l2 api".

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1.7. Video Standards

Video devices typically support one or more different videostandards or variations of standards. Each video input and output maysupport another set of standards. This set is reported by thestd field of struct v4l2_input andstruct v4l2_output returned by theVIDIOC_ENUMINPUT andVIDIOC_ENUMOUTPUTioctl, respectively.

V4L2 defines one bit for each analog video standardcurrently in use worldwide, and sets aside bits for driver definedstandards, e. g. hybrid standards to watch NTSC video tapes on PAL TVsand vice versa. Applications can use the predefined bits to select aparticular standard, although presenting the user a menu of supportedstandards is preferred. To enumerate and query the attributes of thesupported standards applications use theVIDIOC_ENUMSTD ioctl.

Many of the defined standards are actually just variationsof a few major standards. The hardware may in fact not distinguishbetween them, or do so internal and switch automatically. Thereforeenumerated standards also contain sets of one or more standardbits.

Assume a hypothetic tuner capable of demodulating B/PAL,G/PAL and I/PAL signals. The first enumerated standard is a set of Band G/PAL, switched automatically depending on the selected radiofrequency in UHF or VHF band. Enumeration gives a "PAL-B/G" or "PAL-I"choice. Similar a Composite input may collapse standards, enumerating"PAL-B/G/H/I", "NTSC-M" and "SECAM-D/K".[6]

To query and select the standard used by the current videoinput or output applications call theVIDIOC_G_STD andVIDIOC_S_STD ioctl, respectively. Thereceivedstandard can be sensed with theVIDIOC_QUERYSTD ioctl. Note parameter of all these ioctls is a pointer to av4l2_std_id type (a standard set),not an index into the standard enumeration.[7] Drivers must implement all video standard ioctlswhen the device has one or more video inputs or outputs.

Special rules apply to USB cameras where the notion of videostandards makes little sense. More generally any capture device,output devices accordingly, which is

• incapable of capturing fields or frames at the nominalrate of the video standard, or

• where timestamps referto the instant the field or frame was received by the driver, not thecapture time, or

• where sequence numbersrefer to the frames received by the driver, not the capturedframes.

Here the driver shall set the std field of struct  v4l2_input and struct  v4l2_outputto zero, the  VIDIOC_G_STD, VIDIOC_S_STD, VIDIOC_QUERYSTDand VIDIOC_ENUMSTD ioctls shall return the EINVAL error code. [8]

Example 1-5. Information about the current video standard

v4l2_std_id std_id;
struct v4l2_standard standard;

if (-1 == ioctl (fd, VIDIOC_G_STD, &std_id)) {
        /* Note when VIDIOC_ENUMSTD always returns EINVAL this
           is no video device or it falls under the USB exception,
           and VIDIOC_G_STD returning EINVAL is no error. */

        perror ("VIDIOC_G_STD");
        exit (EXIT_FAILURE);
}

memset (&standard, 0, sizeof (standard));
standard.index = 0;

while (0 == ioctl (fd, VIDIOC_ENUMSTD, &standard)) {
        if (standard.id & std_id) {
               printf ("Current video standard: %s\n", standard.name);
               exit (EXIT_SUCCESS);
        }

        standard.index++;
}

/* EINVAL indicates the end of the enumeration, which cannot be
   empty unless this device falls under the USB exception. */

if (errno == EINVAL || standard.index == 0) {
        perror ("VIDIOC_ENUMSTD");
        exit (EXIT_FAILURE);
}
      

Example 1-6. Listing the video standards supported by the currentinput

struct v4l2_input input;
struct v4l2_standard standard;

memset (&input, 0, sizeof (input));

if (-1 == ioctl (fd, VIDIOC_G_INPUT, &input.index)) {
        perror ("VIDIOC_G_INPUT");
        exit (EXIT_FAILURE);
}

if (-1 == ioctl (fd, VIDIOC_ENUMINPUT, &input)) {
        perror ("VIDIOC_ENUM_INPUT");
        exit (EXIT_FAILURE);
}

printf ("Current input %s supports:\n", input.name);

memset (&standard, 0, sizeof (standard));
standard.index = 0;

while (0 == ioctl (fd, VIDIOC_ENUMSTD, &standard)) {
        if (standard.id & input.std)
                printf ("%s\n", standard.name);

        standard.index++;
}

/* EINVAL indicates the end of the enumeration, which cannot be
   empty unless this device falls under the USB exception. */

if (errno != EINVAL || standard.index == 0) {
        perror ("VIDIOC_ENUMSTD");
        exit (EXIT_FAILURE);
}
      

Example 1-7. Selecting a new video standard

struct v4l2_input input;
v4l2_std_id std_id;

memset (&input, 0, sizeof (input));

if (-1 == ioctl (fd, VIDIOC_G_INPUT, &input.index)) {
        perror ("VIDIOC_G_INPUT");
        exit (EXIT_FAILURE);
}

if (-1 == ioctl (fd, VIDIOC_ENUMINPUT, &input)) {
        perror ("VIDIOC_ENUM_INPUT");
        exit (EXIT_FAILURE);
}

if (0 == (input.std & V4L2_STD_PAL_BG)) {
        fprintf (stderr, "Oops. B/G PAL is not supported.\n");
        exit (EXIT_FAILURE);
}

/* Note this is also supposed to work when only B
   or G/PAL is supported. */

std_id = V4L2_STD_PAL_BG;

if (-1 == ioctl (fd, VIDIOC_S_STD, &std_id)) {
        perror ("VIDIOC_S_STD");
        exit (EXIT_FAILURE);
}
      

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1.8. User Controls

Devices typically have a number of user-settable controlssuch as brightness, saturation and so on, which would be presented tothe user on a graphical user interface. But, different deviceswill have different controls available, and furthermore, the range ofpossible values, and the default value will vary from device todevice. The control ioctls provide the information and a mechanism tocreate a nice user interface for these controls that will workcorrectly with any device.

All controls are accessed using an ID value. V4L2 definesseveral IDs for specific purposes. Drivers can also implement theirown custom controls usingV4L2_CID_PRIVATE_BASEand higher values. The pre-defined control IDs have the prefixV4L2_CID_, and are listed inTable 1-1. The ID is used when querying the attributes ofa control, and when getting or setting the current value.

Generally applications should present controls to the userwithout assumptions about their purpose. Each control comes with aname string the user is supposed to understand. When the purpose isnon-intuitive the driver writer should provide a user manual, a userinterface plug-in or a driver specific panel application. PredefinedIDs were introduced to change a few controls programmatically, forexample to mute a device during a channel switch.

Drivers may enumerate different controls after switchingthe current video input or output, tuner or modulator, or audio inputor output. Different in the sense of other bounds, another default andcurrent value, step size or other menu items. A control with a certaincustom ID can also change name andtype.[9]Control values are stored globally, they do notchange when switching except to stay within the reported bounds. Theyalso do not change e. g. when the device is opened or closed, when thetuner radio frequency is changed or generally never withoutapplication request. Since V4L2 specifies no event mechanism, panelapplications intended to cooperate with other panel applications (bethey built into a larger application, as a TV viewer) may need toregularly poll control values to update their userinterface.[10]

Table 1-1. Control IDs

ID	Type	Description
V4L2_CID_BASE		First predefined ID, equal toV4L2_CID_BRIGHTNESS.
V4L2_CID_USER_BASE		Synonym of V4L2_CID_BASE.
V4L2_CID_BRIGHTNESS	integer	Picture brightness, or more precisely, the blacklevel.
V4L2_CID_CONTRAST	integer	Picture contrast or luma gain.
V4L2_CID_SATURATION	integer	Picture color saturation or chroma gain.
V4L2_CID_HUE	integer	Hue or color balance.
V4L2_CID_AUDIO_VOLUME	integer	Overall audio volume. Note some drivers alsoprovide an OSS or ALSA mixer interface.
V4L2_CID_AUDIO_BALANCE	integer	Audio stereo balance. Minimum corresponds to allthe way left, maximum to right.
V4L2_CID_AUDIO_BASS	integer	Audio bass adjustment.
V4L2_CID_AUDIO_TREBLE	integer	Audio treble adjustment.
V4L2_CID_AUDIO_MUTE	boolean	Mute audio, i. e. set the volume to zero, howeverwithout affectingV4L2_CID_AUDIO_VOLUME. LikeALSA drivers, V4L2 drivers must mute at load time to avoid excessivenoise. Actually the entire device should be reset to a low powerconsumption state.
V4L2_CID_AUDIO_LOUDNESS	boolean	Loudness mode (bass boost).
V4L2_CID_BLACK_LEVEL	integer	Another name for brightness (not a synonym ofV4L2_CID_BRIGHTNESS). This control is deprecatedand should not be used in new drivers and applications.
V4L2_CID_AUTO_WHITE_BALANCE	boolean	Automatic white balance (cameras).
V4L2_CID_DO_WHITE_BALANCE	button	This is an action control. When set (the value isignored), the device will do a white balance and then hold the currentsetting. Contrast this with the booleanV4L2_CID_AUTO_WHITE_BALANCE, which, whenactivated, keeps adjusting the white balance.
V4L2_CID_RED_BALANCE	integer	Red chroma balance.
V4L2_CID_BLUE_BALANCE	integer	Blue chroma balance.
V4L2_CID_GAMMA	integer	Gamma adjust.
V4L2_CID_WHITENESS	integer	Whiteness for grey-scale devices. This is a synonymfor V4L2_CID_GAMMA. This control is deprecatedand should not be used in new drivers and applications.
V4L2_CID_EXPOSURE	integer	Exposure (cameras). [Unit?]
V4L2_CID_AUTOGAIN	boolean	Automatic gain/exposure control.
V4L2_CID_GAIN	integer	Gain control.
V4L2_CID_HFLIP	boolean	Mirror the picture horizontally.
V4L2_CID_VFLIP	boolean	Mirror the picture vertically.
V4L2_CID_HCENTER_DEPRECATED (formerlyV4L2_CID_HCENTER)	integer	Horizontal image centering. This control isdeprecated. New drivers and applications should use theCamera class controlsV4L2_CID_PAN_ABSOLUTE,V4L2_CID_PAN_RELATIVEandV4L2_CID_PAN_RESET instead.
V4L2_CID_VCENTER_DEPRECATED (formerlyV4L2_CID_VCENTER)	integer	Vertical image centering. Centering is intended tophysically adjust cameras. For image cropping seeSection 1.11, for clippingSection 4.2. Thiscontrol is deprecated. New drivers and applications should use theCamera class controlsV4L2_CID_TILT_ABSOLUTE,V4L2_CID_TILT_RELATIVEandV4L2_CID_TILT_RESET instead.
V4L2_CID_POWER_LINE_FREQUENCY	integer	Enables a power line frequency filter to avoidflicker. Possible values are:V4L2_CID_POWER_LINE_FREQUENCY_DISABLED(0),V4L2_CID_POWER_LINE_FREQUENCY_50HZ (1) andV4L2_CID_POWER_LINE_FREQUENCY_60HZ (2).
V4L2_CID_HUE_AUTO	boolean	Enables automatic hue control by the device. Theeffect of settingV4L2_CID_HUE while automatichue control is enabled is undefined, drivers should ignore suchrequest.
V4L2_CID_WHITE_BALANCE_TEMPERATURE	integer	This control specifies the white balance settingsas a color temperature in Kelvin. A driver should have a minimum of2800 (incandescent) to 6500 (daylight). For more information aboutcolor temperature seeWikipedia.
V4L2_CID_SHARPNESS	integer	Adjusts the sharpness filters in a camera. Theminimum value disables the filters, higher values give a sharperpicture.
V4L2_CID_BACKLIGHT_COMPENSATION	integer	Adjusts the backlight compensation in a camera. Theminimum value disables backlight compensation.
V4L2_CID_LASTP1		End of the predefined control IDs (currentlyV4L2_CID_BACKLIGHT_COMPENSATION + 1).
V4L2_CID_PRIVATE_BASE		ID of the first custom (driver specific) control.Applications depending on particular custom controls should check thedriver name and version, seeSection 1.2.

Applications can enumerate the available controls with theVIDIOC_QUERYCTRL andVIDIOC_QUERYMENU ioctls, get and set acontrol value with theVIDIOC_G_CTRL andVIDIOC_S_CTRL ioctls.Drivers must implementVIDIOC_QUERYCTRL,VIDIOC_G_CTRL andVIDIOC_S_CTRL when the device has one or morecontrols,VIDIOC_QUERYMENU when it has one ormore menu type controls.

Example 1-8. Enumerating all controls

struct v4l2_queryctrl queryctrl;
struct v4l2_querymenu querymenu;

static void
enumerate_menu (void)
{
        printf ("  Menu items:\n");

        memset (&querymenu, 0, sizeof (querymenu));
        querymenu.id = queryctrl.id;

        for (querymenu.index = queryctrl.minimum;
             querymenu.index <= queryctrl.maximum;
              querymenu.index++) {
                if (0 == ioctl (fd, VIDIOC_QUERYMENU, &querymenu)) {
                        printf ("  %s\n", querymenu.name);
                } else {
                        perror ("VIDIOC_QUERYMENU");
                        exit (EXIT_FAILURE);
                }
        }
}

memset (&queryctrl, 0, sizeof (queryctrl));

for (queryctrl.id = V4L2_CID_BASE;
     queryctrl.id < V4L2_CID_LASTP1;
     queryctrl.id++) {
        if (0 == ioctl (fd, VIDIOC_QUERYCTRL, &queryctrl)) {
                if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
                        continue;

                printf ("Control %s\n", queryctrl.name);

                if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
                        enumerate_menu ();
        } else {
                if (errno == EINVAL)
                        continue;

                perror ("VIDIOC_QUERYCTRL");
                exit (EXIT_FAILURE);
        }
}

for (queryctrl.id = V4L2_CID_PRIVATE_BASE;;
     queryctrl.id++) {
        if (0 == ioctl (fd, VIDIOC_QUERYCTRL, &queryctrl)) {
                if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
                        continue;

                printf ("Control %s\n", queryctrl.name);

                if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
                        enumerate_menu ();
        } else {
                if (errno == EINVAL)
                        break;

                perror ("VIDIOC_QUERYCTRL");
                exit (EXIT_FAILURE);
        }
}

Example 1-9. Changing controls

struct v4l2_queryctrl queryctrl;
struct v4l2_control control;

memset (&queryctrl, 0, sizeof (queryctrl));
queryctrl.id = V4L2_CID_BRIGHTNESS;

if (-1 == ioctl (fd, VIDIOC_QUERYCTRL, &queryctrl)) {
        if (errno != EINVAL) {
                perror ("VIDIOC_QUERYCTRL");
                exit (EXIT_FAILURE);
        } else {
                printf ("V4L2_CID_BRIGHTNESS is not supported\n");
        }
} else if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) {
        printf ("V4L2_CID_BRIGHTNESS is not supported\n");
} else {
        memset (&control, 0, sizeof (control));
        control.id = V4L2_CID_BRIGHTNESS;
        control.value = queryctrl.default_value;

        if (-1 == ioctl (fd, VIDIOC_S_CTRL, &control)) {
                perror ("VIDIOC_S_CTRL");
                exit (EXIT_FAILURE);
        }
}

memset (&control, 0, sizeof (control));
control.id = V4L2_CID_CONTRAST;

if (0 == ioctl (fd, VIDIOC_G_CTRL, &control)) {
        control.value += 1;

        /* The driver may clamp the value or return ERANGE, ignored here */

        if (-1 == ioctl (fd, VIDIOC_S_CTRL, &control)
            && errno != ERANGE) {
                perror ("VIDIOC_S_CTRL");
                exit (EXIT_FAILURE);
        }
/* Ignore if V4L2_CID_CONTRAST is unsupported */
} else if (errno != EINVAL) {
        perror ("VIDIOC_G_CTRL");
        exit (EXIT_FAILURE);
}

control.id = V4L2_CID_AUDIO_MUTE;
control.value = TRUE; /* silence */

/* Errors ignored */
ioctl (fd, VIDIOC_S_CTRL, &control);

---

1.9. Extended Controls

1.9.1. Introduction

The control mechanism as originally designed was meantto be used for user settings (brightness, saturation, etc). However,it turned out to be a very useful model for implementing morecomplicated driver APIs where each driver implements only a subset ofa larger API.

The MPEG encoding API was the driving force behinddesigning and implementing this extended control mechanism: the MPEGstandard is quite large and the currently supported hardware MPEGencoders each only implement a subset of this standard. Further more,many parameters relating to how the video is encoded into an MPEGstream are specific to the MPEG encoding chip since the MPEG standardonly defines the format of the resulting MPEG stream, not how thevideo is actually encoded into that format.

Unfortunately, the original control API lacked somefeatures needed for these new uses and so it was extended into the(not terribly originally named) extended control API.

---

1.9.2. The Extended Control API

Three new ioctls are available: VIDIOC_G_EXT_CTRLS,VIDIOC_S_EXT_CTRLS andVIDIOC_TRY_EXT_CTRLS. These ioctls act onarrays of controls (as opposed to theVIDIOC_G_CTRL andVIDIOC_S_CTRL ioctls that act on a single control). This is neededsince it is often required to atomically change several controls atonce.

Each of the new ioctls expects a pointer to astruct v4l2_ext_controls. This structure contains a pointer to the controlarray, a count of the number of controls in that array and a controlclass. Control classes are used to group similar controls into asingle class. For example, control classV4L2_CTRL_CLASS_USERcontains all user controls(i. e. all controls that can also be set using the oldVIDIOC_S_CTRL ioctl). Control classV4L2_CTRL_CLASS_MPEG contains all controlsrelating to MPEG encoding, etc.

All controls in the control array must belong to thespecified control class. An error is returned if this is not thecase.

It is also possible to use an empty control array (count== 0) to check whether the specified control class issupported.

The control array is a struct v4l2_ext_control array. Thev4l2_ext_control structure is very similar tostruct v4l2_control, except for the fact that it also allows for 64-bitvalues and pointers to be passed (although the latter is not yet usedanywhere).

It is important to realize that due to the flexibility ofcontrols it is necessary to check whether the control you want to setactually is supported in the driver and what the valid range of valuesis. So use theVIDIOC_QUERYCTRL andVIDIOC_QUERYMENU ioctls tocheck this. Also note that it is possible that some of the menuindices in a control of typeV4L2_CTRL_TYPE_MENUmay not be supported (VIDIOC_QUERYMENU willreturn an error). A good example is the list of supported MPEG audiobitrates. Some drivers only support one or two bitrates, otherssupport a wider range.

---

1.9.3. Enumerating Extended Controls

The recommended way to enumerate over the extendedcontrols is by using VIDIOC_QUERYCTRL in combination with theV4L2_CTRL_FLAG_NEXT_CTRLflag:

struct v4l2_queryctrl qctrl;

qctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL;
while (0 == ioctl (fd, VIDIOC_QUERYCTRL, &qctrl)) {
        /* ... */
        qctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL;
}

The initial control ID is set to 0 ORed with theV4L2_CTRL_FLAG_NEXT_CTRL flag. TheVIDIOC_QUERYCTRL ioctl will return the firstcontrol with a higher ID than the specified one. When no such controlsare found an error is returned.

If you want to get all controls within a specific controlclass, then you can set the initialqctrl.id value to the control class and addan extra check to break out of the loop when a control of anothercontrol class is found:

qctrl.id = V4L2_CTRL_CLASS_MPEG | V4L2_CTRL_FLAG_NEXT_CTRL;
while (0 == ioctl (fd, VIDIOC_QUERYCTRL, &qctrl)) {
        if (V4L2_CTRL_ID2CLASS (qctrl.id) != V4L2_CTRL_CLASS_MPEG)
                break;
                /* ... */
                qctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL;
        }

The 32-bit qctrl.id value issubdivided into three bit ranges: the top 4 bits are reserved forflags (e. g.V4L2_CTRL_FLAG_NEXT_CTRL) and are notactually part of the ID. The remaining 28 bits form the control ID, ofwhich the most significant 12 bits define the control class and theleast significant 16 bits identify the control within the controlclass. It is guaranteed that these last 16 bits are always non-zerofor controls. The range of 0x1000 and up are reserved fordriver-specific controls. The macroV4L2_CTRL_ID2CLASS(id) returns the control classID based on a control ID.

If the driver does not support extended controls, thenVIDIOC_QUERYCTRL will fail when used incombination withV4L2_CTRL_FLAG_NEXT_CTRL. Inthat case the old method of enumerating control should be used (see1.8). But if it is supported, then it is guaranteed to enumerate overall controls, including driver-private controls.

---

1.9.4. Creating Control Panels

It is possible to create control panels for a graphicaluser interface where the user can select the various controls.Basically you will have to iterate over all controls using the methoddescribed above. Each control class starts with a control of typeV4L2_CTRL_TYPE_CTRL_CLASS.VIDIOC_QUERYCTRL will return the name of thiscontrol class which can be used as the title of a tab page within acontrol panel.

The flags field of struct v4l2_queryctrl also contains hints onthe behavior of the control. See theVIDIOC_QUERYCTRL documentationfor more details.

---

1.9.5. MPEG Control Reference

Below all controls within the MPEG control class aredescribed. First the generic controls, then controls specific forcertain hardware.

---

1.9.5.1. Generic MPEG Controls

Table 1-2. MPEG Control IDs

ID		Type
	Description
V4L2_CID_MPEG_CLASS		class
	The MPEG classdescriptor. Calling VIDIOC_QUERYCTRL for this control will return adescription of this control class. This description can be used as thecaption of a Tab page in a GUI, for example.
V4L2_CID_MPEG_STREAM_TYPE		enum
	The MPEG-1, -2 or -4output stream type. One cannot assume anything here. Each hardwareMPEG encoder tends to support different subsets of the available MPEGstream types. The currently defined stream types are:
	V4L2_MPEG_STREAM_TYPE_MPEG2_PS  MPEG-2 program stream V4L2_MPEG_STREAM_TYPE_MPEG2_TS  MPEG-2 transport stream V4L2_MPEG_STREAM_TYPE_MPEG1_SS  MPEG-1 system stream V4L2_MPEG_STREAM_TYPE_MPEG2_DVD  MPEG-2 DVD-compatible stream V4L2_MPEG_STREAM_TYPE_MPEG1_VCD  MPEG-1 VCD-compatible stream V4L2_MPEG_STREAM_TYPE_MPEG2_SVCD  MPEG-2 SVCD-compatible stream
V4L2_CID_MPEG_STREAM_PID_PMT		integer
	Program Map TablePacket ID for the MPEG transport stream (default 16)
V4L2_CID_MPEG_STREAM_PID_AUDIO		integer
	Audio Packet ID forthe MPEG transport stream (default 256)
V4L2_CID_MPEG_STREAM_PID_VIDEO		integer
	Video Packet ID forthe MPEG transport stream (default 260)
V4L2_CID_MPEG_STREAM_PID_PCR		integer
	Packet ID for theMPEG transport stream carrying PCR fields (default 259)
V4L2_CID_MPEG_STREAM_PES_ID_AUDIO		integer
	Audio ID for MPEGPES
V4L2_CID_MPEG_STREAM_PES_ID_VIDEO		integer
	Video ID for MPEGPES
V4L2_CID_MPEG_STREAM_VBI_FMT		enum
	Some cards can embedVBI data (e. g. Closed Caption, Teletext) into the MPEG stream. Thiscontrol selects whether VBI data should be embedded, and if so, whatembedding method should be used. The list of possible VBI formatsdepends on the driver. The currently defined VBI format typesare:
	V4L2_MPEG_STREAM_VBI_FMT_NONE  No VBI in the MPEG stream V4L2_MPEG_STREAM_VBI_FMT_IVTV  VBI in private packets, IVTV format (documentedin the kernel sources in the fileDocumentation/video4linux/cx2341x/README.vbi)
V4L2_CID_MPEG_AUDIO_SAMPLING_FREQ		enum
	MPEG Audio samplingfrequency. Possible values are:
	V4L2_MPEG_AUDIO_SAMPLING_FREQ_44100  44.1 kHz V4L2_MPEG_AUDIO_SAMPLING_FREQ_48000  48 kHz V4L2_MPEG_AUDIO_SAMPLING_FREQ_32000  32 kHz
V4L2_CID_MPEG_AUDIO_ENCODING		enum
	MPEG Audio encoding.Possible values are:
	V4L2_MPEG_AUDIO_ENCODING_LAYER_1  MPEG Layer I encoding V4L2_MPEG_AUDIO_ENCODING_LAYER_2  MPEG Layer II encoding V4L2_MPEG_AUDIO_ENCODING_LAYER_3  MPEG Layer III encoding
V4L2_CID_MPEG_AUDIO_L1_BITRATE		enum
	Layer I bitrate.Possible values are:
	V4L2_MPEG_AUDIO_L1_BITRATE_32K  32 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_64K  64 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_96K  96 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_128K  128 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_160K  160 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_192K  192 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_224K  224 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_256K  256 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_288K  288 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_320K  320 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_352K  352 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_384K  384 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_416K  416 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_448K  448 kbit/s
V4L2_CID_MPEG_AUDIO_L2_BITRATE		enum
	Layer II bitrate.Possible values are:
	V4L2_MPEG_AUDIO_L2_BITRATE_32K  32 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_48K  48 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_56K  56 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_64K  64 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_80K  80 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_96K  96 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_112K  112 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_128K  128 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_160K  160 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_192K  192 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_224K  224 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_256K  256 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_320K  320 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_384K  384 kbit/s
V4L2_CID_MPEG_AUDIO_L3_BITRATE		enum
	Layer III bitrate.Possible values are:
	V4L2_MPEG_AUDIO_L3_BITRATE_32K  32 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_40K  40 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_48K  48 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_56K  56 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_64K  64 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_80K  80 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_96K  96 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_112K  112 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_128K  128 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_160K  160 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_192K  192 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_224K  224 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_256K  256 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_320K  320 kbit/s
V4L2_CID_MPEG_AUDIO_MODE		enum
	MPEG Audio mode.Possible values are:
	V4L2_MPEG_AUDIO_MODE_STEREO  Stereo V4L2_MPEG_AUDIO_MODE_JOINT_STEREO  Joint Stereo V4L2_MPEG_AUDIO_MODE_DUAL  Bilingual V4L2_MPEG_AUDIO_MODE_MONO  Mono
V4L2_CID_MPEG_AUDIO_MODE_EXTENSION		enum
	Joint Stereoaudio mode extension. In Layer I and II they indicate which subbandsare in intensity stereo. All other subbands are coded in stereo. LayerIII is not (yet) supported. Possible valuesare:
	V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_4  Subbands 4-31 in intensity stereo V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_8  Subbands 8-31 in intensity stereo V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_12  Subbands 12-31 in intensity stereo V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_16  Subbands 16-31 in intensity stereo
V4L2_CID_MPEG_AUDIO_EMPHASIS		enum
	Audio Emphasis.Possible values are:
	V4L2_MPEG_AUDIO_EMPHASIS_NONE  None V4L2_MPEG_AUDIO_EMPHASIS_50_DIV_15_uS  50/15 microsecond emphasis V4L2_MPEG_AUDIO_EMPHASIS_CCITT_J17  CCITT J.17
V4L2_CID_MPEG_AUDIO_CRC		enum
	CRC method. Possiblevalues are:
	V4L2_MPEG_AUDIO_CRC_NONE  None V4L2_MPEG_AUDIO_CRC_CRC16  16 bit parity check
V4L2_CID_MPEG_AUDIO_MUTE		bool
	Mutes the audio whencapturing. This is not done by muting audio hardware, which can stillproduce a slight hiss, but in the encoder itself, guaranteeing a fixedand reproducable audio bitstream. 0 = unmuted, 1 = muted.
V4L2_CID_MPEG_VIDEO_ENCODING		enum
	MPEG Video encodingmethod. Possible values are:
	V4L2_MPEG_VIDEO_ENCODING_MPEG_1  MPEG-1 Video encoding V4L2_MPEG_VIDEO_ENCODING_MPEG_2  MPEG-2 Video encoding
V4L2_CID_MPEG_VIDEO_ASPECT		enum
	Video aspect.Possible values are:
	V4L2_MPEG_VIDEO_ASPECT_1x1    V4L2_MPEG_VIDEO_ASPECT_4x3    V4L2_MPEG_VIDEO_ASPECT_16x9    V4L2_MPEG_VIDEO_ASPECT_221x100
V4L2_CID_MPEG_VIDEO_B_FRAMES		integer
	Number of B-Frames(default 2)
V4L2_CID_MPEG_VIDEO_GOP_SIZE		integer
	GOP size (default12)
V4L2_CID_MPEG_VIDEO_GOP_CLOSURE		bool
	GOP closure (default1)
V4L2_CID_MPEG_VIDEO_PULLDOWN		bool
	Enable 3:2 pulldown(default 0)
V4L2_CID_MPEG_VIDEO_BITRATE_MODE		enum
	Video bitrate mode.Possible values are:
	V4L2_MPEG_VIDEO_BITRATE_MODE_VBR  Variable bitrate V4L2_MPEG_VIDEO_BITRATE_MODE_CBR  Constant bitrate
V4L2_CID_MPEG_VIDEO_BITRATE		integer
	Video bitrate in bitsper second.
V4L2_CID_MPEG_VIDEO_BITRATE_PEAK		integer
	Peak video bitrate inbits per second. Must be larger or equal to the average video bitrate.It is ignored if the video bitrate mode is set to constantbitrate.
V4L2_CID_MPEG_VIDEO_TEMPORAL_DECIMATION		integer
	For every capturedframe, skip this many subsequent frames (default 0).
V4L2_CID_MPEG_VIDEO_MUTE		bool
	"Mutes" the video to afixed color when capturing. This is useful for testing, to produce afixed video bitstream. 0 = unmuted, 1 = muted.
V4L2_CID_MPEG_VIDEO_MUTE_YUV		integer
	Sets the "mute" colorof the video. The supplied 32-bit integer is interpreted as follows (bit0 = least significant bit):
	Bit 0:7 V chrominance information Bit 8:15 U chrominance information Bit 16:23 Y luminance information Bit 24:31 Must be zero.

---

1.9.5.2. CX2341x MPEG Controls

The following MPEG class controls deal with MPEGencoding settings that are specific to the Conexant CX23415 andCX23416 MPEG encoding chips.

Table 1-3. CX2341x Control IDs

ID		Type
	Description
V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE		enum
	Sets the SpatialFilter mode (default MANUAL). Possible valuesare:
	V4L2_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE_MANUAL  Choose the filter manually V4L2_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE_AUTO  Choose the filter automatically
V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER		integer (0-15)
	The setting for theSpatial Filter. 0 = off, 15 = maximum. (Default is 0.)
V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE		enum
	Select the algorithmto use for the Luma Spatial Filter (default1D_HOR). Possible values:
	V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_OFF  No filter V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_1D_HOR  One-dimensional horizontal V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_1D_VERT  One-dimensional vertical V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_2D_HV_SEPARABLE  Two-dimensional separable V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_2D_SYM_NON_SEPARABLE  Two-dimensional symmetricalnon-separable
V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_SPATIAL_FILTER_TYPE		enum
	Select the algorithmfor the Chroma Spatial Filter (default 1D_HOR).Possible values are:
	V4L2_MPEG_CX2341X_VIDEO_CHROMA_SPATIAL_FILTER_TYPE_OFF  No filter V4L2_MPEG_CX2341X_VIDEO_CHROMA_SPATIAL_FILTER_TYPE_1D_HOR  One-dimensional horizontal
V4L2_CID_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER_MODE		enum
	Sets the TemporalFilter mode (default MANUAL). Possible valuesare:
	V4L2_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER_MODE_MANUAL  Choose the filter manually V4L2_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER_MODE_AUTO  Choose the filter automatically
V4L2_CID_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER		integer (0-31)
	The setting for theTemporal Filter. 0 = off, 31 = maximum. (Default is 8 for full-scalecapturing and 0 for scaled capturing.)
V4L2_CID_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE		enum
	Median Filter Type(default OFF). Possible values are:
	V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_OFF  No filter V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_HOR  Horizontal filter V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_VERT  Vertical filter V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_HOR_VERT  Horizontal and vertical filter V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_DIAG  Diagonal filter
V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_MEDIAN_FILTER_BOTTOM		integer (0-255)
	Threshold above whichthe luminance median filter is enabled (default 0)
V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_MEDIAN_FILTER_TOP		integer (0-255)
	Threshold below whichthe luminance median filter is enabled (default 255)
V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_MEDIAN_FILTER_BOTTOM		integer (0-255)
	Threshold above whichthe chroma median filter is enabled (default 0)
V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_MEDIAN_FILTER_TOP		integer (0-255)
	Threshold below whichthe chroma median filter is enabled (default 255)
V4L2_CID_MPEG_CX2341X_STREAM_INSERT_NAV_PACKETS		bool
	The CX2341X MPEG encodercan insert one empty MPEG-2 PES packet into the stream between everyfour video frames. The packet size is 2048 bytes, including thepacket_start_code_prefix and stream_id fields. The stream_id is 0xBF(private stream 2). The payload consists of 0x00 bytes, to be filledin by the application. 0 = do not insert, 1 = insert packets.

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1.9.6. Camera Control Reference

The Camera class includes controls for mechanical (orequivalent digital) features of a device such as controllable lensesor sensors.

Table 1-4. Camera Control IDs

ID		Type
	Description
V4L2_CID_CAMERA_CLASS		class
	The Camera classdescriptor. Calling VIDIOC_QUERYCTRL for this control will return adescription of this control class.
V4L2_CID_EXPOSURE_AUTO		integer
	Enables automaticadjustments of the exposure time and/or iris aperture. The effect ofmanual changes of the exposure time or iris aperture while thesefeatures are enabled is undefined, drivers should ignore suchrequests. Possible values are:
	V4L2_EXPOSURE_AUTO  Automatic exposure time, automatic irisaperture. V4L2_EXPOSURE_MANUAL  Manual exposure time, manual iris. V4L2_EXPOSURE_SHUTTER_PRIORITY  Manual exposure time, auto iris. V4L2_EXPOSURE_APERTURE_PRIORITY  Auto exposure time, manual iris.
V4L2_CID_EXPOSURE_ABSOLUTE		integer
	Determines the exposuretime of the camera sensor. The exposure time is limited by the frameinterval. Drivers should interpret the values as 100 µs units,where the value 1 stands for 1/10000th of a second, 10000 for 1 secondand 100000 for 10 seconds.
V4L2_CID_EXPOSURE_AUTO_PRIORITY		boolean
	WhenV4L2_CID_EXPOSURE_AUTO is set toAUTO orSHUTTER_PRIORITY,this control determines if the device may dynamically vary the framerate. By default this feature is disabled (0) and the frame rate mustremain constant.
V4L2_CID_PAN_RELATIVE		integer
	This control turns thecamera horizontally by the specified amount. The unit is undefined. Apositive value moves the camera to the right (clockwise when viewedfrom above), a negative value to the left. A value of zero does notcause motion.
V4L2_CID_TILT_RELATIVE		integer
	This control turns thecamera vertically by the specified amount. The unit is undefined. Apositive value moves the camera up, a negative value down. A value ofzero does not cause motion.
V4L2_CID_PAN_RESET		boolean
	When this control is setto TRUE (1), the camera moves horizontally to thedefault position.
V4L2_CID_TILT_RESET		boolean
	When this control is setto TRUE (1), the camera moves vertically to thedefault position.
V4L2_CID_PAN_ABSOLUTE		integer
	This controlturns the camera horizontally to the specified position. Positivevalues move the camera to the right (clockwise when viewed from above),negative values to the left. Drivers should interpret the values as arcseconds, with valid values between -180 * 3600 and +180 * 3600inclusive.
V4L2_CID_TILT_ABSOLUTE		integer
	This controlturns the camera vertically to the specified position. Positive valuesmove the camera up, negative values down. Drivers should interpret thevalues as arc seconds, with valid values between -180 * 3600 and +180* 3600 inclusive.
V4L2_CID_FOCUS_ABSOLUTE		integer
	This control sets thefocal point of the camera to the specified position. The unit isundefined. Positive values set the focus closer to the camera,negative values towards infinity.
V4L2_CID_FOCUS_RELATIVE		integer
	This control moves thefocal point of the camera by the specified amount. The unit isundefined. Positive values move the focus closer to the camera,negative values towards infinity.
V4L2_CID_FOCUS_AUTO		boolean
	Enables automatic focusadjustments. The effect of manual focus adjustments while this featureis enabled is undefined, drivers should ignore such requests.

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1.10. Data Formats

1.10.1. Data Format Negotiation

Different devices exchange different kinds of data withapplications, for example video images, raw or sliced VBI data, RDSdatagrams. Even within one kind many different formats are possible,in particular an abundance of image formats. Although drivers mustprovide a default and the selection persists across closing andreopening a device, applications should always negotiate a data formatbefore engaging in data exchange. Negotiation means the applicationasks for a particular format and the driver selects and reports thebest the hardware can do to satisfy the request. Of courseapplications can also just query the current selection.

A single mechanism exists to negotiate all data formatsusing the aggregate struct v4l2_format and theVIDIOC_G_FMT andVIDIOC_S_FMT ioctls. Additionally theVIDIOC_TRY_FMT ioctl can beused to examine what the hardwarecould do,without actually selecting a new data format. The data formatssupported by the V4L2 API are covered in the respective device sectioninChapter 4. For a closer look at image formats seeChapter 2.

The VIDIOC_S_FMT ioctl is a majorturning-point in the initialization sequence. Prior to this pointmultiple panel applications can access the same device concurrently toselect the current input, change controls or modify other properties.The first VIDIOC_S_FMT assigns a logical stream(video data, VBI data etc.) exclusively to one file descriptor.

Exclusive means no other application, more precisely noother file descriptor, can grab this stream or change deviceproperties inconsistent with the negotiated parameters. A videostandard change for example, when the new standard uses a differentnumber of scan lines, can invalidate the selected image format.Therefore only the file descriptor owning the stream can makeinvalidating changes. Accordingly multiple file descriptors whichgrabbed different logical streams prevent each other from interferingwith their settings. When for example video overlay is about to startor already in progress, simultaneous video capturing may be restrictedto the same cropping and image size.

When applications omit theVIDIOC_S_FMT ioctl its locking side effects areimplied by the next step, the selection of an I/O method with theVIDIOC_REQBUFS ioctl or implicit with the first read() orwrite() call.

Generally only one logical stream can be assigned to afile descriptor, the exception being drivers permitting simultaneousvideo capturing and overlay using the same file descriptor forcompatibility with V4L and earlier versions of V4L2. Switching thelogical stream or returning into "panel mode" is possible by closingand reopening the device. Driversmay support aswitch usingVIDIOC_S_FMT.

All drivers exchanging data withapplications must support the VIDIOC_G_FMT andVIDIOC_S_FMT ioctl. Implementation of theVIDIOC_TRY_FMT is highly recommended butoptional.

---

1.10.2. Image Format Enumeration

Apart of the generic format negotiation functionsa special ioctl to enumerate all image formats supported by videocapture, overlay or output devices is available.[11]

The VIDIOC_ENUM_FMT ioctl must be supportedby all drivers exchanging image data with applications.

Important: Drivers are not supposed to convert image formats inkernel space. They must enumerate only formats directly supported bythe hardware. If necessary driver writers should publish an exampleconversion routine or library for integration into applications.

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1.11. Image Cropping, Insertion and Scaling

Some video capture devices can sample a subsection of thepicture and shrink or enlarge it to an image of arbitrary size. Wecall these abilities cropping and scaling. Some video output devicescan scale an image up or down and insert it at an arbitrary scan lineand horizontal offset into a video signal.

Applications can use the following API to select an area inthe video signal, query the default area and the hardware limits.Despite their name, theVIDIOC_CROPCAP,VIDIOC_G_CROPandVIDIOC_S_CROP ioctls apply to input as well as outputdevices.

Scaling requires a source and a target. On a video captureor overlay device the source is the video signal, and the croppingioctls determine the area actually sampled. The target are imagesread by the application or overlaid onto the graphics screen. Theirsize (and position for an overlay) is negotiated with theVIDIOC_G_FMT andVIDIOC_S_FMT ioctls.

On a video output device the source are the images passed inby the application, and their size is again negotiated with theVIDIOC_G/S_FMT ioctls, or may be encoded in acompressed video stream. The target is the video signal, and thecropping ioctls determine the area where the images areinserted.

Source and target rectangles are defined even if the devicedoes not support scaling or theVIDIOC_G/S_CROPioctls. Their size (and position where applicable) will be fixed inthis case.All capture and output device must support theVIDIOC_CROPCAP ioctl such that applications candetermine if scaling takes place.

---

1.11.1. Cropping Structures

Figure 1-1. Image Cropping, Insertion and Scaling

For capture devices the coordinates of the top leftcorner, width and height of the area which can be sampled is given bythebounds substructure of thestruct v4l2_cropcap returned by theVIDIOC_CROPCAPioctl. To support a wide range of hardware this specification does notdefine an origin or units. However by convention drivers shouldhorizontally count unscaled samples relative to 0H (the leading edgeof the horizontal sync pulse, see Figure 4-1).Vertically ITU-R linenumbers of the first field (Figure 4-2,Figure 4-3), multiplied by two if the driver can capture bothfields.

The top left corner, width and height of the sourcerectangle, that is the area actually sampled, is given by struct v4l2_cropusing the same coordinate system as struct v4l2_cropcap. Applications canuse the VIDIOC_G_CROP andVIDIOC_S_CROP ioctls to get and set thisrectangle. It must lie completely within the capture boundaries andthe driver may further adjust the requested size and/or positionaccording to hardware limitations.

Each capture device has a default source rectangle, givenby the defrect substructure ofstruct v4l2_cropcap. The center of this rectangle shall align with thecenter of the active picture area of the video signal, and cover whatthe driver writer considers the complete picture. Drivers shall resetthe source rectangle to the default when the driver is first loaded,but not later.

For output devices these structures and ioctls are usedaccordingly, defining thetarget rectangle wherethe images will be inserted into the video signal.

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1.11.2. Scaling Adjustments

Video hardware can have various cropping, insertion andscaling limitations. It may only scale up or down, support onlydiscrete scaling factors, or have different scaling abilities inhorizontal and vertical direction. Also it may not support scaling atall. At the same time the struct v4l2_crop rectangle may have to bealigned, and both the source and target rectangles may have arbitraryupper and lower size limits. In particular the maximumwidth and heightin struct v4l2_crop may be smaller than thestruct v4l2_cropcap.bounds area. Therefore, asusual, drivers are expected to adjust the requested parameters andreturn the actual values selected.

Applications can change the source or the target rectanglefirst, as they may prefer a particular image size or a certain area inthe video signal. If the driver has to adjust both to satisfy hardwarelimitations, the last requested rectangle shall take priority, and thedriver should preferably adjust the opposite one. TheVIDIOC_TRY_FMTioctl however shall not change the driver state and therefore onlyadjust the requested rectangle.

Suppose scaling on a video capture device is restricted toa factor 1:1 or 2:1 in either direction and the target image size mustbe a multiple of 16 × 16 pixels. The source croppingrectangle is set to defaults, which are also the upper limit in thisexample, of 640 × 400 pixels at offset 0, 0. Anapplication requests an image size of 300 × 225pixels, assuming video will be scaled down from the "full picture"accordingly. The driver sets the image size to the closest possiblevalues 304 × 224, then chooses the cropping rectangleclosest to the requested size, that is 608 × 224(224 × 2:1 would exceed the limit 400). The offset0, 0 is still valid, thus unmodified. Given the default croppingrectangle reported byVIDIOC_CROPCAP theapplication can easily propose another offset to center the croppingrectangle.

Now the application may insist on covering an area using apicture aspect ratio closer to the original request, so it asks for acropping rectangle of 608 × 456 pixels. The presentscaling factors limit cropping to 640 × 384, so thedriver returns the cropping size 608 × 384 and adjuststhe image size to closest possible 304 × 192.

---

1.11.3. Examples

Source and target rectangles shall remain unchanged acrossclosing and reopening a device, such that piping data into or out of adevice will work without special preparations. More advancedapplications should ensure the parameters are suitable before startingI/O.

Example 1-10. Resetting the cropping parameters

(A video capture device is assumed; changeV4L2_BUF_TYPE_VIDEO_CAPTURE for otherdevices.)

struct v4l2_cropcap cropcap;
struct v4l2_crop crop;

memset (&cropcap, 0, sizeof (cropcap));
cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

if (-1 == ioctl (fd, VIDIOC_CROPCAP, &cropcap)) {
        perror ("VIDIOC_CROPCAP");
        exit (EXIT_FAILURE);
}

memset (&crop, 0, sizeof (crop));
crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
crop.c = cropcap.defrect; 

/* Ignore if cropping is not supported (EINVAL). */

if (-1 == ioctl (fd, VIDIOC_S_CROP, &crop)
    && errno != EINVAL) {
        perror ("VIDIOC_S_CROP");
        exit (EXIT_FAILURE);
}
      

Example 1-11. Simple downscaling

(A video capture device is assumed.)

struct v4l2_cropcap cropcap;
struct v4l2_format format;

reset_cropping_parameters ();

/* Scale down to 1/4 size of full picture. */

memset (&format, 0, sizeof (format)); /* defaults */

format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

format.fmt.pix.width = cropcap.defrect.width >> 1;
format.fmt.pix.height = cropcap.defrect.height >> 1;
format.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;

if (-1 == ioctl (fd, VIDIOC_S_FMT, &format)) {
        perror ("VIDIOC_S_FORMAT");
        exit (EXIT_FAILURE);
}

/* We could check the actual image size now, the actual scaling factor
   or if the driver can scale at all. */
        

Example 1-12. Selecting an output area

struct v4l2_cropcap cropcap;
struct v4l2_crop crop;

memset (&cropcap, 0, sizeof (cropcap));
cropcap.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;

if (-1 == ioctl (fd, VIDIOC_CROPCAP, &cropcap)) {
        perror ("VIDIOC_CROPCAP");
        exit (EXIT_FAILURE);
}

memset (&crop, 0, sizeof (crop));

crop.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;
crop.c = cropcap.defrect;

/* Scale the width and height to 50 % of their original size
   and center the output. */

crop.c.width /= 2;
crop.c.height /= 2;
crop.c.left += crop.c.width / 2;
crop.c.top += crop.c.height / 2;

/* Ignore if cropping is not supported (EINVAL). */

if (-1 == ioctl (fd, VIDIOC_S_CROP, &crop)
    && errno != EINVAL) {
        perror ("VIDIOC_S_CROP");
        exit (EXIT_FAILURE);
}

Example 1-13. Current scaling factor and pixel aspect

(A video capture device is assumed.)

struct v4l2_cropcap cropcap;
struct v4l2_crop crop;
struct v4l2_format format;
double hscale, vscale;
double aspect;
int dwidth, dheight;

memset (&cropcap, 0, sizeof (cropcap));
cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

if (-1 == ioctl (fd, VIDIOC_CROPCAP, &cropcap)) {
        perror ("VIDIOC_CROPCAP");
        exit (EXIT_FAILURE);
}

memset (&crop, 0, sizeof (crop));
crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

if (-1 == ioctl (fd, VIDIOC_G_CROP, &crop)) {
        if (errno != EINVAL) {
                perror ("VIDIOC_G_CROP");
                exit (EXIT_FAILURE);
        }

        /* Cropping not supported. */
        crop.c = cropcap.defrect;
}

memset (&format, 0, sizeof (format));
format.fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

if (-1 == ioctl (fd, VIDIOC_G_FMT, &format)) {
        perror ("VIDIOC_G_FMT");
        exit (EXIT_FAILURE);
}

/* The scaling applied by the driver. */

hscale = format.fmt.pix.width / (double) crop.c.width;
vscale = format.fmt.pix.height / (double) crop.c.height;

aspect = cropcap.pixelaspect.numerator /
         (double) cropcap.pixelaspect.denominator;
aspect = aspect * hscale / vscale;

/* Devices following ITU-R BT.601 do not capture
   square pixels. For playback on a computer monitor
   we should scale the images to this size. */

dwidth = format.fmt.pix.width / aspect;
dheight = format.fmt.pix.height;
        

---

1.12. Streaming Parameters

Streaming parameters are intended to optimize the videocapture process as well as I/O. Presently applications can request ahigh quality capture mode with theVIDIOC_S_PARM ioctl.

The current video standard determines a nominal number offrames per second. If less than this number of frames is to becaptured or output, applications can request frame skipping orduplicating on the driver side. This is especially useful when usingtheread() orwrite(), which are not augmented by timestampsor sequence counters, and to avoid unneccessary data copying.

Finally these ioctls can be used to determine the number ofbuffers used internally by a driver in read/write mode. Forimplications see the section discussing theread()function.

To get and set the streaming parameters applications callthe VIDIOC_G_PARM and VIDIOC_S_PARM ioctl, respectively. They takea pointer to a struct v4l2_streamparm, which contains a union holdingseparate parameters for input and output devices.

These ioctls are optional, drivers need not implementthem. If so, they return theEINVAL error code.

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Chapter 2. Image Formats

The V4L2 API was primarily designed for devices exchangingimage data with applications. Thev4l2_pix_format structure defines the formatand layout of an image in memory. Image formats are negotiated withtheVIDIOC_S_FMT ioctl. (The explanations here focus on videocapturing and output, for overlay frame buffer formats see alsoVIDIOC_G_FBUF.)

Table 2-1. struct v4l2_pix_format

__u32	width	Image width in pixels.
__u32	height	Image height in pixels.
Applications set these fields torequest an image size, drivers return the closest possible values. Incase of planar formats thewidth andheight applies to the largest plane. Toavoid ambiguities drivers must return values rounded up to a multipleof the scale factor of any smaller planes. For example when the imageformat is YUV 4:2:0,width andheight must be multiples of two.
__u32	pixelformat	The pixel format or type of compression, set by theapplication. This is a little endianfour character code. V4L2 definesstandard RGB formats inTable 2-1, YUV formats inSection 2.5, and reserved codes inTable 2-8
enum v4l2_field	field	Video images are typically interlaced. Applicationscan request to capture or output only the top or bottom field, or bothfields interlaced or sequentially stored in one buffer or alternatingin separate buffers. Drivers return the actual field order selected.For details see Section 3.6.
__u32	bytesperline	Distance in bytes between the leftmost pixels in twoadjacent lines.
Both applications and driverscan set this field to request padding bytes at the end of each line.Drivers however may ignore the value requested by the application,returningwidth times bytes per pixel or alarger value required by the hardware. That implies applications canjust set this field to zero to get a reasonabledefault. Video hardware may access padding bytes,therefore they must reside in accessible memory. Consider cases wherepadding bytes after the last line of an image cross a system pageboundary. Input devices may write padding bytes, the value isundefined. Output devices ignore the contents of paddingbytes. When the image format is planar thebytesperline value applies to the largestplane and is divided by the same factor as thewidth field for any smaller planes. Forexample the Cb and Cr planes of a YUV 4:2:0 image have half as manypadding bytes following each line as the Y plane. To avoid ambiguitiesdrivers must return abytesperline valuerounded up to a multiple of the scale factor.
__u32	sizeimage	Size in bytes of the buffer to hold a complete image,set by the driver. Usually this isbytesperline timesheight. When the image consists of variablelength compressed data this is the maximum number of bytes required tohold an image.
enum v4l2_colorspace	colorspace	This information supplements thepixelformat and must be set by the driver,seeSection 2.2.
__u32	priv	Reserved for custom (driver defined) additionalinformation about formats. When not used drivers and applications mustset this field to zero.

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2.1. Standard Image Formats

In order to exchange images between drivers andapplications, it is necessary to have standard image data formatswhich both sides will interpret the same way. V4L2 includes severalsuch formats, and this section is intended to be an unambiguousspecification of the standard image data formats in V4L2.

V4L2 drivers are not limited to these formats, however.Driver-specific formats are possible. In that case the application maydepend on a codec to convert images to one of the standard formatswhen needed. But the data can still be stored and retrieved in theproprietary format. For example, a device may support a proprietarycompressed format. Applications can still capture and save the data inthe compressed format, saving much disk space, and later use a codecto convert the images to the X Windows screen format when the video isto be displayed.

Even so, ultimately, some standard formats are needed, sothe V4L2 specification would not be complete without well-definedstandard formats.

The V4L2 standard formats are mainly uncompressed formats. Thepixels are always arranged in memory from left to right, and from topto bottom. The first byte of data in the image buffer is always forthe leftmost pixel of the topmost row. Following that is the pixelimmediately to its right, and so on until the end of the top row ofpixels. Following the rightmost pixel of the row there may be zero ormore bytes of padding to guarantee that each row of pixel data has acertain alignment. Following the pad bytes, if any, is data for theleftmost pixel of the second row from the top, and so on. The last rowhas just as many pad bytes after it as the other rows.

In V4L2 each format has an identifier which looks likePIX_FMT_XXX, defined in thevideodev.h header file. These identifiersrepresentfour character codeswhich are also listed below, however they are not the same as thoseused in the Windows world.

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2.2. Colorspaces

[intro]

Gamma Correction

[to do]

E'_R = f(R)

E'_G = f(G)

E'_B = f(B)

Construction of luminance and color-differencesignals

[to do]

E'_Y =Coeff_R E'_R+ Coeff_G E'_G+ Coeff_B E'_B

(E'_R - E'_Y) = E'_R- Coeff_R E'_R- Coeff_G E'_G- Coeff_B E'_B

(E'_B - E'_Y) = E'_B- Coeff_R E'_R- Coeff_G E'_G- Coeff_B E'_B

Re-normalized color-difference signals

The color-difference signals are scaled back to unityrange [-0.5;+0.5]:

K_B = 0.5 / (1 - Coeff_B)

K_R = 0.5 / (1 - Coeff_R)

P_B =K_B (E'_B - E'_Y) = 0.5 (Coeff_R / Coeff_B) E'_R+ 0.5 (Coeff_G / Coeff_B) E'_G+ 0.5 E'_B

P_R =K_R (E'_R - E'_Y) = 0.5 E'_R+ 0.5 (Coeff_G / Coeff_R) E'_G+ 0.5 (Coeff_B / Coeff_R) E'_B

Quantization

[to do]

Y' = (Lum. Levels - 1) · E'_Y + Lum. Offset

C_B = (Chrom. Levels - 1)· P_B + Chrom. Offset

C_R = (Chrom. Levels - 1)· P_R + Chrom. Offset

Rounding to the nearest integer and clamping to the range[0;255] finally yields the digital color components Y'CbCrstored in YUV images.

Example 2-1. ITU-R Rec. BT.601 color conversion

Forward Transformation

int ER, EG, EB;         /* gamma corrected RGB input [0;255] */
int Y1, Cb, Cr;         /* output [0;255] */

double r, g, b;         /* temporaries */
double y1, pb, pr;

int
clamp (double x)
{
        int r = x;      /* round to nearest */

        if (r < 0)         return 0;
        else if (r > 255)  return 255;
        else               return r;
}

r = ER / 255.0;
g = EG / 255.0;
b = EB / 255.0;

y1  =  0.299  * r + 0.587 * g + 0.114  * b;
pb  = -0.169  * r - 0.331 * g + 0.5    * b;
pr  =  0.5    * r - 0.419 * g - 0.081  * b;

Y1 = clamp (219 * y1 + 16);
Cb = clamp (224 * pb + 128);
Cr = clamp (224 * pr + 128);

/* or shorter */

y1 = 0.299 * ER + 0.587 * EG + 0.114 * EB;

Y1 = clamp ( (219 / 255.0)                    *       y1  + 16);
Cb = clamp (((224 / 255.0) / (2 - 2 * 0.114)) * (EB - y1) + 128);
Cr = clamp (((224 / 255.0) / (2 - 2 * 0.299)) * (ER - y1) + 128);
      

Inverse Transformation

int Y1, Cb, Cr;         /* gamma pre-corrected input [0;255] */
int ER, EG, EB;         /* output [0;255] */

double r, g, b;         /* temporaries */
double y1, pb, pr;

int
clamp (double x)
{
        int r = x;      /* round to nearest */

        if (r < 0)         return 0;
        else if (r > 255)  return 255;
        else               return r;
}

y1 = (255 / 219.0) * (Y1 - 16);
pb = (255 / 224.0) * (Cb - 128);
pr = (255 / 224.0) * (Cr - 128);

r = 1.0 * y1 + 0     * pb + 1.402 * pr;
g = 1.0 * y1 - 0.344 * pb - 0.714 * pr;
b = 1.0 * y1 + 1.772 * pb + 0     * pr;

ER = clamp (r * 255); /* [ok? one should prob. limit y1,pb,pr] */
EG = clamp (g * 255);
EB = clamp (b * 255);
      

Table 2-2. enum v4l2_colorspace

Identifier	Value	Description	Chromaticities[a]			White Point	Gamma Correction	Luminance E'Y	Quantization
			Red	Green	Blue				Y'	Cb, Cr
V4L2_COLORSPACE_SMPTE170M	1	NTSC/PAL according toSMPTE 170M,ITU BT.601	x = 0.630, y = 0.340	x = 0.310, y = 0.595	x = 0.155, y = 0.070	x = 0.3127, y = 0.3290, Illuminant D65	E' = 4.5 I for I ≤0.018,1.099 I0.45 - 0.099 for 0.018 < I	0.299 E'R+ 0.587 E'G+ 0.114 E'B	219 E'Y + 16	224 PB,R + 128
V4L2_COLORSPACE_SMPTE240M	2	1125-Line (US) HDTV, see SMPTE 240M	x = 0.630, y = 0.340	x = 0.310, y = 0.595	x = 0.155, y = 0.070	x = 0.3127, y = 0.3290, Illuminant D65	E' = 4 I for I ≤0.0228,1.1115 I0.45 - 0.1115 for 0.0228 < I	0.212 E'R+ 0.701 E'G+ 0.087 E'B	219 E'Y + 16	224 PB,R + 128
V4L2_COLORSPACE_REC709	3	HDTV and modern devices, see ITU BT.709	x = 0.640, y = 0.330	x = 0.300, y = 0.600	x = 0.150, y = 0.060	x = 0.3127, y = 0.3290, Illuminant D65	E' = 4.5 I for I ≤0.018,1.099 I0.45 - 0.099 for 0.018 < I	0.2125 E'R+ 0.7154 E'G+ 0.0721 E'B	219 E'Y + 16	224 PB,R + 128
V4L2_COLORSPACE_BT878	4	Broken Bt878 extents[b],ITU BT.601	?	?	?	?	?	0.299 E'R+ 0.587 E'G+ 0.114 E'B	237 E'Y + 16	224 PB,R + 128 (probably)
V4L2_COLORSPACE_470_SYSTEM_M	5	M/NTSC[c] according toITU BT.470,ITU BT.601	x = 0.67, y = 0.33	x = 0.21, y = 0.71	x = 0.14, y = 0.08	x = 0.310, y = 0.316, Illuminant C	?	0.299 E'R+ 0.587 E'G+ 0.114 E'B	219 E'Y + 16	224 PB,R + 128
V4L2_COLORSPACE_470_SYSTEM_BG	6	625-line PAL and SECAM systems according to ITU BT.470,ITU BT.601	x = 0.64, y = 0.33	x = 0.29, y = 0.60	x = 0.15, y = 0.06	x = 0.313, y = 0.329,Illuminant D65	?	0.299 E'R+ 0.587 E'G+ 0.114 E'B	219 E'Y + 16	224 PB,R + 128
V4L2_COLORSPACE_JPEG	7	JPEG Y'CbCr, see JFIF,ITU BT.601	?	?	?	?	?	0.299 E'R+ 0.587 E'G+ 0.114 E'B	256 E'Y + 16[d]	256 PB,R + 128
V4L2_COLORSPACE_SRGB	8	[?]	x = 0.640, y = 0.330	x = 0.300, y = 0.600	x = 0.150, y = 0.060	x = 0.3127, y = 0.3290, Illuminant D65	E' = 4.5 I for I ≤0.018,1.099 I0.45 - 0.099 for 0.018 < I	n/a
Notes: a. The coordinates of the color primaries aregiven in the CIE system (1931) b. The ubiquitous Bt878 video capture chipquantizes E'Y to 238 levels, yielding a rangeof Y' = 16 … 253, unlike Rec. 601 Y' = 16 …235. This is not a typo in the Bt878 documentation, it has beenimplemented in silicon. The chroma extents are unclear. c. No identifier exists for M/PAL which usesthe chromaticities of M/NTSC, the remaining parameters are equal to B andG/PAL. d. Note JFIF quantizesY'PBPR in range [0;+1] and[-0.5;+0.5] to257 levels, however Y'CbCr signalsare still clamped to [0;255].

---

2.3. Indexed Format

In this format each pixel is represented by an 8 bit indexinto a 256 entry ARGB palette. It is intended forVideo Output Overlays only. There are no ioctls toaccess the palette, this must be done with ioctls of the Linux framebuffer API.

Table 2-3. Indexed Image Format

Identifier	Code		Byte 0
		Bit	7	6	5	4	3	2	1	0
V4L2_PIX_FMT_PAL8	'PAL8'		i7	i6	i5	i4	i3	i2	i1	i0

---

2.4. RGB Formats

Packed RGB formats

Name

Packed RGB formats -- Packed RGB formats

Description

These formats are designed to match the pixel formats oftypical PC graphics frame buffers. They occupy 8, 16, 24 or 32 bitsper pixel. These are all packed-pixel formats, meaning all the datafor a pixel lie next to each other in memory.

When one of these formats is used, drivers shall report thecolorspace V4L2_COLORSPACE_SRGB.

Table 2-1. Packed RGB Image Formats

Identifier	Code		Byte 0 in memory									Byte 1									Byte 2									Byte 3
		Bit	7	6	5	4	3	2	1	0		7	6	5	4	3	2	1	0		7	6	5	4	3	2	1	0		7	6	5	4	3	2	1	0
V4L2_PIX_FMT_RGB332	'RGB1'		b1	b0	g2	g1	g0	r2	r1	r0
V4L2_PIX_FMT_RGB444	'R444'		g3	g2	g1	g0	b3	b2	b1	b0		a3	a2	a1	a0	r3	r2	r1	r0
V4L2_PIX_FMT_RGB555	'RGBO'		g2	g1	g0	r4	r3	r2	r1	r0		a	b4	b3	b2	b1	b0	g4	g3
V4L2_PIX_FMT_RGB565	'RGBP'		g2	g1	g0	r4	r3	r2	r1	r0		b4	b3	b2	b1	b0	g5	g4	g3
V4L2_PIX_FMT_RGB555X	'RGBQ'		a	b4	b3	b2	b1	b0	g4	g3		g2	g1	g0	r4	r3	r2	r1	r0
V4L2_PIX_FMT_RGB565X	'RGBR'		b4	b3	b2	b1	b0	g5	g4	g3		g2	g1	g0	r4	r3	r2	r1	r0
V4L2_PIX_FMT_BGR24	'BGR3'		b7	b6	b5	b4	b3	b2	b1	b0		g7	g6	g5	g4	g3	g2	g1	g0		r7	r6	r5	r4	r3	r2	r1	r0
V4L2_PIX_FMT_RGB24	'RGB3'		r7	r6	r5	r4	r3	r2	r1	r0		g7	g6	g5	g4	g3	g2	g1	g0		b7	b6	b5	b4	b3	b2	b1	b0
V4L2_PIX_FMT_BGR32	'BGR4'		b7	b6	b5	b4	b3	b2	b1	b0		g7	g6	g5	g4	g3	g2	g1	g0		r7	r6	r5	r4	r3	r2	r1	r0		a7	a6	a5	a4	a3	a2	a1	a0
V4L2_PIX_FMT_RGB32	'RGB4'		r7	r6	r5	r4	r3	r2	r1	r0		g7	g6	g5	g4	g3	g2	g1	g0		b7	b6	b5	b4	b3	b2	b1	b0		a7	a6	a5	a4	a3	a2	a1	a0

Bit 7 is the most significant bit. The value of a = alphabits is undefined when reading from the driver, ignored when writingto the driver, except when alpha blending has been negotiated for aVideo Overlay or Video Output Overlay.

Example 2-1. V4L2_PIX_FMT_BGR24 4 × 4 pixelimage

Byte Order. Each cell is one byte.

start + 0:

B00

G00

R00

B01

G01

R01

B02

G02

R02

B03

G03

R03

start + 12:

B10

G10

R10

B11

G11

R11

B12

G12

R12

B13

G13

R13

start + 24:

B20

G20

R20

B21

G21

R21

B22

G22

R22

B23

G23

R23

start + 36:

B30

G30

R30

B31

G31

R31

B32

G32

R32

B33

G33

R33

Important: Drivers may interpret these formats differently.

Some RGB formats above are uncommon and were probablydefined in error. Drivers may interpret them as inTable 2-2.

Table 2-2. Packed RGB Image Formats (corrected)

Identifier	Code		Byte 0 in memory									Byte 1									Byte 2									Byte 3
		Bit	7	6	5	4	3	2	1	0		7	6	5	4	3	2	1	0		7	6	5	4	3	2	1	0		7	6	5	4	3	2	1	0
V4L2_PIX_FMT_RGB332	'RGB1'		r2	r1	r0	g2	g1	g0	b1	b0
V4L2_PIX_FMT_RGB444	'R444'		g3	g2	g1	g0	b3	b2	b1	b0		a3	a2	a1	a0	r3	r2	r1	r0
V4L2_PIX_FMT_RGB555	'RGBO'		g2	g1	g0	b4	b3	b2	b1	b0		a	r4	r3	r2	r1	r0	g4	g3
V4L2_PIX_FMT_RGB565	'RGBP'		g2	g1	g0	b4	b3	b2	b1	b0		r4	r3	r2	r1	r0	g5	g4	g3
V4L2_PIX_FMT_RGB555X	'RGBQ'		a	r4	r3	r2	r1	r0	g4	g3		g2	g1	g0	b4	b3	b2	b1	b0
V4L2_PIX_FMT_RGB565X	'RGBR'		r4	r3	r2	r1	r0	g5	g4	g3		g2	g1	g0	b4	b3	b2	b1	b0
V4L2_PIX_FMT_BGR24	'BGR3'		b7	b6	b5	b4	b3	b2	b1	b0		g7	g6	g5	g4	g3	g2	g1	g0		r7	r6	r5	r4	r3	r2	r1	r0
V4L2_PIX_FMT_RGB24	'RGB3'		r7	r6	r5	r4	r3	r2	r1	r0		g7	g6	g5	g4	g3	g2	g1	g0		b7	b6	b5	b4	b3	b2	b1	b0
V4L2_PIX_FMT_BGR32	'BGR4'		b7	b6	b5	b4	b3	b2	b1	b0		g7	g6	g5	g4	g3	g2	g1	g0		r7	r6	r5	r4	r3	r2	r1	r0		a7	a6	a5	a4	a3	a2	a1	a0
V4L2_PIX_FMT_RGB32	'RGB4'		a7	a6	a5	a4	a3	a2	a1	a0		r7	r6	r5	r4	r3	r2	r1	r0		g7	g6	g5	g4	g3	g2	g1	g0		b7	b6	b5	b4	b3	b2	b1	b0

A test utility to determine which RGB formats a driveractually supports is available from the LinuxTV v4l-dvb repository.Seehttp://linuxtv.org/repo/ for access instructions.

V4L2_PIX_FMT_SBGGR8 ('BA81')

Name

V4L2_PIX_FMT_SBGGR8 -- Bayer RGB format

Description

This is commonly the native format of digital cameras,reflecting the arrangement of sensors on the CCD device. Only one red,green or blue value is given for each pixel. Missing components mustbe interpolated from neighbouring pixels. From left to right the firstrow consists of a blue and green value, the second row of a green andred value. This scheme repeats to the right and down for every twocolumns and rows.

Example 2-1. V4L2_PIX_FMT_SBGGR8 4 × 4pixel image

Byte Order. Each cell is one byte.

start + 0:	B00	G01	B02	G03
start + 4:	G10	R11	G12	R13
start + 8:	B20	G21	B22	G23
start + 12:	G30	R31	G32	R33

V4L2_PIX_FMT_SBGGR16 ('BA82')

Name

V4L2_PIX_FMT_SBGGR16 -- Bayer RGB format

Description

This format is similar to V4L2_PIX_FMT_SBGGR8, except each pixel hasa depth of 16 bits. The least significant byte is stored at lowermemory addresses (little-endian). Note the actual sampling precisionmay be lower than 16 bits, for example 10 bits per pixel with valuesin range 0 to 1023.

Example 2-1. V4L2_PIX_FMT_SBGGR16 4 × 4pixel image

Byte Order. Each cell is one byte.

start + 0:	B00low	B00high	G01low	G01high	B02low	B02high	G03low	G03high
start + 8:	G10low	G10high	R11low	R11high	G12low	G12high	R13low	R13high
start + 16:	B20low	B20high	G21low	G21high	B22low	B22high	G23low	G23high
start + 24:	G30low	G30high	R31low	R31high	G32low	G32high	R33low	R33high

---

2.5. YUV Formats

YUV is the format native to TV broadcast and composite videosignals. It separates the brightness information (Y) from the colorinformation (U and V or Cb and Cr). The color information consists ofred and bluecolor difference signals, this waythe green component can be reconstructed by subtracting from thebrightness component. SeeSection 2.2 for conversionexamples. YUV was chosen because early television would only transmitbrightness information. To add color in a way compatible with existingreceivers a new signal carrier was added to transmit the colordifference signals. Secondary in the YUV format the U and V componentsusually have lower resolution than the Y component. This is an analogvideo compression technique taking advantage of a property of thehuman visual system, being more sensitive to brightnessinformation.

Packed YUV formats

Name

Packed YUV formats -- Packed YUV formats

Description

Similar to the packed RGB formats these formats storethe Y, Cb and Cr component of each pixel in one 16 or 32 bitword.

Table 2-1. Packed YUV Image Formats

Identifier	Code		Byte 0 in memory									Byte 1									Byte 2									Byte 3
		Bit	7	6	5	4	3	2	1	0		7	6	5	4	3	2	1	0		7	6	5	4	3	2	1	0		7	6	5	4	3	2	1	0
V4L2_PIX_FMT_YUV444	'Y444'		Cb3	Cb2	Cb1	Cb0	Cr3	Cr2	Cr1	Cr0		a3	a2	a1	a0	Y'3	Y'2	Y'1	Y'0
V4L2_PIX_FMT_YUV555	'YUVO'		Cb2	Cb1	Cb0	Cr4	Cr3	Cr2	Cr1	Cr0		a	Y'4	Y'3	Y'2	Y'1	Y'0	Cb4	Cb3
V4L2_PIX_FMT_YUV565	'YUVP'		Cb2	Cb1	Cb0	Cr4	Cr3	Cr2	Cr1	Cr0		Y'4	Y'3	Y'2	Y'1	Y'0	Cb5	Cb4	Cb3
V4L2_PIX_FMT_YUV32	'YUV4'		a7	a6	a5	a4	a3	a2	a1	a0		Y'7	Y'6	Y'5	Y'4	Y'3	Y'2	Y'1	Y'0		Cb7	Cb6	Cb5	Cb4	Cb3	Cb2	Cb1	Cb0		Cr7	Cr6	Cr5	Cr4	Cr3	Cr2	Cr1	Cr0

Bit 7 is the most significant bit. The value of a = alphabits is undefined when reading from the driver, ignored when writingto the driver, except when alpha blending has been negotiated for aVideo Overlay or Video Output Overlay.

V4L2_PIX_FMT_GREY ('GREY')

Name

V4L2_PIX_FMT_GREY -- Grey-scale image

Description

This is a grey-scale image. It is really a degenerateY'CbCr format which simply contains no Cb or Cr data.

Example 2-1. V4L2_PIX_FMT_GREY 4 × 4pixel image

Byte Order. Each cell is one byte.

start + 0:	Y'00	Y'01	Y'02	Y'03
start + 4:	Y'10	Y'11	Y'12	Y'13
start + 8:	Y'20	Y'21	Y'22	Y'23
start + 12:	Y'30	Y'31	Y'32	Y'33

V4L2_PIX_FMT_Y16 ('Y16 ')

Name

V4L2_PIX_FMT_Y16 -- Grey-scale image

Description

This is a grey-scale image with a depth of 16 bits perpixel. The least significant byte is stored at lower memory addresses(little-endian). Note the actual sampling precision may be lower than16 bits, for example 10 bits per pixel with values in range 0 to1023.

Example 2-1. V4L2_PIX_FMT_Y16 4 × 4pixel image

Byte Order. Each cell is one byte.

start + 0:	Y'00low	Y'00high	Y'01low	Y'01high	Y'02low	Y'02high	Y'03low	Y'03high
start + 8:	Y'10low	Y'10high	Y'11low	Y'11high	Y'12low	Y'12high	Y'13low	Y'13high
start + 16:	Y'20low	Y'20high	Y'21low	Y'21high	Y'22low	Y'22high	Y'23low	Y'23high
start + 24:	Y'30low	Y'30high	Y'31low	Y'31high	Y'32low	Y'32high	Y'33low	Y'33high

V4L2_PIX_FMT_YUYV ('YUYV')

Name

V4L2_PIX_FMT_YUYV -- Packed format with ½ horizontal chromaresolution, also known as YUV 4:2:2

Description

In this format each four bytes is two pixels. Each fourbytes is two Y's, a Cb and a Cr. Each Y goes to one of the pixels, andthe Cb and Cr belong to both pixels. As you can see, the Cr and Cbcomponents have half the horizontal resolution of the Y component.V4L2_PIX_FMT_YUYVis known in the Windowsenvironment as YUY2.

Example 2-1. V4L2_PIX_FMT_YUYV 4 × 4pixel image

Byte Order. Each cell is one byte.

start + 0:	Y'00	Cb00	Y'01	Cr00	Y'02	Cb01	Y'03	Cr01
start + 8:	Y'10	Cb10	Y'11	Cr10	Y'12	Cb11	Y'13	Cr11
start + 16:	Y'20	Cb20	Y'21	Cr20	Y'22	Cb21	Y'23	Cr21
start + 24:	Y'30	Cb30	Y'31	Cr30	Y'32	Cb31	Y'33	Cr31

Color Sample Location.

	0		1		2		3
0	Y	C	Y		Y	C	Y
1	Y	C	Y		Y	C	Y
2	Y	C	Y		Y	C	Y
3	Y	C	Y		Y	C	Y

V4L2_PIX_FMT_UYVY ('UYVY')

Name

V4L2_PIX_FMT_UYVY -- Variation of V4L2_PIX_FMT_YUYV with different order of samplesin memory

Description

In this format each four bytes is two pixels. Each fourbytes is two Y's, a Cb and a Cr. Each Y goes to one of the pixels, andthe Cb and Cr belong to both pixels. As you can see, the Cr and Cbcomponents have half the horizontal resolution of the Ycomponent.

Example 2-1. V4L2_PIX_FMT_UYVY 4 × 4pixel image

Byte Order. Each cell is one byte.

start + 0:	Cb00	Y'00	Cr00	Y'01	Cb01	Y'02	Cr01	Y'03
start + 8:	Cb10	Y'10	Cr10	Y'11	Cb11	Y'12	Cr11	Y'13
start + 16:	Cb20	Y'20	Cr20	Y'21	Cb21	Y'22	Cr21	Y'23
start + 24:	Cb30	Y'30	Cr30	Y'31	Cb31	Y'32	Cr31	Y'33

Color Sample Location.

	0		1		2		3
0	Y	C	Y		Y	C	Y
1	Y	C	Y		Y	C	Y
2	Y	C	Y		Y	C	Y
3	Y	C	Y		Y	C	Y

V4L2_PIX_FMT_Y41P ('Y41P')

Name

V4L2_PIX_FMT_Y41P -- Format with ¼ horizontal chromaresolution, also known as YUV 4:1:1

Description

In this format each 12 bytes is eight pixels. In thetwelve bytes are two CbCr pairs and eight Y's. The first CbCr pairgoes with the first four Y's, and the second CbCr pair goes with theother four Y's. The Cb and Cr components have one fourth thehorizontal resolution of the Y component.

Do not confuse this format with V4L2_PIX_FMT_YUV411P. Y41P is derived from "YUV 4:1:1packed", whileYUV411P stands for "YUV 4:1:1planar".

Example 2-1. V4L2_PIX_FMT_Y41P 8 × 4pixel image

Byte Order. Each cell is one byte.

start + 0:

Cb00

Y'00

Cr00

Y'01

Cb01

Y'02

Cr01

Y'03

Y'04

Y'05

Y'06

Y'07

start + 12:

Cb10

Y'10

Cr10

Y'11

Cb11

Y'12

Cr11

Y'13

Y'14

Y'15

Y'16

Y'17

start + 24:

Cb20

Y'20

Cr20

Y'21

Cb21

Y'22

Cr21

Y'23

Y'24

Y'25

Y'26

Y'27

start + 36:

Cb30

Y'30

Cr30

Y'31

Cb31

Y'32

Cr31

Y'33

Y'34

Y'35

Y'36

Y'37

Color Sample Location.

0

1

2

3

4

5

6

7

0

Y

Y

C

Y

Y

Y

Y

C

Y

Y

1

Y

Y

C

Y

Y

Y

Y

C

Y

Y

2

Y

Y

C

Y

Y

Y

Y

C

Y

Y

3

Y

Y

C

Y

Y

Y

Y

C

Y

Y

V4L2_PIX_FMT_YVU420 ('YV12'), V4L2_PIX_FMT_YUV420 ('YU12')

Name

V4L2_PIX_FMT_YVU420,  V4L2_PIX_FMT_YUV420 -- Planar formats with ½ horizontal andvertical chroma resolution, also known as YUV 4:2:0

Description

These are planar formats, as opposed to a packed format.The three components are separated into three sub- images or planes.The Y plane is first. The Y plane has one byte per pixel. ForV4L2_PIX_FMT_YVU420, the Cr plane immediatelyfollows the Y plane in memory. The Cr plane is half the width and halfthe height of the Y plane (and of the image). Each Cr belongs to fourpixels, a two-by-two square of the image. For example,Cr₀ belongs to Y'₀₀,Y'₀₁, Y'₁₀, andY'₁₁. Following the Cr plane is the Cb plane,just like the Cr plane.V4L2_PIX_FMT_YUV420 isthe same except the Cb plane comes first, then the Cr plane.

If the Y plane has pad bytes after each row, then the Crand Cb planes have half as many pad bytes after their rows. In otherwords, two Cx rows (including padding) is exactly as long as one Y row(including padding).

Example 2-1. V4L2_PIX_FMT_YVU420 4 × 4pixel image

Byte Order. Each cell is one byte.

start + 0:	Y'00	Y'01	Y'02	Y'03
start + 4:	Y'10	Y'11	Y'12	Y'13
start + 8:	Y'20	Y'21	Y'22	Y'23
start + 12:	Y'30	Y'31	Y'32	Y'33
start + 16:	Cr00	Cr01
start + 18:	Cr10	Cr11
start + 20:	Cb00	Cb01
start + 22:	Cb10	Cb11

Color Sample Location.

	0		1		2		3
0	Y		Y		Y		Y
		C				C
1	Y		Y		Y		Y
2	Y		Y		Y		Y
		C				C
3	Y		Y		Y		Y

V4L2_PIX_FMT_YVU410 ('YVU9'), V4L2_PIX_FMT_YUV410 ('YUV9')

Name

V4L2_PIX_FMT_YVU410,  V4L2_PIX_FMT_YUV410 -- Planar formats with ¼ horizontal andvertical chroma resolution, also known as YUV 4:1:0

Description

These are planar formats, as opposed to a packed format.The three components are separated into three sub-images or planes.The Y plane is first. The Y plane has one byte per pixel. ForV4L2_PIX_FMT_YVU410, the Cr plane immediatelyfollows the Y plane in memory. The Cr plane is ¼ the width and¼ the height of the Y plane (and of the image). Each Cr belongsto 16 pixels, a four-by-four square of the image. Following the Crplane is the Cb plane, just like the Cr plane.V4L2_PIX_FMT_YUV410 is the same, except the Cbplane comes first, then the Cr plane.

If the Y plane has pad bytes after each row, then the Crand Cb planes have ¼ as many pad bytes after their rows. Inother words, four Cx rows (including padding) are exactly as long asone Y row (including padding).

Example 2-1. V4L2_PIX_FMT_YVU410 4 × 4pixel image

Byte Order. Each cell is one byte.

start + 0:	Y'00	Y'01	Y'02	Y'03
start + 4:	Y'10	Y'11	Y'12	Y'13
start + 8:	Y'20	Y'21	Y'22	Y'23
start + 12:	Y'30	Y'31	Y'32	Y'33
start + 16:	Cr00
start + 17:	Cb00

Color Sample Location.

	0		1		2		3
0	Y		Y		Y		Y
1	Y		Y		Y		Y
				C
2	Y		Y		Y		Y
3	Y		Y		Y		Y

V4L2_PIX_FMT_YUV422P ('422P')

Name

V4L2_PIX_FMT_YUV422P -- Format with ½ horizontal chroma resolution,also known as YUV 4:2:2. Planar layout as opposed to V4L2_PIX_FMT_YUYV

Description

This format is not commonly used. This is a planarversion of the YUYV format. The three components are separated intothree sub-images or planes. The Y plane is first. The Y plane has onebyte per pixel. The Cb plane immediately follows the Y plane inmemory. The Cb plane is half the width of the Y plane (and of theimage). Each Cb belongs to two pixels. For example,Cb₀ belongs to Y'₀₀,Y'₀₁. Following the Cb plane is the Cr plane,just like the Cb plane.

If the Y plane has pad bytes after each row, then the Crand Cb planes have half as many pad bytes after their rows. In otherwords, two Cx rows (including padding) is exactly as long as one Y row(including padding).

Example 2-1. V4L2_PIX_FMT_YUV422P 4 × 4pixel image

Byte Order. Each cell is one byte.

start + 0:	Y'00	Y'01	Y'02	Y'03
start + 4:	Y'10	Y'11	Y'12	Y'13
start + 8:	Y'20	Y'21	Y'22	Y'23
start + 12:	Y'30	Y'31	Y'32	Y'33
start + 16:	Cb00	Cb01
start + 18:	Cb10	Cb11
start + 20:	Cb20	Cb21
start + 22:	Cb30	Cb31
start + 24:	Cr00	Cr01
start + 26:	Cr10	Cr11
start + 28:	Cr20	Cr21
start + 30:	Cr30	Cr31

Color Sample Location.

	0		1		2		3
0	Y	C	Y		Y	C	Y
1	Y	C	Y		Y	C	Y
2	Y	C	Y		Y	C	Y
3	Y	C	Y		Y	C	Y

V4L2_PIX_FMT_YUV411P ('411P')

Name

V4L2_PIX_FMT_YUV411P -- Format with ¼ horizontal chroma resolution,also known as YUV 4:1:1. Planar layout as opposed to V4L2_PIX_FMT_Y41P

Description

This format is not commonly used. This is a planarformat similar to the 4:2:2 planar format except with half as manychroma. The three components are separated into three sub-images orplanes. The Y plane is first. The Y plane has one byte per pixel. TheCb plane immediately follows the Y plane in memory. The Cb plane is¼ the width of the Y plane (and of the image). Each Cb belongsto 4 pixels all on the same row. For example,Cb₀ belongs to Y'₀₀,Y'₀₁, Y'₀₂andY'₀₃. Following the Cb plane is the Cr plane,just like the Cb plane.

If the Y plane has pad bytes after each row, then the Crand Cb planes have ¼ as many pad bytes after their rows. Inother words, four C x rows (including padding) is exactly as long asone Y row (including padding).

Example 2-1. V4L2_PIX_FMT_YUV411P 4 × 4pixel image

Byte Order. Each cell is one byte.

start + 0:	Y'00	Y'01	Y'02	Y'03
start + 4:	Y'10	Y'11	Y'12	Y'13
start + 8:	Y'20	Y'21	Y'22	Y'23
start + 12:	Y'30	Y'31	Y'32	Y'33
start + 16:	Cb00
start + 17:	Cb10
start + 18:	Cb20
start + 19:	Cb30
start + 20:	Cr00
start + 21:	Cr10
start + 22:	Cr20
start + 23:	Cr30

Color Sample Location.

	0		1		2		3
0	Y		Y	C	Y		Y
1	Y		Y	C	Y		Y
2	Y		Y	C	Y		Y
3	Y		Y	C	Y		Y

V4L2_PIX_FMT_NV12 ('NV12'), V4L2_PIX_FMT_NV21 ('NV21')

Name

V4L2_PIX_FMT_NV12,  V4L2_PIX_FMT_NV21 -- Formats with ½ horizontal and verticalchroma resolution, also known as YUV 4:2:0. One luminance and onechrominance plane with alternating chroma samples as opposed to V4L2_PIX_FMT_YVU420

Description

These are two-plane versions of the YUV 4:2:0 format.The three components are separated into two sub-images or planes. TheY plane is first. The Y plane has one byte per pixel. ForV4L2_PIX_FMT_NV12, a combined CbCr planeimmediately follows the Y plane in memory. The CbCr plane is the samewidth, in bytes, as the Y plane (and of the image), but is half astall in pixels. Each CbCr pair belongs to four pixels. For example,Cb₀/Cr₀ belongs toY'₀₀, Y'₀₁,Y'₁₀, Y'₁₁.V4L2_PIX_FMT_NV21 is the same except the Cb andCr bytes are swapped, the CrCb plane starts with a Cr byte.

If the Y plane has pad bytes after each row, then theCbCr plane has as many pad bytes after its rows.

Example 2-1. V4L2_PIX_FMT_NV12 4 × 4pixel image

Byte Order. Each cell is one byte.

start + 0:	Y'00	Y'01	Y'02	Y'03
start + 4:	Y'10	Y'11	Y'12	Y'13
start + 8:	Y'20	Y'21	Y'22	Y'23
start + 12:	Y'30	Y'31	Y'32	Y'33
start + 16:	Cb00	Cr00	Cb01	Cr01
start + 20:	Cb10	Cr10	Cb11	Cr11

Color Sample Location.

	0		1		2		3
0	Y		Y		Y		Y
		C				C
1	Y		Y		Y		Y
2	Y		Y		Y		Y
		C				C
3	Y		Y		Y		Y

---

2.6. Compressed Formats

Table 2-7. Compressed Image Formats

Identifier	Code	Details
V4L2_PIX_FMT_JPEG	'JPEG'	TBD. See also VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP.
V4L2_PIX_FMT_MPEG	'MPEG'	MPEG stream. The actual format is determined byextended controlV4L2_CID_MPEG_STREAM_TYPE, seeTable 1-2.

---

2.7. Reserved Format Identifiers

These formats are not defined by this specification, theyare just listed for reference and to avoid naming conflicts. If youwant to register your own format, send an e-mail to the V4L mailinglisthttps://listman.redhat.com/mailman/listinfo/video4linux-list for inclusion in thevideodev.hfile. If you want to share your format with other developers add alink to your documentation and send a copy to the maintainer of thisdocument, Michael Schimek, forinclusion in this section. If you think your format should be listedin a standard format section please make a proposal on the V4L mailinglist.

Table 2-8. Reserved Image Formats

Identifier	Code	Details
V4L2_PIX_FMT_DV	'dvsd'	unknown
V4L2_PIX_FMT_ET61X251	'E625'	Compressed format of the ET61X251 driver.
V4L2_PIX_FMT_HI240	'HI24'	8 bit RGB format used by the BTTV driver,http://bytesex.org/bttv/
V4L2_PIX_FMT_HM12	'HM12'	YUV 4:2:0 format used by theIVTV driver, http://www.ivtvdriver.org/ The format is documented in thekernel sources in the fileDocumentation/video4linux/cx2341x/README.hm12
V4L2_PIX_FMT_MJPEG	'MJPG'	Compressed format used by the Zoran driver
V4L2_PIX_FMT_PWC1	'PWC1'	Compressed format of the PWC driver.
V4L2_PIX_FMT_PWC2	'PWC2'	Compressed format of the PWC driver.
V4L2_PIX_FMT_SN9C10X	'S910'	Compressed format of the SN9C102 driver.
V4L2_PIX_FMT_WNVA	'WNVA'	Used by the Winnov Videum driver, http://www.thedirks.org/winnov/
V4L2_PIX_FMT_YYUV	'YYUV'	unknown

---

Chapter 3. Input/Output

The V4L2 API defines several different methods to read from orwrite to a device. All drivers exchanging data with applications mustsupport at least one of them.

The classic I/O method using the read()and write() function is automatically selectedafter opening a V4L2 device. When the driver does not support thismethod attempts to read or write will fail at any time.

Other methods must be negotiated. To select the streaming I/Omethod with memory mapped or user buffers applications call theVIDIOC_REQBUFS ioctl. The asynchronous I/O method is not definedyet.

Video overlay can be considered another I/O method, althoughthe application does not directly receive the image data. It isselected by initiating video overlay with theVIDIOC_S_FMT ioctl.For more information seeSection 4.2.

Generally exactly one I/O method, including overlay, isassociated with each file descriptor. The only exceptions areapplications not exchanging data with a driver ("panel applications",seeSection 1.1) and drivers permitting simultaneous video capturingand overlay using the same file descriptor, for compatibility with V4Land earlier versions of V4L2.

VIDIOC_S_FMT andVIDIOC_REQBUFS would permit this to some degree,but for simplicity drivers need not support switching the I/O method(after first switching away from read/write) other than by closingand reopening the device.

The following sections describe the various I/O methods inmore detail.

---

3.1. Read/Write

Input and output devices support theread() and write() function,respectively, when the V4L2_CAP_READWRITE flag inthecapabilities field of struct v4l2_capabilityreturned by theVIDIOC_QUERYCAP ioctl is set.

Drivers may need the CPU to copy the data, but they may alsosupport DMA to or from user memory, so this I/O method is notnecessarily less efficient than other methods merely exchanging bufferpointers. It is considered inferior though because no meta-informationlike frame counters or timestamps are passed. This information isnecessary to recognize frame dropping and to synchronize with otherdata streams. However this is also the simplest I/O method, requiringlittle or no setup to exchange data. It permits command line stuntslike this (the vidctrl tool isfictitious):

> vidctrl /dev/video --input=0 --format=YUYV --size=352x288
> dd if=/dev/video of=myimage.422 bs=202752 count=1

To read from the device applications use theread() function, to write thewrite() function.Drivers must implement one I/O method if theyexchange data with applications, but it need not be this.[12] When reading or writing is supported, the drivermust also support the select() and poll()function.[13]

---

3.2. Streaming I/O (Memory Mapping)

Input and output devices support this I/O method when theV4L2_CAP_STREAMING flag in thecapabilities field of struct v4l2_capabilityreturned by theVIDIOC_QUERYCAP ioctl is set. There are twostreaming methods, to determine if the memory mapping flavor issupported applications must call theVIDIOC_REQBUFS ioctl.

Streaming is an I/O method where only pointers to buffersare exchanged between application and driver, the data itself is notcopied. Memory mapping is primarily intended to map buffers in devicememory into the application's address space. Device memory can be forexample the video memory on a graphics card with a video captureadd-on. However, being the most efficient I/O method available for along time, many other drivers support streaming as well, allocatingbuffers in DMA-able main memory.

A driver can support many sets of buffers. Each set isidentified by a unique buffer type value. The sets are independent andeach set can hold a different type of data. To access different setsat the same time different file descriptors must be used.[14]

To allocate device buffers applications call theVIDIOC_REQBUFS ioctl with the desired number of buffers and buffertype, for exampleV4L2_BUF_TYPE_VIDEO_CAPTURE.This ioctl can also be used to change the number of buffers or to freethe allocated memory, provided none of the buffers are stillmapped.

Before applications can access the buffers they must mapthem into their address space with themmap() function. Thelocation of the buffers in device memory can be determined with theVIDIOC_QUERYBUF ioctl. Them.offset andlength returned in a struct v4l2_buffer arepassed as sixth and second parameter to themmap() function. The offset and length valuesmust not be modified. Remember the buffers are allocated in physicalmemory, as opposed to virtual memory which can be swapped out to disk.Applications should free the buffers as soon as possible with themunmap() function.

Example 3-1. Mapping buffers

struct v4l2_requestbuffers reqbuf;
struct {
        void *start;
        size_t length;
} *buffers;
unsigned int i;

memset (&reqbuf, 0, sizeof (reqbuf));
reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
reqbuf.memory = V4L2_MEMORY_MMAP;
reqbuf.count = 20;

if (-1 == ioctl (fd, VIDIOC_REQBUFS, &reqbuf)) {
        if (errno == EINVAL)
                printf ("Video capturing or mmap-streaming is not supported\n");
        else
                perror ("VIDIOC_REQBUFS");

        exit (EXIT_FAILURE);
}

/* We want at least five buffers. */

if (reqbuf.count < 5) {
        /* You may need to free the buffers here. */
        printf ("Not enough buffer memory\n");
        exit (EXIT_FAILURE);
}

buffers = calloc (reqbuf.count, sizeof (*buffers));
assert (buffers != NULL);

for (i = 0; i < reqbuf.count; i++) {
        struct v4l2_buffer buffer;

        memset (&buffer, 0, sizeof (buffer));
        buffer.type = reqbuf.type;
	buffer.memory = V4L2_MEMORY_MMAP;
        buffer.index = i;

        if (-1 == ioctl (fd, VIDIOC_QUERYBUF, &buffer)) {
                perror ("VIDIOC_QUERYBUF");
                exit (EXIT_FAILURE);
        }

        buffers[i].length = buffer.length; /* remember for munmap() */

        buffers[i].start = mmap (NULL, buffer.length,
                                 PROT_READ | PROT_WRITE, /* recommended */
                                 MAP_SHARED,             /* recommended */
                                 fd, buffer.m.offset);

        if (MAP_FAILED == buffers[i].start) {
                /* If you do not exit here you should unmap() and free()
                   the buffers mapped so far. */
                perror ("mmap");
                exit (EXIT_FAILURE);
        }
}

/* Cleanup. */

for (i = 0; i < reqbuf.count; i++)
        munmap (buffers[i].start, buffers[i].length);
      

Conceptually streaming drivers maintain two buffer queues, an incomingand an outgoing queue. They separate the synchronous capture or outputoperation locked to a video clock from the application which issubject to random disk or network delays and preemption byother processes, thereby reducing the probability of data loss.The queues are organized as FIFOs, buffers will beoutput in the order enqueued in the incoming FIFO, and werecaptured in the order dequeued from the outgoing FIFO.

The driver may require a minimum number of buffers enqueuedat all times to function, apart of this no limit exists on the numberof buffers applications can enqueue in advance, or dequeue andprocess. They can also enqueue in a different order than buffers havebeen dequeued, and the driver can fillenqueuedempty buffers in any order.[15] The index number of a buffer (struct v4l2_bufferindex) plays no role here, it onlyidentifies the buffer.

Initially all mapped buffers are in dequeued state,inaccessible by the driver. For capturing applications it is customaryto first enqueue all mapped buffers, then to start capturing and enterthe read loop. Here the application waits until a filled buffer can bedequeued, and re-enqueues the buffer when the data is no longerneeded. Output applications fill and enqueue buffers, when enoughbuffers are stacked up the output is started withVIDIOC_STREAMON. In the write loop, whenthe application runs out of free buffers, it must wait until an emptybuffer can be dequeued and reused.

To enqueue and dequeue a buffer applications use theVIDIOC_QBUF andVIDIOC_DQBUF ioctl. The status of a buffer beingmapped, enqueued, full or empty can be determined at any time using theVIDIOC_QUERYBUF ioctl. Two methods exist to suspend execution of theapplication until one or more buffers can be dequeued. By defaultVIDIOC_DQBUF blocks when no buffer is in theoutgoing queue. When theO_NONBLOCK flag wasgiven to theopen()function,VIDIOC_DQBUFreturns immediately with anEAGAIN error code when no buffer is available. Theselect() orpoll() function are always available.

To start and stop capturing or output applications call theVIDIOC_STREAMON andVIDIOC_STREAMOFF ioctl. NoteVIDIOC_STREAMOFF removes all buffers from bothqueues as a side effect. Since there is no notion of doing anything"now" on a multitasking system, if an application needs to synchronizewith another event it should examine the struct v4l2_buffertimestamp of captured buffers, or set thefield before enqueuing buffers for output.

Drivers implementing memory mapping I/O mustsupport the VIDIOC_REQBUFS,VIDIOC_QUERYBUF,VIDIOC_QBUF,VIDIOC_DQBUF,VIDIOC_STREAMONandVIDIOC_STREAMOFF ioctl, themmap(),munmap(),select() andpoll()function.[16]

[capture example]

---

3.3. Streaming I/O (User Pointers)

Input and output devices support this I/O method when theV4L2_CAP_STREAMING flag in thecapabilities field of struct v4l2_capabilityreturned by theVIDIOC_QUERYCAP ioctl is set. If the particular userpointer method (not only memory mapping) is supported must bedetermined by calling theVIDIOC_REQBUFS ioctl.

This I/O method combines advantages of the read/write andmemory mapping methods. Buffers are allocated by the applicationitself, and can reside for example in virtual or shared memory. Onlypointers to data are exchanged, these pointers and meta-informationare passed in struct v4l2_buffer. The driver must be switchedinto user pointer I/O mode by calling theVIDIOC_REQBUFS with thedesired buffer type. No buffers are allocated beforehands,consequently they are not indexed and cannot be queried like mappedbuffers with theVIDIOC_QUERYBUF ioctl.

Example 3-2. Initiating streaming I/O with user pointers

struct v4l2_requestbuffers reqbuf;

memset (&reqbuf, 0, sizeof (reqbuf));
reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
reqbuf.memory = V4L2_MEMORY_USERPTR;

if (ioctl (fd, VIDIOC_REQBUFS, &reqbuf) == -1) {
        if (errno == EINVAL)
                printf ("Video capturing or user pointer streaming is not supported\n");
        else
                perror ("VIDIOC_REQBUFS");

        exit (EXIT_FAILURE);
}
      

Buffer addresses and sizes are passed on the fly with theVIDIOC_QBUF ioctl. Although buffers are commonly cycled,applications can pass different addresses and sizes at eachVIDIOC_QBUF call. If required by the hardware thedriver swaps memory pages within physical memory to create acontinuous area of memory. This happens transparently to theapplication in the virtual memory subsystem of the kernel. When bufferpages have been swapped out to disk they are brought back and finallylocked in physical memory for DMA.[17]

Filled or displayed buffers are dequeued with theVIDIOC_DQBUF ioctl. The driver can unlock the memory pages at anytime between the completion of the DMA and this ioctl. The memory isalso unlocked when VIDIOC_STREAMOFF is called, VIDIOC_REQBUFS, orwhen the device is closed. Applications must take care not to freebuffers without dequeuing. For once, the buffers remain locked untilfurther, wasting physical memory. Second the driver will not benotified when the memory is returned to the application's free listand subsequently reused for other purposes, possibly completing therequested DMA and overwriting valuable data.

For capturing applications it is customary to enqueue anumber of empty buffers, to start capturing and enter the read loop.Here the application waits until a filled buffer can be dequeued, andre-enqueues the buffer when the data is no longer needed. Outputapplications fill and enqueue buffers, when enough buffers are stackedup output is started. In the write loop, when the applicationruns out of free buffers it must wait until an empty buffer can bedequeued and reused. Two methods exist to suspend execution of theapplication until one or more buffers can be dequeued. By defaultVIDIOC_DQBUF blocks when no buffer is in theoutgoing queue. When theO_NONBLOCK flag wasgiven to theopen() function,VIDIOC_DQBUFreturns immediately with anEAGAIN error code when no buffer is available. Theselect() orpoll() function are always available.

To start and stop capturing or output applications call theVIDIOC_STREAMON andVIDIOC_STREAMOFF ioctl. NoteVIDIOC_STREAMOFF removes all buffers from bothqueues and unlocks all buffers as a side effect. Since there is nonotion of doing anything "now" on a multitasking system, if anapplication needs to synchronize with another event it should examinethe struct v4l2_buffertimestamp of capturedbuffers, or set the field before enqueuing buffers for output.

Drivers implementing user pointer I/O mustsupport the VIDIOC_REQBUFS,VIDIOC_QBUF,VIDIOC_DQBUF,VIDIOC_STREAMON andVIDIOC_STREAMOFFioctl, theselect() andpoll() function.[18]

---

3.4. Asynchronous I/O

This method is not defined yet.

---

3.5. Buffers

A buffer contains data exchanged by application anddriver using one of the Streaming I/O methods. Only pointers tobuffers are exchanged, the data itself is not copied. These pointers,together with meta-information like timestamps or field parity, arestored in a struct v4l2_buffer, argument totheVIDIOC_QUERYBUF, VIDIOC_QBUF and VIDIOC_DQBUF ioctl.

Nominally timestamps refer to the first data byte transmitted.In practice however the wide range of hardware covered by the V4L2 APIlimits timestamp accuracy. Often an interrupt routine willsample the system clock shortly after the field or frame was storedcompletely in memory. So applications must expect a constantdifference up to one field or frame period plus a small (few scanlines) random error. The delay and error can be muchlarger due to compression or transmission over an external bus whenthe frames are not properly stamped by the sender. This is frequentlythe case with USB cameras. Here timestamps refer to the instant thefield or frame was received by the driver, not the capture time. Thesedevices identify by not enumerating any video standards, seeSection 1.7.

Similar limitations apply to output timestamps. Typicallythe video hardware locks to a clock controlling the video timing, thehorizontal and vertical synchronization pulses. At some point in theline sequence, possibly the vertical blanking, an interrupt routinesamples the system clock, compares against the timestamp and programsthe hardware to repeat the previous field or frame, or to display thebuffer contents.

Apart of limitations of the video device and naturalinaccuracies of all clocks, it should be noted system time itself isnot perfectly stable. It can be affected by power saving cycles,warped to insert leap seconds, or even turned back or forth by thesystem administrator affecting long term measurements. [19]

Table 3-1. struct v4l2_buffer

__u32	index		Number of the buffer, set by the application. Thisfield is only used for memory mapping I/Oand can range from zero to the number of buffers allocatedwith theVIDIOC_REQBUFS ioctl (struct v4l2_requestbufferscount) minus one.
enum v4l2_buf_type	type		Type of the buffer, same as struct v4l2_formattype or struct v4l2_requestbufferstype, set by the application.
__u32	bytesused		The number of bytes occupied by the data in thebuffer. It depends on the negotiated data format and may change witheach buffer for compressed variable size data like JPEG images.Drivers must set this field whentyperefers to an input stream, applications when an output stream.
__u32	flags		Flags set by the application or driver, see Table 3-3.
enum v4l2_field	field		Indicates the field order of the image in thebuffer, see Table 3-8. This field is not used whenthe buffer contains VBI data. Drivers must set it whentype refers to an input stream,applications when an output stream.
struct timeval	timestamp		For input streams this is thesystem time (as returned by thegettimeofday()function) when the first data byte was captured. For output streamsthe data will not be displayed before this time, secondary to thenominal frame rate determined by the current video standard inenqueued order. Applications can for example zero this field todisplay frames as soon as possible. The driver stores the time atwhich the first data byte was actually sent out in thetimestamp field. This permitsapplications to monitor the drift between the video and systemclock.
struct v4l2_timecode	timecode		When type isV4L2_BUF_TYPE_VIDEO_CAPTURE and theV4L2_BUF_FLAG_TIMECODE flag is set inflags, this structure contains a frametimecode. InV4L2_FIELD_ALTERNATEmode the top and bottom field contain the same timecode.Timecodes are intended to help video editing and are typically recorded onvideo tapes, but also embedded in compressed formats like MPEG. Thisfield is independent of the timestamp andsequence fields.
__u32	sequence		Set by the driver, counting the frames in thesequence.
In V4L2_FIELD_ALTERNATE mode the top andbottom field have the same sequence number. The count starts at zeroand includes dropped or repeated frames. A dropped frame was receivedby an input device but could not be stored due to lack of free bufferspace. A repeated frame was displayed again by an output devicebecause the application did not pass new data intime. Note this may count the frames receivede.g. over USB, without taking into account the frames dropped by theremote hardware due to limited compression throughput or busbandwidth. These devices identify by not enumerating any videostandards, seeSection 1.7.
enum v4l2_memory	memory		This field must be set by applications and/or driversin accordance with the selected I/O method.
union	m
	__u32	offset	When memory isV4L2_MEMORY_MMAP this is the offset of the bufferfrom the start of the device memory. The value is returned by thedriver and apart of serving as parameter to themmap() functionnot useful for applications. SeeSection 3.2 for details.
	unsigned long	userptr	When memory isV4L2_MEMORY_USERPTR this is a pointer to thebuffer (casted to unsigned long type) in virtual memory, set by theapplication. SeeSection 3.3 for details.
__u32	length		Size of the buffer (not the payload) in bytes.
__u32	input		Some video capture drivers support rapid andsynchronous video input changes, a function useful for example invideo surveillance applications. For this purpose applications set theV4L2_BUF_FLAG_INPUT flag, and this field to thenumber of a video input as in struct v4l2_input fieldindex.
__u32	reserved		A place holder for future extensions and custom(driver defined) buffer typesV4L2_BUF_TYPE_PRIVATE and higher.

Table 3-2. enum v4l2_buf_type

V4L2_BUF_TYPE_VIDEO_CAPTURE	1	Buffer of a video capture stream, see Section 4.1.
V4L2_BUF_TYPE_VIDEO_OUTPUT	2	Buffer of a video output stream, see Section 4.3.
V4L2_BUF_TYPE_VIDEO_OVERLAY	3	Buffer for video overlay, see Section 4.2.
V4L2_BUF_TYPE_VBI_CAPTURE	4	Buffer of a raw VBI capture stream, see Section 4.7.
V4L2_BUF_TYPE_VBI_OUTPUT	5	Buffer of a raw VBI output stream, see Section 4.7.
V4L2_BUF_TYPE_SLICED_VBI_CAPTURE	6	Buffer of a sliced VBI capture stream, see Section 4.8.
V4L2_BUF_TYPE_SLICED_VBI_OUTPUT	7	Buffer of a sliced VBI output stream, see Section 4.8.
V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY	8	Buffer for video output overlay (OSD), see Section 4.4. Status: Experimental.
V4L2_BUF_TYPE_PRIVATE	0x80	This and higher values are reserved for custom(driver defined) buffer types.

Table 3-3. Buffer Flags

V4L2_BUF_FLAG_MAPPED	0x0001	The buffer resides in device memory and has been mappedinto the application's address space, seeSection 3.2 for details.Drivers set or clear this flag when theVIDIOC_QUERYBUF,VIDIOC_QBUF orVIDIOC_DQBUFioctl is called. Set by the driver.
V4L2_BUF_FLAG_QUEUED	0x0002	Internally drivers maintain two buffer queues, anincoming and outgoing queue. When this flag is set, the buffer iscurrently on the incoming queue. It automatically moves to theoutgoing queue after the buffer has been filled (capture devices) ordisplayed (output devices). Drivers set or clear this flag when theVIDIOC_QUERYBUF ioctl is called. After(successful) calling theVIDIOC_QBUFioctl it isalways set and afterVIDIOC_DQBUFalwayscleared.
V4L2_BUF_FLAG_DONE	0x0004	When this flag is set, the buffer is currently onthe outgoing queue, ready to be dequeued from the driver. Drivers setor clear this flag when theVIDIOC_QUERYBUF ioctlis called. After calling theVIDIOC_QBUForVIDIOC_DQBUF it is always cleared. Of course abuffer cannot be on both queues at the same time, theV4L2_BUF_FLAG_QUEUEDandV4L2_BUF_FLAG_DONE flag are mutually exclusive.They can be both cleared however, then the buffer is in "dequeued"state, in the application domain to say so.
V4L2_BUF_FLAG_KEYFRAME	0x0008	Drivers set or clear this flag when calling theVIDIOC_DQBUF ioctl. It may be set by videocapture devices when the buffer contains a compressed image which is akey frame (or field), i. e. can be decompressed on its own.
V4L2_BUF_FLAG_PFRAME	0x0010	Similar to V4L2_BUF_FLAG_KEYFRAMEthis flags predicted frames or fields which contain only differences to aprevious key frame.
V4L2_BUF_FLAG_BFRAME	0x0020	Similar to V4L2_BUF_FLAG_PFRAME this is a bidirectional predicted frame or field. [ooc tbd]
V4L2_BUF_FLAG_TIMECODE	0x0100	The timecode field is valid.Drivers set or clear this flag when theVIDIOC_DQBUFioctl is called.
V4L2_BUF_FLAG_INPUT	0x0200	The input field is valid.Applications set or clear this flag before calling theVIDIOC_QBUF ioctl.

Table 3-4. enum v4l2_memory

V4L2_MEMORY_MMAP	1	The buffer is used for memorymapping I/O.
V4L2_MEMORY_USERPTR	2	The buffer is used for userpointer I/O.
V4L2_MEMORY_OVERLAY	3	[to do]

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3.5.1. Timecodes

The v4l2_timecode structure isdesigned to hold aSMPTE 12M or similar timecode.(structtimeval timestamps are stored instruct v4l2_bufferfieldtimestamp.)

Table 3-5. struct v4l2_timecode

__u32	type	Frame rate the timecodes are based on, see Table 3-6.
__u32	flags	Timecode flags, see Table 3-7.
__u8	frames	Frame count, 0 ... 23/24/29/49/59, depending on the type of timecode.
__u8	seconds	Seconds count, 0 ... 59. This is a binary, not BCD number.
__u8	minutes	Minutes count, 0 ... 59. This is a binary, not BCD number.
__u8	hours	Hours count, 0 ... 29. This is a binary, not BCD number.
__u8	userbits[4]	The "user group" bits from the timecode.

Table 3-6. Timecode Types

V4L2_TC_TYPE_24FPS	1	24 frames per second, i. e. film.
V4L2_TC_TYPE_25FPS	2	25 frames per second, i. e. PAL or SECAM video.
V4L2_TC_TYPE_30FPS	3	30 frames per second, i. e. NTSC video.
V4L2_TC_TYPE_50FPS	4
V4L2_TC_TYPE_60FPS	5

Table 3-7. Timecode Flags

V4L2_TC_FLAG_DROPFRAME	0x0001	Indicates "drop frame" semantics for counting framesin 29.97 fps material. When set, frame numbers 0 and 1 at the start ofeach minute, except minutes 0, 10, 20, 30, 40, 50 are omitted from thecount.
V4L2_TC_FLAG_COLORFRAME	0x0002	The "color frame" flag.
V4L2_TC_USERBITS_field	0x000C	Field mask for the "binary group flags".
V4L2_TC_USERBITS_USERDEFINED	0x0000	Unspecified format.
V4L2_TC_USERBITS_8BITCHARS	0x0008	8-bit ISO characters.

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3.6. Field Order

We have to distinguish between progressive and interlacedvideo. Progressive video transmits all lines of a video imagesequentially. Interlaced video divides an image into two fields,containing only the odd and even lines of the image, respectively.Alternating the so called odd and even field are transmitted, and dueto a small delay between fields a cathode ray TV displays the linesinterleaved, yielding the original frame. This curious technique wasinvented because at refresh rates similar to film the image wouldfade out too quickly. Transmitting fields reduces the flicker withoutthe necessity of doubling the frame rate and with it the bandwidthrequired for each channel.

It is important to understand a video camera does not exposeone frame at a time, merely transmitting the frames separated intofields. The fields are in fact captured at two different instances intime. An object on screen may well move between one field and thenext. For applications analysing motion it is of paramount importanceto recognize which field of a frame is older, thetemporalorder.

When the driver provides or accepts images field by fieldrather than interleaved, it is also important applications understandhow the fields combine to frames. We distinguish between top andbottom fields, thespatial order: The first lineof the top field is the first line of an interlaced frame, the firstline of the bottom field is the second line of that frame.

However because fields were captured one after the other,arguing whether a frame commences with the top or bottom field ispointless. Any two successive top and bottom, or bottom and top fieldsyield a valid frame. Only when the source was progressive to beginwith, e. g. when transferring film to video, two fields may come fromthe same frame, creating a natural order.

Counter to intuition the top field is not necessarily theolder field. Whether the older field contains the top or bottom linesis a convention determined by the video standard. Hence thedistinction between temporal and spatial order of fields. The diagramsbelow should make this clearer.

All video capture and output devices must report the currentfield order. Some drivers may permit the selection of a differentorder, to this end applications initialize thefield field of struct v4l2_pix_format beforecalling the VIDIOC_S_FMT ioctl. If this is not desired it shouldhave the valueV4L2_FIELD_ANY (0).

Table 3-8. enum v4l2_field

V4L2_FIELD_ANY	0	Applications request this field order when anyone of theV4L2_FIELD_NONE,V4L2_FIELD_TOP,V4L2_FIELD_BOTTOM, orV4L2_FIELD_INTERLACED formats is acceptable.Drivers choose depending on hardware capabilities or e. g. therequested image size, and return the actual field order. struct v4l2_bufferfield can never beV4L2_FIELD_ANY.
V4L2_FIELD_NONE	1	Images are in progressive format, not interlaced.The driver may also indicate this order when it cannot distinguishbetweenV4L2_FIELD_TOPandV4L2_FIELD_BOTTOM.
V4L2_FIELD_TOP	2	Images consist of the top field only.
V4L2_FIELD_BOTTOM	3	Images consist of the bottom field only.Applications may wish to prevent a device from capturing interlacedimages because they will have "comb" or "feathering" artefacts aroundmoving objects.
V4L2_FIELD_INTERLACED	4	Images contain both fields, interleaved line byline. The temporal order of the fields (whether the top or bottomfield is first transmitted) depends on the current video standard.M/NTSC transmits the bottom field first, all other standards the topfield first.
V4L2_FIELD_SEQ_TB	5	Images contain both fields, the top field linesare stored first in memory, immediately followed by the bottom fieldlines. Fields are always stored in temporal order, the older one firstin memory. Image sizes refer to the frame, not fields.
V4L2_FIELD_SEQ_BT	6	Images contain both fields, the bottom fieldlines are stored first in memory, immediately followed by the topfield lines. Fields are always stored in temporal order, the older onefirst in memory. Image sizes refer to the frame, not fields.
V4L2_FIELD_ALTERNATE	7	The two fields of a frame are passed in separatebuffers, in temporal order, i. e. the older one first. To indicate the fieldparity (whether the current field is a top or bottom field) the driveror application, depending on data direction, must set struct v4l2_bufferfield toV4L2_FIELD_TOPorV4L2_FIELD_BOTTOM. Any two successive fields pairto build a frame. If fields are successive, without any dropped fieldsbetween them (fields can drop individually), can be determined fromthe struct v4l2_buffersequencefield. Imagesizes refer to the frame, not fields. This format cannot be selectedwhen using the read/write I/O method.
V4L2_FIELD_INTERLACED_TB	8	Images contain both fields, interleaved line byline, top field first. The top field is transmitted first.
V4L2_FIELD_INTERLACED_BT	9	Images contain both fields, interleaved line byline, top field first. The bottom field is transmitted first.

Figure 3-1. Field Order, Top Field First Transmitted

Figure 3-2. Field Order, Bottom Field First Transmitted

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Chapter 4. Interfaces

4.1. Video Capture Interface

Video capture devices sample an analog video signal and storethe digitized images in memory. Today nearly all devices can captureat full 25 or 30 frames/second. With this interface applications cancontrol the capture process and move images from the driver into userspace.

Conventionally V4L2 video capture devices are accessed throughcharacter device special files named/dev/videoand/dev/video0 to/dev/video63with major number 81 and minornumbers 0 to 63./dev/video is typically asymbolic link to the preferred video device. Note the same devicefiles are used for video output devices.

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4.1.1. Querying Capabilities

Devices supporting the video capture interface set theV4L2_CAP_VIDEO_CAPTURE flag in thecapabilities field of struct v4l2_capabilityreturned by theVIDIOC_QUERYCAP ioctl. As secondary device functionsthey may also support thevideo overlay(V4L2_CAP_VIDEO_OVERLAY) and theraw VBI capture(V4L2_CAP_VBI_CAPTURE) interface. At least one ofthe read/write or streaming I/O methods must be supported. Tuners andaudio inputs are optional.

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4.1.2. Supplemental Functions

Video capture devices shall support audio input, tuner, controls,cropping and scaling andstreaming parameter ioctls as needed.Thevideo input andvideo standard ioctls must be supported byall video capture devices.

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4.1.3. Image Format Negotiation

The result of a capture operation is determined bycropping and image format parameters. The former select an area of thevideo picture to capture, the latter how images are stored in memory,i. e. in RGB or YUV format, the number of bits per pixel or width andheight. Together they also define how images are scaled in theprocess.

As usual these parameters are not resetatopen() time to permit Unix tool chains, programming a deviceand then reading from it as if it was a plain file. Well written V4L2applications ensure they really get what they want, including croppingand scaling.

Cropping initialization at minimum requires to reset theparameters to defaults. An example is given inSection 1.11.

To query the current image format applications set thetype field of a struct v4l2_format toV4L2_BUF_TYPE_VIDEO_CAPTURE and call theVIDIOC_G_FMTioctl with a pointer to this structure. Drivers fillthe struct v4l2_pix_formatpix member of thefmt union.

To request different parameters applications set thetype field of a struct v4l2_format as above andinitialize all fields of the struct v4l2_pix_formatvbimember of thefmt union, or better just modify theresults ofVIDIOC_G_FMT, and call theVIDIOC_S_FMT ioctl with a pointer to this structure. Drivers mayadjust the parameters and finally return the actual parameters asVIDIOC_G_FMT does.

Like VIDIOC_S_FMT theVIDIOC_TRY_FMT ioctl can be used to learn about hardware limitationswithout disabling I/O or possibly time consuming hardwarepreparations.

The contents of struct v4l2_pix_format are discussed inChapter 2. See also the specification of theVIDIOC_G_FMT,VIDIOC_S_FMTandVIDIOC_TRY_FMTioctls for details. Videocapture devices must implement both theVIDIOC_G_FMT andVIDIOC_S_FMT ioctl, even ifVIDIOC_S_FMT ignores all requests and alwaysreturns default parameters asVIDIOC_G_FMT does.VIDIOC_TRY_FMT is optional.

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4.1.4. Reading Images

A video capture device may support the read() function and/or streaming (memory mapping oruser pointer) I/O. SeeChapter 3 for details.

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4.2. Video Overlay Interface

Also known as Framebuffer Overlay or Previewing

Video overlay devices have the ability to genlock (TV-)videointo the (VGA-)video signal of a graphics card, or to store capturedimages directly in video memory of a graphics card, typically withclipping. This can be considerable more efficient than capturingimages and displaying them by other means. In the old days when onlynuclear power plants needed cooling towers this used to be the onlyway to put live video into a window.

Video overlay devices are accessed through the same characterspecial files as video capture devices.Note the default function of a /dev/video deviceis video capturing. The overlay function is only available aftercalling theVIDIOC_S_FMT ioctl.

The driver may support simultaneous overlay and capturingusing the read/write and streaming I/O methods. If so, operation atthe nominal frame rate of the video standard is not guaranteed. Framesmay be directed away from overlay to capture, or one field may be usedfor overlay and the other for capture if the capture parameters permitthis.

Applications should use different file descriptors forcapturing and overlay. This must be supported by all drivers capableof simultaneous capturing and overlay. Optionally these drivers mayalso permit capturing and overlay with a single file descriptor forcompatibility with V4L and earlier versions of V4L2.[20]

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4.2.1. Querying Capabilities

Devices supporting the video overlay interface set theV4L2_CAP_VIDEO_OVERLAY flag in thecapabilities field of struct v4l2_capabilityreturned by theVIDIOC_QUERYCAP ioctl. The overlay I/O method specifiedbelow must be supported. Tuners and audio inputs are optional.

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4.2.2. Supplemental Functions

Video overlay devices shall support audio input, tuner, controls,cropping and scaling andstreaming parameter ioctls as needed.Thevideo input andvideo standard ioctls must be supported byall video overlay devices.

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4.2.3. Setup

Before overlay can commence applications must program thedriver with frame buffer parameters, namely the address and size ofthe frame buffer and the image format, for example RGB 5:6:5. TheVIDIOC_G_FBUF and VIDIOC_S_FBUF ioctls are available to getand set these parameters, respectively. TheVIDIOC_S_FBUF ioctl is privileged because itallows to set up DMA into physical memory, bypassing the memoryprotection mechanisms of the kernel. Only the superuser can change theframe buffer address and size. Users are not supposed to run TVapplications as root or with SUID bit set. A small helper applicationwith suitable privileges should query the graphics system and programthe V4L2 driver at the appropriate time.

Some devices add the video overlay to the output signalof the graphics card. In this case the frame buffer is not modified bythe video device, and the frame buffer address and pixel format arenot needed by the driver. TheVIDIOC_S_FBUF ioctlis not privileged. An application can check for this type of device bycalling theVIDIOC_G_FBUF ioctl.

A driver may support any (or none) of five clipping/blendingmethods:

1. Chroma-keying displays the overlaid image only wherepixels in the primary graphics surface assume a certain color.

2. A bitmap can be specified where each bit correspondsto a pixel in the overlaid image. When the bit is set, thecorresponding video pixel is displayed, otherwise a pixel of thegraphics surface.

3. A list of clipping rectangles can be specified. Inthese regions no video is displayed, so thegraphics surface can be seen here.

4. The framebuffer has an alpha channel that can be usedto clip or blend the framebuffer with the video.

5. A global alpha value can be specified to blend theframebuffer contents with video images.

When simultaneous capturing and overlay is supported andthe hardware prohibits different image and frame buffer formats, theformat requested first takes precedence. The attempt to capture(VIDIOC_S_FMT) or overlay (VIDIOC_S_FBUF) may fail with anEBUSY error code or return accordingly modified parameters..

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4.2.4. Overlay Window

The overlaid image is determined by cropping and overlaywindow parameters. The former select an area of the video picture tocapture, the latter how images are overlaid and clipped. Croppinginitialization at minimum requires to reset the parameters todefaults. An example is given in Section 1.11.

The overlay window is described by a struct v4l2_window. Itdefines the size of the image, its position over the graphics surfaceand the clipping to be applied. To get the current parametersapplications set the type field of astruct v4l2_format toV4L2_BUF_TYPE_VIDEO_OVERLAY andcall theVIDIOC_G_FMT ioctl. The driver fills thev4l2_window substructure namedwin. It is not possible to retrieve apreviously programmed clipping list or bitmap.

To program the overlay window applications set thetype field of a struct v4l2_format toV4L2_BUF_TYPE_VIDEO_OVERLAY, initialize thewin substructure and call theVIDIOC_S_FMT ioctl. The driver adjusts the parameters againsthardware limits and returns the actual parameters asVIDIOC_G_FMT does. LikeVIDIOC_S_FMT, theVIDIOC_TRY_FMT ioctl can beused to learn about driver capabilities without actually changingdriver state. UnlikeVIDIOC_S_FMTthis also worksafter the overlay has been enabled.

The scaling factor of the overlaid image is implied by thewidth and height given in struct v4l2_window and the size of the croppingrectangle. For more information seeSection 1.11.

When simultaneous capturing and overlay is supported andthe hardware prohibits different image and window sizes, the sizerequested first takes precedence. The attempt to capture or overlay aswell (VIDIOC_S_FMT) may fail with an EBUSY error code or return accordinglymodified parameters.

Table 4-1. struct v4l2_window

struct v4l2_rect	w	Size and position of the window relative to thetop, left corner of the frame buffer defined withVIDIOC_S_FBUF. Thewindow can extend the frame buffer width and height, thex andycoordinates can be negative, and it can lie completely outside theframe buffer. The driver clips the window accordingly, or if that isnot possible, modifies its size and/or position.
enum v4l2_field	field	Applications set this field to determine whichvideo field shall be overlaid, typically one ofV4L2_FIELD_ANY(0),V4L2_FIELD_TOP,V4L2_FIELD_BOTTOMorV4L2_FIELD_INTERLACED. Drivers may have to choosea different field order and return the actual setting here.
__u32	chromakey	When chroma-keying has been negotiated withVIDIOC_S_FBUFapplications set this field to the desired pixel valuefor the chroma key. The format is the same as the pixel format of theframebuffer (struct v4l2_framebufferfmt.pixelformat field), with bytes in hostorder. E. g. forV4L2_PIX_FMT_BGR24 the value should be 0xRRGGBB on a little endian, 0xBBGGRR on a bigendian host.
struct v4l2_clip *	clips	When chroma-keying has notbeen negotiated andVIDIOC_G_FBUFindicated this capability,applications can set this field to point to an array ofclipping rectangles.
Like the window coordinatesw, clipping rectangles are defined relativeto the top, left corner of the frame buffer. However clippingrectangles must not extend the frame buffer width and height, and theymust not overlap. If possible applications should merge adjacentrectangles. Whether this must create x-y or y-x bands, or the order ofrectangles, is not defined. When clip lists are not supported thedriver ignores this field. Its contents after callingVIDIOC_S_FMTare undefined.
__u32	clipcount	When the application set theclips field, this field must contain thenumber of clipping rectangles in the list. When clip lists are notsupported the driver ignores this field, its contents after callingVIDIOC_S_FMT are undefined. When clip lists aresupported but no clipping is desired this field must be set tozero.
void *	bitmap	When chroma-keying hasnot been negotiated andVIDIOC_G_FBUFindicatedthis capability, applications can set this field to point to aclipping bit mask.
It must be of the same sizeas the window, w.width andw.height. Each bit corresponds to a pixelin the overlaid image, which is displayed only when the bit isset. Pixel coordinates translate to bits like: ((__u8 *) bitmap)[w.width * y + x / 8] & (1 << (x & 7)) where 0 ≤ x <w.width and0 ≤y <w.height.a When a clippingbit mask is not supported the driver ignores this field, its contentsafter callingVIDIOC_S_FMT are undefined. When a bit mask is supportedbut no clipping is desired this field must be set toNULL. Applications need not create aclip list or bit mask. When they pass both, or despite negotiatingchroma-keying, the results are undefined. Regardless of the chosenmethod, the clipping abilities of the hardware may be limited inquantity or quality. The results when these limits are exceeded areundefined.b
__u8	global_alpha	The global alpha value used to blend theframebuffer with video images, if global alpha blending has beennegotiated (V4L2_FBUF_FLAG_GLOBAL_ALPHA, seeVIDIOC_S_FBUF,Table 3). Notethis field was added in Linux 2.6.23, extending the structure. HowevertheVIDIOC_G/S/TRY_FMT ioctls,which take a pointer to a v4l2_format parent structure with paddingbytes at the end, are not affected.
Notes: a. Should we require w.width to be a multiple of eight? b. When the image is written into frame buffermemory it will be undesirable if the driver clips out less pixelsthan expected, because the application and graphics system are notaware these regions need to be refreshed. The driver should clip outmore pixels or not write the image at all.

Table 4-2. struct v4l2_clip[21]

struct v4l2_rect	c	Coordinates of the clipping rectangle, relative tothe top, left corner of the frame buffer. Only window pixelsoutside all clipping rectangles aredisplayed.
struct v4l2_clip *	next	Pointer to the next clipping rectangle, NULL whenthis is the last rectangle. Drivers ignore this field, it cannot beused to pass a linked list of clipping rectangles.

Table 4-3. struct v4l2_rect

__s32	left	Horizontal offset of the top, left corner of therectangle, in pixels.
__s32	top	Vertical offset of the top, left corner of therectangle, in pixels. Offsets increase to the right and down.
__s32	width	Width of the rectangle, in pixels.
__s32	height	Height of the rectangle, in pixels. Width andheight cannot be negative, the fields are signed for hystericalreasons.

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4.2.5. Enabling Overlay

To start or stop the frame buffer overlay applications callthe VIDIOC_OVERLAY ioctl.

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4.3. Video Output Interface

Video output devices encode stills or image sequences asanalog video signal. With this interface applications cancontrol the encoding process and move images from user space tothe driver.

Conventionally V4L2 video output devices are accessed throughcharacter device special files named/dev/videoand/dev/video0 to/dev/video63with major number 81 and minornumbers 0 to 63./dev/video is typically asymbolic link to the preferred video device. Note the same devicefiles are used for video capture devices.

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4.3.1. Querying Capabilities

Devices supporting the video output interface set theV4L2_CAP_VIDEO_OUTPUT flag in thecapabilities field of struct v4l2_capabilityreturned by theVIDIOC_QUERYCAP ioctl. As secondary device functionsthey may also support theraw VBIoutput (V4L2_CAP_VBI_OUTPUT) interface. Atleast one of the read/write or streaming I/O methods must besupported. Modulators and audio outputs are optional.

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4.3.2. Supplemental Functions

Video output devices shall support audio output, modulator, controls,cropping and scaling andstreaming parameter ioctls as needed.Thevideo outputandvideo standard ioctls must be supported byall video output devices.

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4.3.3. Image Format Negotiation

The output is determined by cropping and image formatparameters. The former select an area of the video picture where theimage will appear, the latter how images are stored in memory, i. e. inRGB or YUV format, the number of bits per pixel or width and height.Together they also define how images are scaled in the process.

As usual these parameters are not resetatopen() time to permit Unix tool chains, programming a deviceand then writing to it as if it was a plain file. Well written V4L2applications ensure they really get what they want, including croppingand scaling.

Cropping initialization at minimum requires to reset theparameters to defaults. An example is given inSection 1.11.

To query the current image format applications set thetype field of a struct v4l2_format toV4L2_BUF_TYPE_VIDEO_OUTPUT and call theVIDIOC_G_FMTioctl with a pointer to this structure. Drivers fillthe struct v4l2_pix_formatpix member of thefmt union.

To request different parameters applications set thetype field of a struct v4l2_format as above andinitialize all fields of the struct v4l2_pix_formatvbimember of thefmt union, or better just modify theresults ofVIDIOC_G_FMT, and call theVIDIOC_S_FMT ioctl with a pointer to this structure. Drivers mayadjust the parameters and finally return the actual parameters asVIDIOC_G_FMT does.

Like VIDIOC_S_FMT theVIDIOC_TRY_FMT ioctl can be used to learn about hardware limitationswithout disabling I/O or possibly time consuming hardwarepreparations.

The contents of struct v4l2_pix_format are discussed inChapter 2. See also the specification of theVIDIOC_G_FMT,VIDIOC_S_FMTandVIDIOC_TRY_FMTioctls for details. Videooutput devices must implement both theVIDIOC_G_FMT andVIDIOC_S_FMT ioctl, even ifVIDIOC_S_FMT ignores all requests and alwaysreturns default parameters asVIDIOC_G_FMT does.VIDIOC_TRY_FMT is optional.

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4.3.4. Writing Images

A video output device may support the write() function and/or streaming (memory mapping oruser pointer) I/O. SeeChapter 3 for details.

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4.4. Video Output Overlay Interface

Also known as On-Screen Display (OSD)

Experimental: This is an experimentalinterface and may change in the future.

Some video output devices can overlay a framebuffer image ontothe outgoing video signal. Applications can set up such an overlayusing this interface, which borrows structures and ioctls of theVideo Overlay interface.

The OSD function is accessible through the same characterspecial file as the Video Output function.Note the default function of such a /dev/videodeviceis video capturing or output. The OSD function is only available aftercalling theVIDIOC_S_FMT ioctl.

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4.4.1. Querying Capabilities

Devices supporting the Video OutputOverlay interface set theV4L2_CAP_VIDEO_OUTPUT_OVERLAY flag in thecapabilities field of struct v4l2_capabilityreturned by the VIDIOC_QUERYCAP ioctl.

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4.4.2. Framebuffer

Contrary to the Video Overlayinterface the framebuffer is normally implemented on the TV card andnot the graphics card. On Linux it is accessible as a framebufferdevice (/dev/fbN). Given a V4L2 device,applications can find the corresponding framebuffer device by callingthe VIDIOC_G_FBUF ioctl. It returns, amongst other information, thephysical address of the framebuffer in thebase field of struct v4l2_framebuffer. Theframebuffer device ioctlFBIOGET_FSCREENINFOreturns the same address in thesmem_startfield of structfb_fix_screeninfo. TheFBIOGET_FSCREENINFO ioctl and structfb_fix_screeninfo are defined in thelinux/fb.h header file.

The width and height of the framebuffer depends on thecurrent video standard. A V4L2 driver may reject attempts to changethe video standard (or any other ioctl which would imply a framebuffersize change) with anEBUSY error code until all applications closed theframebuffer device.

Example 4-1. Finding a framebuffer device for OSD

#include 

struct v4l2_framebuffer fbuf;
unsigned int i;
int fb_fd;

if (-1 == ioctl (fd, VIDIOC_G_FBUF, &fbuf)) {
        perror ("VIDIOC_G_FBUF");
        exit (EXIT_FAILURE);
}

for (i = 0; i < 30; ++i) {
        char dev_name[16];
        struct fb_fix_screeninfo si;

        snprintf (dev_name, sizeof (dev_name), "/dev/fb%u", i);

        fb_fd = open (dev_name, O_RDWR);
        if (-1 == fb_fd) {
                switch (errno) {
                case ENOENT: /* no such file */
                case ENXIO:  /* no driver */
                        continue;

                default:
                        perror ("open");
                        exit (EXIT_FAILURE);
                }
        }

        if (0 == ioctl (fb_fd, FBIOGET_FSCREENINFO, &si)) {
                if (si.smem_start == (unsigned long) fbuf.base)
                        break;
        } else {
                /* Apparently not a framebuffer device. */
        }

        close (fb_fd);
        fb_fd = -1;
}

/* fb_fd is the file descriptor of the framebuffer device
   for the video output overlay, or -1 if no device was found. */

---

4.4.3. Overlay Window and Scaling

The overlay is controlled by source and target rectangles.The source rectangle selects a subsection of the framebuffer image tobe overlaid, the target rectangle an area in the outgoing video signalwhere the image will appear. Drivers may or may not support scaling,and arbitrary sizes and positions of these rectangles. Further driversmay support any (or none) of the clipping/blending methods defined fortheVideo Overlay interface.

A struct v4l2_window defines the size of the source rectangle,its position in the framebuffer and the clipping/blending method to beused for the overlay. To get the current parameters applications setthe type field of a struct v4l2_format toV4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY and call theVIDIOC_G_FMT ioctl. The driver fills thev4l2_window substructure namedwin. It is not possible to retrieve apreviously programmed clipping list or bitmap.

To program the source rectangle applications set thetype field of a struct v4l2_format toV4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY, initializethe winsubstructure and call theVIDIOC_S_FMT ioctl. The driver adjusts the parameters againsthardware limits and returns the actual parameters asVIDIOC_G_FMT does. LikeVIDIOC_S_FMT, theVIDIOC_TRY_FMT ioctl can beused to learn about driver capabilities without actually changingdriver state. UnlikeVIDIOC_S_FMT this also worksafter the overlay has been enabled.

A struct v4l2_crop defines the size and position of the targetrectangle. The scaling factor of the overlay is implied by the widthand height given in struct v4l2_window and struct v4l2_crop. The cropping APIapplies toVideo Output andVideoOutput Overlay devices in the same way as toVideo CaptureandVideoOverlay devices, merely reversing the direction of thedata flow. For more information seeSection 1.11.

---

4.4.4. Enabling Overlay

There is no V4L2 ioctl to enable or disable the overlay,however the framebuffer interface of the driver may support theFBIOBLANK ioctl.

---

4.5. Codec Interface

Suspended: This interface has been be suspended from the V4L2 APIimplemented in Linux 2.6 until we have more experience with codecdevice interfaces.

A V4L2 codec can compress, decompress, transform, or otherwiseconvert video data from one format into another format, in memory.Applications send data to be converted to the driver through awrite() call, and receive the converted data through aread() call. For efficiency a driver may also support streamingI/O.

[to do]

---

4.6. Effect Devices Interface

Suspended: This interface has been be suspended from the V4L2 APIimplemented in Linux 2.6 until we have more experience with effectdevice interfaces.

A V4L2 video effect device can do image effects, filtering, orcombine two or more images or image streams. For example videotransitions or wipes. Applications send data to be processed andreceive the result data either withread() andwrite()functions, or through the streaming I/O mechanism.

[to do]

---

4.7. Raw VBI Data Interface

VBI is an abbreviation of Vertical Blanking Interval, a gapin the sequence of lines of an analog video signal. During VBIno picture information is transmitted, allowing some time while theelectron beam of a cathode ray tube TV returns to the top of thescreen. Using an oscilloscope you will find here the verticalsynchronization pulses and short data packages ASKmodulated[22]onto the video signal. These are transmissions of services such asTeletext or Closed Caption.

Subject of this interface type is raw VBI data, as sampled offa video signal, or to be added to a signal for output.The data format is similar to uncompressed video images, a number oflines times a number of samples per line, we call this a VBI image.

Conventionally V4L2 VBI devices are accessed through characterdevice special files named/dev/vbi and/dev/vbi0 to/dev/vbi31 withmajor number 81 and minor numbers 224 to 255./dev/vbi is typically a symbolic link to thepreferred VBI device. This convention applies to both input and outputdevices.

To address the problems of finding related video and VBIdevices VBI capturing and output is also available as device functionunder/dev/video. To capture or output raw VBIdata with these devices applications must call theVIDIOC_S_FMTioctl. Accessed as/dev/vbi, raw VBI capturingor output is the default device function.

---

4.7.1. Querying Capabilities

Devices supporting the raw VBI capturing or output API setthe V4L2_CAP_VBI_CAPTURE orV4L2_CAP_VBI_OUTPUT flags, respectively, in thecapabilities field of struct v4l2_capabilityreturned by the VIDIOC_QUERYCAP ioctl. At least one of theread/write, streaming or asynchronous I/O methods must besupported. VBI devices may or may not have a tuner or modulator.

---

4.7.2. Supplemental Functions

VBI devices shall support videoinput or output, tuner ormodulator, and controls ioctlsas needed. The video standard ioctls provideinformation vital to program a VBI device, therefore must besupported.

---

4.7.3. Raw VBI Format Negotiation

Raw VBI sampling abilities can vary, in particular thesampling frequency. To properly interpret the data V4L2 specifies anioctl to query the sampling parameters. Moreover, to allow for someflexibility applications can also suggest different parameters.

As usual these parameters are notreset atopen() time to permit Unix tool chains, programming adevice and then reading from it as if it was a plain file. Wellwritten V4L2 applications should always ensure they really get whatthey want, requesting reasonable parameters and then checking if theactual parameters are suitable.

To query the current raw VBI capture parametersapplications set the type field of astruct v4l2_format toV4L2_BUF_TYPE_VBI_CAPTUREorV4L2_BUF_TYPE_VBI_OUTPUT, and call theVIDIOC_G_FMT ioctl with a pointer to this structure. Drivers fillthe struct v4l2_vbi_formatvbi member of thefmt union.

To request different parameters applications set thetype field of a struct v4l2_format as above andinitialize all fields of the struct v4l2_vbi_formatvbimember of thefmt union, or better just modify theresults ofVIDIOC_G_FMT, and call theVIDIOC_S_FMT ioctl with a pointer to this structure. Drivers returnanEINVAL error code only when the given parameters are ambiguous, otherwisethey modify the parameters according to the hardware capabilites andreturn the actual parameters. When the driver allocates resources atthis point, it may return an EBUSY error code to indicate the returnedparameters are valid but the required resources are currently notavailable. That may happen for instance when the video and VBI areasto capture would overlap, or when the driver supports multiple opensand another process already requested VBI capturing or output. Anyway,applications must expect other resource allocation points which mayreturnEBUSY, at theVIDIOC_STREAMON ioctland the first read(), write() and select() call.

VBI devices must implement both theVIDIOC_G_FMT andVIDIOC_S_FMT ioctl, even ifVIDIOC_S_FMT ignores all requests and alwaysreturns default parameters asVIDIOC_G_FMT does.VIDIOC_TRY_FMT is optional.

Table 4-4. struct v4l2_vbi_format

__u32	sampling_rate	Samples per second, i. e. unit 1 Hz.
__u32	offset	Horizontal offset of the VBI image,relative to the leading edge of the line synchronization pulse andcounted in samples: The first sample in the VBI image will be locatedoffset /sampling_rate seconds following the leadingedge. See also Figure 4-1.
__u32	samples_per_line
__u32	sample_format	Defines the sample format as in Chapter 2, a four-character-code.a Usually this isV4L2_PIX_FMT_GREY, i. e. each sampleconsists of 8 bits with lower values oriented towards the black level.Do not assume any other correlation of values with the signal level.For example, the MSB does not necessarily indicate if the signal is'high' or 'low' because 128 may not be the mean value of thesignal. Drivers shall not convert the sample format by software.
__u32	start[2]	This is the scanning system line numberassociated with the first line of the VBI image, of the first and thesecond field respectively. SeeFigure 4-2andFigure 4-3 for valid values. VBI input drivers canreturn start values 0 if the hardware cannot reliable identifyscanning lines, VBI acquisition may not require thisinformation.
__u32	count[2]	The number of lines in the first and secondfield image, respectively.
Drivers should be asflexibility as possible. For example, it may be possible to extend ormove the VBI capture window down to the picture area, implementing a'full field mode' to capture data service transmissions embedded inthe picture. An application can set the first or secondcount value to zero if no data is requiredfrom the respective field;count[1] if thescanning system is progressive, i. e. not interlaced. Thecorresponding start value shall be ignored by the application anddriver. Anyway, drivers may not support single field capturing andreturn both count values non-zero. Bothcount values set to zero, or line numbersoutside the bounds depicted inFigure 4-2 andFigure 4-3, or a field image coveringlines of two fields, are invalid and shall not be returned by thedriver. To initialize the startand count fields, applications must firstdetermine the current video standard selection. Thev4l2_std_id ortheframelines field of struct v4l2_standard canbe evaluated for this purpose.
__u32	flags	See Table 4-5 below. Currentlyonly drivers set flags, applications must set this field tozero.
__u32	reserved[2]	This array is reserved for future extensions.Drivers and applications must set it to zero.
Notes: a. A few devices may be unable tosample VBI data at all but can extend the video capture window to theVBI region.

Table 4-5. Raw VBI Format Flags

V4L2_VBI_UNSYNC	0x0001	This flag indicates hardware which does notproperly distinguish between fields. Normally the VBI image stores thefirst field (lower scanning line numbers) first in memory. This may bea top or bottom field depending on the video standard. When this flagis set the first or second field may be stored first, however thefields are still in correct temporal order with the older field firstin memory.a
V4L2_VBI_INTERLACED	0x0002	By default the two field images will be passedsequentially; all lines of the first field followed by all lines ofthe second field (compareSection 3.6V4L2_FIELD_SEQ_TB andV4L2_FIELD_SEQ_BT, whether the top or bottomfield is first in memory depends on the video standard). When thisflag is set, the two fields are interlaced (cf.V4L2_FIELD_INTERLACED). The first line of thefirst field followed by the first line of the second field, then thetwo second lines, and so on. Such a layout may be necessary when thehardware has been programmed to capture or output interlaced videoimages and is unable to separate the fields for VBI capturing atthe same time. For simplicity setting this flag implies that bothcount values are equal and non-zero.
Notes: a. Most VBI services transmit on both fields, butsome have different semantics depending on the field number. Thesecannot be reliable decoded or encoded whenV4L2_VBI_UNSYNC is set.

Figure 4-1. Line synchronization

Figure 4-2. ITU-R 525 line numbering (M/NTSC and M/PAL)

(1) For the purpose of this specification field 2starts in line 264 and not 263.5 because half line capturing is notsupported.

Figure 4-3. ITU-R 625 line numbering

(1) For the purpose of this specification field 2starts in line 314 and not 313.5 because half line capturing is notsupported.

Remember the VBI image format depends on the selectedvideo standard, therefore the application must choose a new standard orquery the current standard first. Attempts to read or write data aheadof format negotiation, or after switching the video standard which mayinvalidate the negotiated VBI parameters, should be refused by thedriver. A format change during active I/O is not permitted.

---

4.7.4. Reading and writing VBI images

To assure synchronization with the field number and easierimplementation, the smallest unit of data passed at a time is oneframe, consisting of two fields of VBI images immediately following inmemory.

The total size of a frame computes as follows:

(count[0] + count[1]) *
samples_per_line * sample size in bytes

The sample size is most likely always one byte,applications must check the sample_formatfield though, to function properly with other drivers.

A VBI device may support read/write and/or streaming (memory mapping oruser pointer) I/O. The latter bears thepossibility of synchronizing video andVBI data by using buffer timestamps.

Remember the VIDIOC_STREAMON ioctl and the first read(),write() and select() call can be resource allocation points returninganEBUSY error code if the required hardware resources are temporarilyunavailable, for example the device is already in use by anotherprocess.

---

4.8. Sliced VBI Data Interface

VBI stands for Vertical Blanking Interval, a gap in thesequence of lines of an analog video signal. During VBI no pictureinformation is transmitted, allowing some time while the electron beamof a cathode ray tube TV returns to the top of the screen.

Sliced VBI devices use hardware to demodulate data transmittedin the VBI. V4L2 drivers shallnot do this bysoftware, see also theraw VBIinterface. The data is passed as short packets of fixed size,covering one scan line each. The number of packets per video frame isvariable.

Sliced VBI capture and output devices are accessed through thesame character special files as raw VBI devices. When a driversupports both interfaces, the default function of a/dev/vbi device israw VBIcapturing or output, and the sliced VBI function is only availableafter calling theVIDIOC_S_FMT ioctl as defined below. Likewise a/dev/video device may support the sliced VBI API,however the default function here is video capturing or output.Different file descriptors must be used to pass raw and sliced VBIdata simultaneously, if this is supported by the driver.

---

4.8.1. Querying Capabilities

Devices supporting the sliced VBI capturing or output APIset the V4L2_CAP_SLICED_VBI_CAPTURE orV4L2_CAP_SLICED_VBI_OUTPUT flag respectively, inthecapabilities field of struct v4l2_capabilityreturned by theVIDIOC_QUERYCAP ioctl. At least one of theread/write, streaming or asynchronousI/Omethods must be supported. Sliced VBI devices may have a tuneror modulator.

---

4.8.2. Supplemental Functions

Sliced VBI devices shall support videoinput or output and tuner ormodulator ioctls if they have these capabilities, and they maysupportcontrol ioctls. Thevideo standard ioctls provide informationvital to program a sliced VBI device, therefore must besupported.

---

4.8.3. Sliced VBI Format Negotiation

To find out which data services are supported by thehardware applications can call theVIDIOC_G_SLICED_VBI_CAP ioctl.All drivers implementing the sliced VBI interface must support thisioctl. The results may differ from those of theVIDIOC_S_FMT ioctlwhen the number of VBI lines the hardware can capture or output perframe, or the number of services it can identify on a given line arelimited. For example on PAL line 16 the hardware may be able to lookfor a VPS or Teletext signal, but not both at the same time.

To determine the currently selected services applicationsset the type field of struct v4l2_format to V4L2_BUF_TYPE_SLICED_VBI_CAPTUREorV4L2_BUF_TYPE_SLICED_VBI_OUTPUT, and theVIDIOC_G_FMTioctl fills thefmt.sliced member, astruct v4l2_sliced_vbi_format.

Applications can request different parameters byinitializing or modifying the fmt.slicedmember and calling the VIDIOC_S_FMT ioctl with a pointer to thev4l2_format structure.

The sliced VBI API is more complicated than the raw VBI APIbecause the hardware must be told which VBI service to expect on eachscan line. Not all services may be supported by the hardware on alllines (this is especially true for VBI output where Teletext is oftenunsupported and other services can only be inserted in one specificline). In many cases, however, it is sufficient to just set theservice_set field to the required servicesand let the driver fill theservice_linesarray according to hardware capabilities. Only if more precise controlis needed should the programmer set theservice_lines array explicitly.

The VIDIOC_S_FMT ioctl returns anEINVAL error code only when thegiven parameters are ambiguous, otherwise it modifies the parametersaccording to hardware capabilities. When the driver allocatesresources at this point, it may return anEBUSY error code if the requiredresources are temporarily unavailable. Other resource allocationpoints which may returnEBUSY can be theVIDIOC_STREAMON ioctl and the firstread(),write() andselect()call.

Table 4-6. structv4l2_sliced_vbi_format

__u32	service_set	Ifservice_set is non-zero when passed withVIDIOC_S_FMTorVIDIOC_TRY_FMT, theservice_lines array will be filled by thedriver according to the services specified in this field. For example,ifservice_setis initialized withV4L2_SLICED_TELETEXT_B | V4L2_SLICED_WSS_625, adriver for the cx25840 video decoder sets lines 7-22 of bothfieldsatoV4L2_SLICED_TELETEXT_Band line 23 of the first field toV4L2_SLICED_WSS_625. Ifservice_set is set to zero, then the valuesofservice_lines will be used instead. On return the driver sets this field to the union of allelements of the returnedservice_linesarray. It may contain less services than requested, perhaps just one,if the hardware cannot handle more services simultaneously. It may beempty (zero) if none of the requested services are supported by thehardware.
__u16	service_lines[2][24]	Applications initialize thisarray with sets of data services the driver shall look for or inserton the respective scan line. Subject to hardware capabilities driversreturn the requested set, a subset, which may be just a singleservice, or an empty set. When the hardware cannot handle multipleservices on the same line the driver shall choose one. No assumptionscan be made on which service the driver chooses. Dataservices are defined in Table 4-7. Array indicesmap to ITU-R line numbers (see also Figure 4-2 and Figure 4-3) as follows:
		Element	525 line systems	625 line systems
		service_lines[0][1]	1	1
		service_lines[0][23]	23	23
		service_lines[1][1]	264	314
		service_lines[1][23]	286	336
		Drivers must setservice_lines[0][0] andservice_lines[1][0] to zero.
__u32	io_size	Maximum number of bytes passed byone read() or write() call, and the buffer size in bytes fortheVIDIOC_QBUF andVIDIOC_DQBUF ioctl. Drivers set this field tothe size of struct v4l2_sliced_vbi_data times the number of non-zeroelements in the returnedservice_linesarray (that is the number of lines potentially carrying data).
__u32	reserved[2]	This array is reserved for futureextensions. Applications and drivers must set it to zero.
Notes: a. According to ETS 300 706 lines 6-22 of thefirst field and lines 5-22 of the second field may carry Teletextdata.

Table 4-7. Sliced VBI services

Symbol	Value	Reference	Lines, usually	Payload
V4L2_SLICED_TELETEXT_B(Teletext System B)	0x0001	ETS 300 706,ITU BT.653	PAL/SECAM line 7-22, 320-335 (second field 7-22)	Last 42 of the 45 byte Teletext packet, that iswithout clock run-in and framing code, lsb first transmitted.
V4L2_SLICED_VPS	0x0400	ETS 300 231	PAL line 16	Byte number 3 to 15 according to Figure 9 ofETS 300 231, lsb first transmitted.
V4L2_SLICED_CAPTION_525	0x1000	EIA 608-B	NTSC line 21, 284 (second field 21)	Two bytes in transmission order, including paritybit, lsb first transmitted.
V4L2_SLICED_WSS_625	0x4000	ITU BT.1119,EN 300 294	PAL/SECAM line 23	Byte 0 1 msb lsb msb lsb Bit 7 6 5 4 3 2 1 0 x x 13 12 11 10 9
V4L2_SLICED_VBI_525	0x1000	Set of services applicable to 525line systems.
V4L2_SLICED_VBI_625	0x4401	Set of services applicable to 625line systems.

Drivers may return an EINVAL error code when applications attempt toread or write data without prior format negotiation, after switchingthe video standard (which may invalidate the negotiated VBIparameters) and after switching the video input (which may change thevideo standard as a side effect). TheVIDIOC_S_FMT ioctl may returnan EBUSY error code when applications attempt to change the format while i/o isin progress (between aVIDIOC_STREAMON andVIDIOC_STREAMOFF call,and after the firstread() orwrite() call).

---

4.8.4. Reading and writing sliced VBI data

A single read() or write() call must pass all databelonging to one video frame. That is an array ofv4l2_sliced_vbi_data structures with one ormore elements and a total size not exceedingio_size bytes. Likewise in streaming I/Omode one buffer of io_size bytes mustcontain data of one video frame. Theid ofunusedv4l2_sliced_vbi_data elements must bezero.

Table 4-8. structv4l2_sliced_vbi_data

__u32	id	A flag from Table 2identifying the type of data in this packet. Only a single bit must beset. When theid of a captured packet iszero, the packet is empty and the contents of other fields areundefined. Applications shall ignore empty packets. When theid of a packet for output is zero thecontents of thedatafield are undefinedand the driver must no longer insert data on the requestedfield andline.
__u32	field	The video field number this data has been capturedfrom, or shall be inserted at.0 for the firstfield,1 for the second field.
__u32	line	The field (as opposed to frame) line number thisdata has been captured from, or shall be inserted at. SeeFigure 4-2 andFigure 4-3 for validvalues. Sliced VBI capture devices can set the line number of allpackets to0 if the hardware cannot reliablyidentify scan lines. The field number must always be valid.
__u32	reserved	This field is reserved for future extensions.Applications and drivers must set it to zero.
__u8	data[48]	The packet payload. See Table 2 for the contents and number ofbytes passed for each data type. The contents of padding bytes at theend of this array are undefined, drivers and applications shall ignorethem.

Packets are always passed in ascending line number order,without duplicate line numbers. Thewrite() function and theVIDIOC_QBUF ioctl must return an EINVAL error code when applications violatethis rule. They must also return anEINVAL error code when applications pass anincorrect field or line number, or a combination offield,line andid which has not been negotiated with theVIDIOC_G_FMT orVIDIOC_S_FMT ioctl. When the line numbers areunknown the driver must pass the packets in transmitted order. Thedriver can insert empty packets withid setto zero anywhere in the packet array.

To assure synchronization and to distinguish from framedropping, when a captured frame does not carry any of the requesteddata services drivers must pass one or more empty packets. When anapplication fails to pass VBI data in time for output, the drivermust output the last VPS and WSS packet again, and disable the outputof Closed Caption and Teletext data, or output data which is ignoredby Closed Caption and Teletext decoders.

A sliced VBI device may support read/write and/or streaming (memory mapping and/oruserpointer) I/O. The latter bears the possibility of synchronizingvideo and VBI data by using buffer timestamps.

---

4.9. Teletext Interface

This interface aims at devices receiving and demodulatingTeletext data [ETS 300 706,ITU BT.653], evaluating theTeletext packages and storing formatted pages in cache memory. Suchdevices are usually implemented as microcontrollers with serialinterface (I²C) and can be found on olderTV cards, dedicated Teletext decoding cards and home-brew devicesconnected to the PC parallel port.

The Teletext API was designed by Martin Buck. It is defined inthe kernel header filelinux/videotext.h, thespecification is available fromhttp://home.pages.de/~videotext/. (Videotext is the name ofthe German public television Teletext service.) Conventional characterdevice file names are/dev/vtx and/dev/vttuner, with device number 83, 0 and 83, 16respectively. A similar interface exists for the Philips SAA5249Teletext decoder [specification?] with character device file names/dev/tlkN, device number 102, N.

Eventually the Teletext API was integrated into the V4L APIwith character device file names/dev/vtx0 to/dev/vtx31, device major number 81, minor numbers192 to 223. For reference the V4L Teletext API specification isreproduced here in full: "Teletext interfaces talk the existing VTXAPI." Teletext devices with major number 83 and 102 will be removed inLinux 2.6.

There are no plans to replace the Teletext API or to integrateit into V4L2. Please write to the Video4Linux mailing list:https://listman.redhat.com/mailman/listinfo/video4linux-list when the need arises.

---

4.10. Radio Interface

This interface is intended for AM and FM (analog) radioreceivers.

Conventionally V4L2 radio devices are accessed throughcharacter device special files named/dev/radioand/dev/radio0 to/dev/radio63 with major number 81 and minornumbers 64 to 127.

---

4.10.1. Querying Capabilities

Devices supporting the radio interface set theV4L2_CAP_RADIO andV4L2_CAP_TUNER flag in thecapabilities field of struct v4l2_capabilityreturned by the VIDIOC_QUERYCAP ioctl. Other combinations ofcapability flags are reserved for future extensions.

---

4.10.2. Supplemental Functions

Radio devices can support controls, and must support the tuner ioctls.

They do not support the video input or output, audio inputor output, video standard, cropping and scaling, compression andstreaming parameter, or overlay ioctls. All other ioctls and I/Omethods are reserved for future extensions.

---

4.10.3. Programming

Radio devices may have a couple audio controls (as discussedin Section 1.8) such as a volume control, possibly customcontrols. Further all radio devices have one tuner (these arediscussed inSection 1.6) with index number zero to selectthe radio frequency and to determine if a monaural or FM stereoprogram is received. Drivers switch automatically between AM and FMdepending on the selected frequency. TheVIDIOC_G_TUNER ioctlreports the supported frequency range.

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4.11. RDS Interface

The Radio Data System transmits supplementaryinformation in binary format, for example the station name or travelinformation, on a inaudible audio subcarrier of a radio program. Thisinterface aims at devices capable of receiving and decoding RDSinformation.

The V4L API defines its RDS API as follows.

From radio devices supporting it, RDS data can be readwith the read() function. The data is packed in groups of three,as follows:

1. First Octet Least Significant Byte of RDS Block

2. Second Octet Most Significant Byte of RDS Block

3. Third Octet Bit 7: Error bit. Indicates that anuncorrectable error occurred during reception of this block. Bit 6:Corrected bit. Indicates that an error was corrected for this datablock. Bits 5-3: Received Offset. Indicates the offset received by thesync system. Bits 2-0: Offset Name. Indicates the offset applied tothis data.

It was argued the RDS API should beextended before integration into V4L2, no new API has been devised yet.Please write to the Video4Linux mailing list for discussion:https://listman.redhat.com/mailman/listinfo/video4linux-list. Meanwhile no V4L2 driver should set theV4L2_CAP_RDS_CAPTURE capability flag.

I. Function Reference

V4L2 close()

Name

v4l2-close -- Close a V4L2 device

Synopsis

#include 

int close(int fd);

Arguments

fd

File descriptor returned by open().

Description

Closes the device. Any I/O in progress is terminated andresources associated with the file descriptor are freed. However dataformat parameters, current input or output, control values or otherproperties remain unchanged.

Return Value

The function returns 0 onsuccess, -1 on failure and theerrno is set appropriately. Possible errorcodes:

EBADF

fd is not a valid open filedescriptor.

V4L2 ioctl()

Name

v4l2-ioctl -- Program a V4L2 device

Synopsis

#include 

int ioctl(int fd, int request, void *argp);

Arguments

fd

File descriptor returned by open().

request

V4L2 ioctl request code as defined in the videodev.h header file, for exampleVIDIOC_QUERYCAP.

argp

Pointer to a function parameter, usually a structure.

Description

The ioctl() function is used to programV4L2 devices. The argumentfd must be an openfile descriptor. An ioctlrequest has encodedin it whether the argument is an input, output or read/writeparameter, and the size of the argumentargp inbytes. Macros and defines specifying V4L2 ioctl requests are locatedin thevideodev.h header file.Applications should use their own copy, not include the version in thekernel sources on the system they compile on. All V4L2 ioctl requests,their respective function and parameters are specified inReference I,Function Reference.

Return Value

On success the ioctl() function returns0 and does not reset theerrno variable. On failure-1 is returned, when the ioctl takes anoutput or read/write parameter it remains unmodified, and theerrno variable is set appropriately. See below forpossible error codes. Generic errors likeEBADForEFAULT are not listed in the sectionsdiscussing individual ioctl requests.

Note ioctls may return undefined error codes. Since errorsmay have side effects such as a driver reset applications shouldabort on unexpected errors.

EBADF

fd is not a valid open filedescriptor.

EBUSY

The property cannot be changed right now. Typicallythis error code is returned when I/O is in progress or the driversupports multiple opens and another process locked the property.

EFAULT

argp references an inaccessiblememory area.

ENOTTY

fd is not associated with acharacter special device.

EINVAL

The request or the data pointedto by argp is not valid. This is a very commonerror code, see the individual ioctl requests listed inReference I,Function Reference for actual causes.

ENOMEM

Not enough physical or virtual memory was available tocomplete the request.

ERANGE

The application attempted to set a control with theVIDIOC_S_CTRL ioctl to a value which is out of bounds.

ioctl VIDIOC_CROPCAP

Name

VIDIOC_CROPCAP -- Information about the video cropping and scaling abilities

Synopsis

int ioctl(int fd, int request, struct v4l2_cropcap*argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_CROPCAP

argp

Description

Applications use this function to query the croppinglimits, the pixel aspect of images and to calculate scale factors.They set thetype field of a v4l2_cropcapstructure to the respective buffer (stream) type and call theVIDIOC_CROPCAP ioctl with a pointer to thisstructure. Drivers fill the rest of the structure. The results areconstant except when switching the video standard. Remember thisswitch can occur implicit when switching the video input oroutput.

Table 1. struct v4l2_cropcap

enum v4l2_buf_type	type	Type of the data stream, set by the application.Only these types are valid here:V4L2_BUF_TYPE_VIDEO_CAPTURE,V4L2_BUF_TYPE_VIDEO_OUTPUT,V4L2_BUF_TYPE_VIDEO_OVERLAY, and custom (driverdefined) types with code V4L2_BUF_TYPE_PRIVATEand higher.
struct v4l2_rect	bounds	Defines the window within capturing or output ispossible, this may exclude for example the horizontal and verticalblanking areas. The cropping rectangle cannot exceed these limits.Width and height are defined in pixels, the driver writer is free tochoose origin and units of the coordinate system in the analogdomain.
struct v4l2_rect	defrect	Default cropping rectangle, it shall cover the"whole picture". Assuming pixel aspect 1/1 this could be for example a640 × 480 rectangle for NTSC, a768 × 576 rectangle for PAL and SECAM centered overthe active picture area. The same co-ordinate system as for bounds is used.
struct v4l2_fract	pixelaspect	This is the pixel aspect (y / x) when noscaling is applied, the ratio of the actual samplingfrequency and the frequency required to get squarepixels. When cropping coordinates refer to square pixels,the driver sets pixelaspect to 1/1. Othercommon values are 54/59 for PAL and SECAM, 11/10 for NTSC sampledaccording to [ITU BT.601].

Table 2. struct v4l2_rect

__s32	left	Horizontal offset of the top, left corner of therectangle, in pixels.
__s32	top	Vertical offset of the top, left corner of therectangle, in pixels.
__s32	width	Width of the rectangle, in pixels.
__s32	height	Height of the rectangle, in pixels. Widthand height cannot be negative, the fields are signed forhysterical reasons.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_cropcaptype isinvalid or the ioctl is not supported. This is not permitted forvideo capture, output and overlay devices, which must supportVIDIOC_CROPCAP.

ioctl VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER

Name

VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER -- Read or write hardware registers

Synopsis

int ioctl(int fd, int request, struct v4l2_register *argp);

int ioctl(int fd, int request, const struct v4l2_register*argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER

argp

Description

Experimental: This is an experimentalinterface and may change in the future.

For driver debugging purposes these ioctls allow testapplications to access hardware registers directly. Regularapplications should not use them.

Since writing or even reading registers can jeopardize thesystem security, its stability and damage the hardware, both ioctlsrequire superuser privileges. Additionally the Linux kernel must becompiled with theCONFIG_VIDEO_ADV_DEBUG optionto enable these ioctls.

To write a register applications must initialize all fieldsof a struct v4l2_register and callVIDIOC_DBG_S_REGISTER with a pointer to thisstructure. Thematch_type andmatch_chip fields select a chip on the TVcard, thereg field specifies a registernumber and theval field the value to bewritten into the register.

To read a register applications must initialize thematch_type,match_chip andreg fields, and callVIDIOC_DBG_G_REGISTER with a pointer to thisstructure. On success the driver stores the register value in theval field. On failure the structure remainsunchanged.

When match_type isV4L2_CHIP_MATCH_HOST,match_chip selects the nth non-I²C chipon the TV card. Drivers may also interpretmatch_chip as a random ID, but we recommendagainst that. The number zero always selects the host chip, e. g. thechip connected to the PCI bus. You can find out which chips arepresent with theVIDIOC_G_CHIP_IDENT ioctl.

When match_type isV4L2_CHIP_MATCH_I2C_DRIVER,match_chip contains a driver ID as definedin thelinux/i2c-id.h header file. For instanceI2C_DRIVERID_SAA7127 will match any chipsupported by the saa7127 driver, regardless of its I²C bus address.When multiple chips supported by the same driver are present, theeffect of these ioctls is undefined. Again with theVIDIOC_G_CHIP_IDENT ioctl you can find out which I²C chips arepresent.

When match_type isV4L2_CHIP_MATCH_I2C_ADDR,match_chip selects a chip by its 7 bit I²Cbus address.

Success not guaranteed: Due to a flaw in the Linux I²C bus driver these ioctls mayreturn successfully without actually reading or writing a register. Tocatch the most likely failure we recommend aVIDIOC_G_CHIP_IDENTcall confirming the presence of the selected I²C chip.

These ioctls are optional, not all drivers may support them.However when a driver supports these ioctls it must also supportVIDIOC_G_CHIP_IDENT. Conversely it may supportVIDIOC_G_CHIP_IDENT but not these ioctls.

VIDIOC_DBG_G_REGISTER andVIDIOC_DBG_S_REGISTER were introduced in Linux2.6.21.

We recommended the v4l2-dbgutility over calling these ioctls directly. It is available from theLinuxTV v4l-dvb repository; seehttp://linuxtv.org/repo/ foraccess instructions.

Table 1. struct v4l2_register

__u32	match_type	See Table 2 for a list of possible types.
__u32	match_chip	Match a chip by this number, interpreted accordingto the match_type field.
__u64	reg	A register number.
__u64	val	The value read from, or to be written into theregister.

Table 2. Chip Match Types

V4L2_CHIP_MATCH_HOST	0	Match the nth chip on the card, zero for the host chip. Does not match I2C chips.
V4L2_CHIP_MATCH_I2C_DRIVER	1	Match an I2C chip by its driver ID from thelinux/i2c-id.h header file.
V4L2_CHIP_MATCH_I2C_ADDR	2	Match a chip by its 7 bit I2C bus address.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The driver does not support this ioctl, or the kernelwas not compiled with theCONFIG_VIDEO_ADV_DEBUGoption, or thematch_type is invalid, or theselected chip or register does not exist.

EPERM

Insufficient permissions. Root privileges are requiredto execute these ioctls.

ioctl VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD

Name

VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD -- Execute an encoder command

Synopsis

int ioctl(int fd, int request, struct v4l2_encoder_cmd *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD

argp

Description

Experimental: This is an experimentalinterface and may change in the future.

These ioctls control an audio/video (usually MPEG-) encoder.VIDIOC_ENCODER_CMD sends a command to theencoder,VIDIOC_TRY_ENCODER_CMD can be used totry a command without actually executing it.

To send a command applications must initialize all fields of a struct v4l2_encoder_cmd and callVIDIOC_ENCODER_CMD orVIDIOC_TRY_ENCODER_CMDwith a pointer to this structure.

The cmd field must contain thecommand code. Theflags field is currentlyonly used by the STOP command and contains one bit: If theV4L2_ENC_CMD_STOP_AT_GOP_END flag is set,encoding will continue until the end of the currentGroupOf Pictures, otherwise it will stop immediately.

A read() call sends a START command tothe encoder if it has not been started yet. After a STOP command,read() calls will read the remaining databuffered by the driver. When the buffer is empty,read() will return zero and the nextread() call will restart the encoder.

A close() call sends an immediate STOPto the encoder, and all buffered data is discarded.

These ioctls are optional, not all drivers may supportthem. They were introduced in Linux 2.6.21.

Table 1. struct v4l2_encoder_cmd

__u32	cmd	The encoder command, see Table 2.
__u32	flags	Flags to go with the command, see Table 3. If no flags are defined forthis command, drivers and applications must set this field tozero.
__u32	data[8]	Reserved for future extensions. Drivers andapplications must set the array to zero.

Table 2. Encoder Commands

V4L2_ENC_CMD_START	0	Start the encoder. When the encoder is alreadyrunning or paused, this command does nothing. No flags are defined forthis command.
V4L2_ENC_CMD_STOP	1	Stop the encoder. When theV4L2_ENC_CMD_STOP_AT_GOP_END flag is set,encoding will continue until the end of the currentGroupOf Pictures, otherwise encoding will stop immediately.When the encoder is already stopped, this command doesnothing.
V4L2_ENC_CMD_PAUSE	2	Pause the encoder. When the encoder has not beenstarted yet, the driver will return anEPERM error code. When the encoder isalready paused, this command does nothing. No flags are defined forthis command.
V4L2_ENC_CMD_RESUME	3	Resume encoding after a PAUSE command. When theencoder has not been started yet, the driver will return anEPERM error code.When the encoder is already running, this command does nothing. Noflags are defined for this command.

Table 3. Encoder Command Flags

V4L2_ENC_CMD_STOP_AT_GOP_END

0x0001

Stop encoding at the end of the current Group OfPictures, rather than immediately.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The driver does not support this ioctl, or thecmd field is invalid.

EPERM

The application sent a PAUSE or RESUME command whenthe encoder was not running.

ioctl VIDIOC_ENUMAUDIO

Name

VIDIOC_ENUMAUDIO -- Enumerate audio inputs

Synopsis

int ioctl(int fd, int request, struct v4l2_audio *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUMAUDIO

argp

Description

To query the attributes of an audio input applicationsinitialize the index field and zero out thereserved array of a struct v4l2_audioand call theVIDIOC_ENUMAUDIO ioctl with a pointerto this structure. Drivers fill the rest of the structure or return anEINVAL error code when the index is out of bounds. To enumerate all audioinputs applications shall begin at index zero, incrementing by oneuntil the driver returnsEINVAL.

See ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO(2) for a description ofstruct v4l2_audio.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The number of the audio input is out of bounds, orthere are no audio inputs at all and this ioctl is notsupported.

ioctl VIDIOC_ENUMAUDOUT

Name

VIDIOC_ENUMAUDOUT -- Enumerate audio outputs

Synopsis

int ioctl(int fd, int request, struct v4l2_audioout *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUMAUDOUT

argp

Description

To query the attributes of an audio output applicationsinitialize the index field and zero out thereserved array of a struct v4l2_audioout andcall theVIDIOC_G_AUDOUT ioctl with a pointerto this structure. Drivers fill the rest of the structure or return anEINVAL error code when the index is out of bounds. To enumerate all audiooutputs applications shall begin at index zero, incrementing by oneuntil the driver returns EINVAL.

Note connectors on a TV card to loop back the received audiosignal to a sound card are not audio outputs in this sense.

See ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT(2) for a description ofstruct v4l2_audioout.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The number of the audio output is out of bounds, orthere are no audio outputs at all and this ioctl is notsupported.

ioctl VIDIOC_ENUM_FMT

Name

VIDIOC_ENUM_FMT -- Enumerate image formats

Synopsis

int ioctl(int fd, int request, struct v4l2_fmtdesc*argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUM_FMT

argp

Description

To enumerate image formats applications initialize thetype andindexfield of struct v4l2_fmtdesc and call theVIDIOC_ENUM_FMT ioctl with a pointer to thisstructure. Drivers fill the rest of the structure or return anEINVAL error code. All formats are enumerable by beginning at index zero andincrementing by one untilEINVAL isreturned.

Table 1. struct v4l2_fmtdesc

__u32	index	Number of the format in the enumeration, set bythe application. This is in no way related to thepixelformat field.
enum v4l2_buf_type	type	Type of the data stream, set by the application.Only these types are valid here:V4L2_BUF_TYPE_VIDEO_CAPTURE,V4L2_BUF_TYPE_VIDEO_OUTPUT,V4L2_BUF_TYPE_VIDEO_OVERLAY, and custom (driverdefined) types with code V4L2_BUF_TYPE_PRIVATEand higher.
__u32	flags	See Table 2
__u8	description[32]	Description of the format, a NUL-terminated ASCIIstring. This information is intended for the user, for example: "YUV4:2:2".
__u32	pixelformat	The image format identifier. This is afour character code as computed by the v4l2_fourcc()macro:
#define v4l2_fourcc(a,b,c,d) (((__u32)(a)<<0)|((__u32)(b)<<8)|((__u32)(c)<<16)|((__u32)(d)<<24)) Several image formats are alreadydefined by this specification in Chapter 2. Note thesecodes are not the same as those used in the Windows world.
__u32	reserved[4]	Reserved for future extensions. Drivers must setthe array to zero.

Table 2. Image Format Description Flags

V4L2_FMT_FLAG_COMPRESSED

0x0001

This is a compressed format.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_fmtdesctypeis not supported or theindex is out ofbounds.

ioctl VIDIOC_ENUM_FRAMESIZES

Name

VIDIOC_ENUM_FRAMESIZES -- Enumerate frame sizes

Synopsis

int ioctl(int fd, int request, struct v4l2_frmsizeenum *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUM_FRAMESIZES

argp

Pointer to a struct v4l2_frmsizeenum that contains an indexand pixel format and receives a frame width and height.

Description

Experimental: This is an experimentalinterface and may change in the future.

This ioctl allows applications to enumerate all frame sizes(i. e. width and height in pixels) that the device supports for thegiven pixel format.

The supported pixel formats can be obtained by using theVIDIOC_ENUM_FMT function.

The return value and the content of thev4l2_frmsizeenum.type field depend on thetype of frame sizes the device supports. Here are the semantics of thefunction for the different cases:

• Discrete: The functionreturns success if the given index value (zero-based) is valid. Theapplication should increase the index by one for each call untilEINVAL is returned. Thev4l2_frmsizeenum.type field is set toV4L2_FRMSIZE_TYPE_DISCRETE by the driver. Of theunion only thediscrete member isvalid.

• Step-wise: The functionreturns success if the given index value is zero andEINVAL for any other index value. Thev4l2_frmsizeenum.type field is set toV4L2_FRMSIZE_TYPE_STEPWISE by the driver. Of theunion only thestepwise member isvalid.

• Continuous: This is aspecial case of the step-wise type above. The function returns successif the given index value is zero andEINVAL forany other index value. Thev4l2_frmsizeenum.type field is set toV4L2_FRMSIZE_TYPE_CONTINUOUS by the driver. Ofthe union only thestepwisemember is validand thestep_width andstep_height values are set to 1.

When the application calls the function with index zero, itmust check the type field to determine thetype of frame size enumeration the device supports. Only for theV4L2_FRMSIZE_TYPE_DISCRETE type does it makesense to increase the index value to receive more frame sizes.

Note that the order in which the frame sizes are returnedhas no special meaning. In particular does it not say anything aboutpotential default format sizes.

Applications can assume that the enumeration data does notchange without any interaction from the application itself. This meansthat the enumeration data is consistent if the application does notperform any other ioctl calls while it runs the frame sizeenumeration.

Structs

In the structs below, IN denotes avalue that has to be filled in by the application,OUT denotes values that the driver fills in. Theapplication should zero out all members except for theIN fields.

Table 1. struct v4l2_frmsize_discrete

__u32	width	Width of the frame [pixel].
__u32	height	Height of the frame [pixel].

Table 2. struct v4l2_frmsize_stepwise

__u32	min_width	Minimum frame width [pixel].
__u32	max_width	Maximum frame width [pixel].
__u32	step_width	Frame width step size [pixel].
__u32	min_height	Minimum frame height [pixel].
__u32	max_height	Maximum frame height [pixel].
__u32	step_height	Frame height step size [pixel].

Table 3. struct v4l2_frmsizeenum

__u32	index		IN: Index of the given frame size in the enumeration.
__u32	pixel_format		IN: Pixel format for which the frame sizes are enumerated.
__u32	type		OUT: Frame size type the device supports.
union			OUT: Frame size with the given index.
	struct v4l2_frmsize_discrete	discrete
	struct v4l2_frmsize_stepwise	stepwise
__u32	reserved[2]		Reserved space for future use.

Enums

Table 4. enum v4l2_frmsizetypes

V4L2_FRMSIZE_TYPE_DISCRETE	1	Discrete frame size.
V4L2_FRMSIZE_TYPE_CONTINUOUS	2	Continuous frame size.
V4L2_FRMSIZE_TYPE_STEPWISE	3	Step-wise defined frame size.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

See the description section above for a list of returnvalues that errno can have.

ioctl VIDIOC_ENUM_FRAMEINTERVALS

Name

VIDIOC_ENUM_FRAMEINTERVALS -- Enumerate frame intervals

Synopsis

int ioctl(int fd, int request, struct v4l2_frmivalenum *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUM_FRAMEINTERVALS

argp

Pointer to a struct v4l2_frmivalenum structure thatcontains a pixel format and size and receives a frame interval.

Description

This ioctl allows applications to enumerate all frameintervals that the device supports for the given pixel format andframe size.

The supported pixel formats and frame sizes can be obtainedby using the VIDIOC_ENUM_FMT and VIDIOC_ENUM_FRAMESIZESfunctions.

The return value and the content of thev4l2_frmivalenum.type field depend on thetype of frame intervals the device supports. Here are the semantics ofthe function for the different cases:

• Discrete: The functionreturns success if the given index value (zero-based) is valid. Theapplication should increase the index by one for each call untilEINVAL is returned. The `v4l2_frmivalenum.type`field is set to `V4L2_FRMIVAL_TYPE_DISCRETE` by the driver. Of theunion only the `discrete` member is valid.

• Step-wise: The functionreturns success if the given index value is zero andEINVAL for any other index value. Thev4l2_frmivalenum.type field is set toV4L2_FRMIVAL_TYPE_STEPWISE by the driver. Of theunion only thestepwise member isvalid.

• Continuous: This is aspecial case of the step-wise type above. The function returns successif the given index value is zero andEINVAL forany other index value. Thev4l2_frmivalenum.type field is set toV4L2_FRMIVAL_TYPE_CONTINUOUS by the driver. Ofthe union only thestepwisemember is validand thestep value is set to 1.

When the application calls the function with index zero, itmust check the type field to determine thetype of frame interval enumeration the device supports. Only for theV4L2_FRMIVAL_TYPE_DISCRETE type does it makesense to increase the index value to receive more frameintervals.

Note that the order in which the frame intervals arereturned has no special meaning. In particular does it not sayanything about potential default frame intervals.

Applications can assume that the enumeration data does notchange without any interaction from the application itself. This meansthat the enumeration data is consistent if the application does notperform any other ioctl calls while it runs the frame intervalenumeration.

Notes

• Frame intervals and framerates: The V4L2 API uses frame intervals instead of framerates. Given the frame interval the frame rate can be computed asfollows:

  frame_rate = 1 / frame_interval

Structs

In the structs below, IN denotes avalue that has to be filled in by the application,OUT denotes values that the driver fills in. Theapplication should zero out all members except for theIN fields.

Table 1. struct v4l2_frmival_stepwise

struct v4l2_fract	min	Minimum frame interval [s].
struct v4l2_fract	max	Maximum frame interval [s].
struct v4l2_fract	step	Frame interval step size [s].

Table 2. struct v4l2_frmivalenum

__u32	index		IN: Index of the given frame interval in theenumeration.
__u32	pixel_format		IN: Pixel format for which the frame intervals areenumerated.
__u32	width		IN: Frame width for which the frame intervals areenumerated.
__u32	height		IN: Frame height for which the frame intervals areenumerated.
__u32	type		OUT: Frame interval type the device supports.
union			OUT: Frame interval with the given index.
	struct v4l2_fract	discrete	Frame interval [s].
	struct v4l2_frmival_stepwise	stepwise
__u32	reserved[2]		Reserved space for future use.

Enums

Table 3. enum v4l2_frmivaltypes

V4L2_FRMIVAL_TYPE_DISCRETE	1	Discrete frame interval.
V4L2_FRMIVAL_TYPE_CONTINUOUS	2	Continuous frame interval.
V4L2_FRMIVAL_TYPE_STEPWISE	3	Step-wise defined frame interval.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

See the description section above for a list of returnvalues that errno can have.

ioctl VIDIOC_ENUMINPUT

Name

VIDIOC_ENUMINPUT -- Enumerate video inputs

Synopsis

int ioctl(int fd, int request, struct v4l2_input*argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUMINPUT

argp

Description

To query the attributes of a video input applicationsinitialize the index field of struct v4l2_inputand call theVIDIOC_ENUMINPUT ioctl with apointer to this structure. Drivers fill the rest of the structure orreturn anEINVAL error code when the index is out of bounds. To enumerate allinputs applications shall begin at index zero, incrementing by oneuntil the driver returnsEINVAL.

Table 1. struct v4l2_input

__u32	index	Identifies the input, set by theapplication.
__u8	name[32]	Name of the video input, a NUL-terminated ASCIIstring, for example: "Vin (Composite 2)". This information is intendedfor the user, preferably the connector label on the device itself.
__u32	type	Type of the input, see Table 2.
__u32	audioset	Drivers can enumerate up to 32 video andaudio inputs. This field shows which audio inputs were selectable asaudio source if this was the currently selected video input. It is abit mask. The LSB corresponds to audio input 0, the MSB to input 31.Any number of bits can be set, or none. When the driverdoes not enumerate audio inputs no bits must be set. Applicationsshall not interpret this as lack of audio support. Some driversautomatically select audio sources and do not enumerate them sincethere is no choice anyway. For details on audio inputs andhow to select the current input see Section 1.5.
__u32	tuner	Capture devices can have zero or more tuners (RFdemodulators). When thetype is set toV4L2_INPUT_TYPE_TUNER this is an RF connector andthis field identifies the tuner. It corresponds tostruct v4l2_tuner fieldindex. For details ontuners see Section 1.6.
v4l2_std_id	std	Every video input supports one or more differentvideo standards. This field is a set of all supported standards. Fordetails on video standards and how to switch seeSection 1.7.
__u32	status	This field provides status information about theinput. See Table 3 for flags.status is only valid when this is thecurrent input.
__u32	reserved[4]	Reserved for future extensions. Drivers must setthe array to zero.

Table 2. Input Types

V4L2_INPUT_TYPE_TUNER	1	This input uses a tuner (RF demodulator).
V4L2_INPUT_TYPE_CAMERA	2	Analog baseband input, for example CVBS /Composite Video, S-Video, RGB.

Table 3. Input Status Flags

General
V4L2_IN_ST_NO_POWER	0x00000001	Attached device is off.
V4L2_IN_ST_NO_SIGNAL	0x00000002
V4L2_IN_ST_NO_COLOR	0x00000004	The hardware supports color decoding, but does notdetect color modulation in the signal.
Analog Video
V4L2_IN_ST_NO_H_LOCK	0x00000100	No horizontal sync lock.
V4L2_IN_ST_COLOR_KILL	0x00000200	A color killer circuit automatically disables colordecoding when it detects no color modulation. When this flag is setthe color killer is enabledandhas shut offcolor decoding.
Digital Video
V4L2_IN_ST_NO_SYNC	0x00010000	No synchronization lock.
V4L2_IN_ST_NO_EQU	0x00020000	No equalizer lock.
V4L2_IN_ST_NO_CARRIER	0x00040000	Carrier recovery failed.
VCR and Set-Top Box
V4L2_IN_ST_MACROVISION	0x01000000	Macrovision is an analog copy prevention systemmangling the video signal to confuse video recorders. When thisflag is set Macrovision has been detected.
V4L2_IN_ST_NO_ACCESS	0x02000000	Conditional access denied.
V4L2_IN_ST_VTR	0x04000000	VTR time constant. [?]

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_inputindex isout of bounds.

ioctl VIDIOC_ENUMOUTPUT

Name

VIDIOC_ENUMOUTPUT -- Enumerate video outputs

Synopsis

int ioctl(int fd, int request, struct v4l2_output *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUMOUTPUT

argp

Description

To query the attributes of a video outputs applicationsinitialize the index field of struct v4l2_outputand call theVIDIOC_ENUMOUTPUT ioctl with apointer to this structure. Drivers fill the rest of the structure orreturn anEINVAL error code when the index is out of bounds. To enumerate alloutputs applications shall begin at index zero, incrementing by oneuntil the driver returnsEINVAL.

Table 1. struct v4l2_output

__u32	index	Identifies the output, set by theapplication.
__u8	name[32]	Name of the video output, a NUL-terminated ASCIIstring, for example: "Vout". This information is intended for theuser, preferably the connector label on the device itself.
__u32	type	Type of the output, see Table 2.
__u32	audioset	Drivers can enumerate up to 32 video andaudio outputs. This field shows which audio outputs wereselectable as the current output if this was the currently selectedvideo output. It is a bit mask. The LSB corresponds to audio output 0,the MSB to output 31. Any number of bits can be set, ornone. When the driver does not enumerate audio outputs nobits must be set. Applications shall not interpret this as lack ofaudio support. Drivers may automatically select audio outputs withoutenumerating them. For details on audio outputs and how toselect the current output see Section 1.5.
__u32	modulator	Output devices can have zero or more RF modulators.When the typeisV4L2_OUTPUT_TYPE_MODULATOR this is an RFconnector and this field identifies the modulator. It corresponds tostruct v4l2_modulator fieldindex. For detailson modulators see Section 1.6.
v4l2_std_id	std	Every video output supports one or more differentvideo standards. This field is a set of all supported standards. Fordetails on video standards and how to switch seeSection 1.7.
__u32	reserved[4]	Reserved for future extensions. Drivers must setthe array to zero.

Table 2. Output Type

V4L2_OUTPUT_TYPE_MODULATOR	1	This output is an analog TV modulator.
V4L2_OUTPUT_TYPE_ANALOG	2	Analog baseband output, for example Composite /CVBS, S-Video, RGB.
V4L2_OUTPUT_TYPE_ANALOGVGAOVERLAY	3	[?]

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_outputindexis out of bounds.

ioctl VIDIOC_ENUMSTD

Name

VIDIOC_ENUMSTD -- Enumerate supported video standards

Synopsis

int ioctl(int fd, int request, struct v4l2_standard *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUMSTD

argp

Description

To query the attributes of a video standard,especially a custom (driver defined) one, applications initialize theindex field of struct v4l2_standard and call theVIDIOC_ENUMSTD ioctl with a pointer to thisstructure. Drivers fill the rest of the structure or return anEINVAL error code when the index is out of bounds. To enumerate all standardsapplications shall begin at index zero, incrementing by one until thedriver returns EINVAL. Drivers may enumerate adifferent set of standards after switching the video input oroutput.[23]

Table 1. struct v4l2_standard

__u32	index	Number of the video standard, set by theapplication.
v4l2_std_id	id	The bits in this field identify the standard asone of the common standards listed inTable 3,or if bits 32 to 63 are set as custom standards. Multiple bits can beset if the hardware does not distinguish between these standards,however separate indices do not indicate the opposite. Theidmust be unique. No other enumeratedv4l2_standard structure, for this input oroutput anyway, can contain the same set of bits.
__u8	name[24]	Name of the standard, a NUL-terminated ASCIIstring, for example: "PAL-B/G", "NTSC Japan". This information isintended for the user.
struct v4l2_fract	frameperiod	The frame period (not field period) is numerator/ denominator. For example M/NTSC has a frame period of 1001 /30000 seconds.
__u32	framelines	Total lines per frame including blanking,e. g. 625 for B/PAL.
__u32	reserved[4]	Reserved for future extensions. Drivers must setthe array to zero.

Table 2. struct v4l2_fract

__u32	numerator
__u32	denominator

Table 3. typedef v4l2_std_id

__u64

v4l2_std_id

This type is a set, each bit representing anothervideo standard as listed below and inTable 4. The 32 most significant bits are reservedfor custom (driver defined) video standards.

#define V4L2_STD_PAL_B          ((v4l2_std_id)0x00000001)
#define V4L2_STD_PAL_B1         ((v4l2_std_id)0x00000002)
#define V4L2_STD_PAL_G          ((v4l2_std_id)0x00000004)
#define V4L2_STD_PAL_H          ((v4l2_std_id)0x00000008)
#define V4L2_STD_PAL_I          ((v4l2_std_id)0x00000010)
#define V4L2_STD_PAL_D          ((v4l2_std_id)0x00000020)
#define V4L2_STD_PAL_D1         ((v4l2_std_id)0x00000040)
#define V4L2_STD_PAL_K          ((v4l2_std_id)0x00000080)

#define V4L2_STD_PAL_M          ((v4l2_std_id)0x00000100)
#define V4L2_STD_PAL_N          ((v4l2_std_id)0x00000200)
#define V4L2_STD_PAL_Nc         ((v4l2_std_id)0x00000400)
#define V4L2_STD_PAL_60         ((v4l2_std_id)0x00000800)

V4L2_STD_PAL_60 isa hybrid standard with 525 lines, 60 Hz refresh rate, and PAL colormodulation with a 4.43 MHz color subcarrier. Some PAL video recorderscan play back NTSC tapes in this mode for display on a 50/60 Hz agnosticPAL TV.

#define V4L2_STD_NTSC_M         ((v4l2_std_id)0x00001000)
#define V4L2_STD_NTSC_M_JP      ((v4l2_std_id)0x00002000)
#define V4L2_STD_NTSC_443       ((v4l2_std_id)0x00004000)

V4L2_STD_NTSC_443is a hybrid standard with 525 lines, 60 Hz refresh rate, and NTSCcolor modulation with a 4.43 MHz colorsubcarrier.

#define V4L2_STD_NTSC_M_KR      ((v4l2_std_id)0x00008000)

#define V4L2_STD_SECAM_B        ((v4l2_std_id)0x00010000)
#define V4L2_STD_SECAM_D        ((v4l2_std_id)0x00020000)
#define V4L2_STD_SECAM_G        ((v4l2_std_id)0x00040000)
#define V4L2_STD_SECAM_H        ((v4l2_std_id)0x00080000)
#define V4L2_STD_SECAM_K        ((v4l2_std_id)0x00100000)
#define V4L2_STD_SECAM_K1       ((v4l2_std_id)0x00200000)
#define V4L2_STD_SECAM_L        ((v4l2_std_id)0x00400000)
#define V4L2_STD_SECAM_LC       ((v4l2_std_id)0x00800000)

/* ATSC/HDTV */
#define V4L2_STD_ATSC_8_VSB     ((v4l2_std_id)0x01000000)
#define V4L2_STD_ATSC_16_VSB    ((v4l2_std_id)0x02000000)

V4L2_STD_ATSC_8_VSB andV4L2_STD_ATSC_16_VSB are U.S. terrestrial digitalTV standards. Presently the V4L2 API does not support digital TV. Seealso the Linux DVB API athttp://linuxtv.org.

#define V4L2_STD_PAL_BG         (V4L2_STD_PAL_B         |\
                                 V4L2_STD_PAL_B1        |\
                                 V4L2_STD_PAL_G)
#define V4L2_STD_B              (V4L2_STD_PAL_B         |\
                                 V4L2_STD_PAL_B1        |\
                                 V4L2_STD_SECAM_B)
#define V4L2_STD_GH             (V4L2_STD_PAL_G         |\
                                 V4L2_STD_PAL_H         |\
                                 V4L2_STD_SECAM_G       |\
                                 V4L2_STD_SECAM_H)
#define V4L2_STD_PAL_DK         (V4L2_STD_PAL_D         |\
                                 V4L2_STD_PAL_D1        |\
                                 V4L2_STD_PAL_K)
#define V4L2_STD_PAL            (V4L2_STD_PAL_BG        |\
                                 V4L2_STD_PAL_DK        |\
                                 V4L2_STD_PAL_H         |\
                                 V4L2_STD_PAL_I)
#define V4L2_STD_NTSC           (V4L2_STD_NTSC_M        |\
                                 V4L2_STD_NTSC_M_JP     |\
                                 V4L2_STD_NTSC_M_KR)
#define V4L2_STD_MN             (V4L2_STD_PAL_M         |\
                                 V4L2_STD_PAL_N         |\
                                 V4L2_STD_PAL_Nc        |\
                                 V4L2_STD_NTSC)
#define V4L2_STD_SECAM_DK       (V4L2_STD_SECAM_D       |\
                                 V4L2_STD_SECAM_K       |\
                                 V4L2_STD_SECAM_K1)
#define V4L2_STD_DK             (V4L2_STD_PAL_DK        |\
                                 V4L2_STD_SECAM_DK)

#define V4L2_STD_SECAM          (V4L2_STD_SECAM_B       |\
                                 V4L2_STD_SECAM_G       |\
                                 V4L2_STD_SECAM_H       |\
                                 V4L2_STD_SECAM_DK      |\
                                 V4L2_STD_SECAM_L       |\
                                 V4L2_STD_SECAM_LC)

#define V4L2_STD_525_60         (V4L2_STD_PAL_M         |\
                                 V4L2_STD_PAL_60        |\
                                 V4L2_STD_NTSC          |\
                                 V4L2_STD_NTSC_443)
#define V4L2_STD_625_50         (V4L2_STD_PAL           |\
                                 V4L2_STD_PAL_N         |\
                                 V4L2_STD_PAL_Nc        |\
                                 V4L2_STD_SECAM)

#define V4L2_STD_UNKNOWN        0
#define V4L2_STD_ALL            (V4L2_STD_525_60        |\
                                 V4L2_STD_625_50)

Table 4. Video Standards (based on [ITU BT.470])

Characteristics	M/NTSCa	M/PAL	N/PALb	B, B1, G/PAL	D, D1, K/PAL	H/PAL	I/PAL	B, G/SECAM	D, K/SECAM	K1/SECAM	L/SECAM
Frame lines	525		625
Frame period (s)	1001/30000		1/25
Chrominance sub-carrier frequency (Hz)	3579545 ± 10	3579611.49 ± 10	4433618.75 ± 5 (3582056.25± 5)	4433618.75 ± 5			4433618.75 ± 1	fOR =4406250 ± 2000, fOB = 4250000± 2000
Nominal radio-frequency channel bandwidth(MHz)	6	6	6	B: 7; B1, G: 8	8	8	8	8	8	8	8
Sound carrier relative to vision carrier(MHz)	+ 4.5	+ 4.5	+ 4.5	+ 5.5 ± 0.001c de f	+ 6.5 ± 0.001	+ 5.5	+ 5.9996 ± 0.0005	+ 5.5 ± 0.001	+ 6.5 ± 0.001	+ 6.5	+ 6.5 g
Notes: a. Japan uses a standardsimilar to M/NTSC(V4L2_STD_NTSC_M_JP). b. The values inbrackets apply to the combination N/PAL a.k.a.NC used in Argentina(V4L2_STD_PAL_Nc). c. In the Federal Republic of Germany, Austria, Italy,the Netherlands, Slovakia and Switzerland a system of two soundcarriers is used, the frequency of the second carrier being242.1875 kHz above the frequency of the first sound carrier. Forstereophonic sound transmissions a similar system is used inAustralia. d. New Zealand uses a soundcarrier displaced 5.4996 ± 0.0005 MHz from the visioncarrier. e. In Denmark, Finland, NewZealand, Sweden and Spain a system of two sound carriers is used. InIceland, Norway and Poland the same system is being introduced. Thesecond carrier is 5.85 MHz above the vision carrier and is DQPSKmodulated with 728 kbit/s sound and data multiplex. (NICAMsystem) f. In the United Kingdom, asystem of two sound carriers is used. The second sound carrier is6.552 MHz above the vision carrier and is DQPSK modulated with a728 kbit/s sound and data multiplex able to carry two soundchannels. (NICAM system) g. In France, adigital carrier 5.85 MHz away from the vision carrier may be used inaddition to the main sound carrier. It is modulated in differentiallyencoded QPSK with a 728 kbit/s sound and data multiplexer capable ofcarrying two sound channels. (NICAMsystem)

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_standardindexis out of bounds.

ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO

Name

VIDIOC_G_AUDIO, VIDIOC_S_AUDIO -- Query or select the current audio input and itsattributes

Synopsis

int ioctl(int fd, int request, struct v4l2_audio *argp);

int ioctl(int fd, int request, const struct v4l2_audio *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_AUDIO, VIDIOC_S_AUDIO

argp

Description

To query the current audio input applications zero out thereserved array of a struct v4l2_audioand call theVIDIOC_G_AUDIO ioctl with a pointerto this structure. Drivers fill the rest of the structure or return anEINVAL error code when the device has no audio inputs, or none which combinewith the current video input.

Audio inputs have one writable property, the audio mode. Toselect the current audio inputand change theaudio mode, applications initialize theindexandmodefields, and thereserved array of av4l2_audio structure and call theVIDIOC_S_AUDIO ioctl. Drivers may switch to adifferent audio mode if the request cannot be satisfied. However, thisis a write-only ioctl, it does not return the actual new audiomode.

Table 1. struct v4l2_audio

__u32	index	Identifies the audio input, set by thedriver or application.
__u8	name[32]	Name of the audio input, a NUL-terminated ASCIIstring, for example: "Line In". This information is intended for theuser, preferably the connector label on the device itself.
__u32	capability	Audio capability flags, see Table 2.
__u32	mode	Audio mode flags set by drivers and applications (on VIDIOC_S_AUDIOioctl), seeTable 3.
__u32	reserved[2]	Reserved for future extensions. Drivers andapplications must set the array to zero.

Table 2. Audio Capability Flags

V4L2_AUDCAP_STEREO	0x00001	This is a stereo input. The flag is intended toautomatically disable stereo recording etc. when the signal is alwaysmonaural. The API provides no means to detect if stereo isreceived, unless the audio input belongs to atuner.
V4L2_AUDCAP_AVL	0x00002	Automatic Volume Level mode is supported.

Table 3. Audio Mode Flags

V4L2_AUDMODE_AVL

0x00001

AVL mode is on.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

No audio inputs combine with the current video input,or the number of the selected audio input is out of bounds or it doesnot combine, or there are no audio inputs at all and the ioctl is notsupported.

EBUSY

I/O is in progress, the input cannot beswitched.

ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT

Name

VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT -- Query or select the current audio output

Synopsis

int ioctl(int fd, int request, struct v4l2_audioout *argp);

int ioctl(int fd, int request, const struct v4l2_audioout *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT

argp

Description

To query the current audio output applications zero out thereserved array of a struct v4l2_audioout andcall theVIDIOC_G_AUDOUT ioctl with a pointerto this structure. Drivers fill the rest of the structure or return anEINVAL error code when the device has no audio inputs, or none which combinewith the current video output.

Audio outputs have no writable properties. Nevertheless, toselect the current audio output applications can initialize theindex field andreserved array (which in the future maycontain writable properties) of av4l2_audioout structure and call theVIDIOC_S_AUDOUT ioctl. Drivers switch to therequested output or return theEINVAL error code when the index is out ofbounds. This is a write-only ioctl, it does not return the currentaudio output attributes asVIDIOC_G_AUDOUTdoes.

Note connectors on a TV card to loop back the received audiosignal to a sound card are not audio outputs in this sense.

Table 1. struct v4l2_audioout

__u32	index	Identifies the audio output, set by thedriver or application.
__u8	name[32]	Name of the audio output, a NUL-terminated ASCIIstring, for example: "Line Out". This information is intended for theuser, preferably the connector label on the device itself.
__u32	capability	Audio capability flags, none defined yet. Driversmust set this field to zero.
__u32	mode	Audio mode, none defined yet. Drivers andapplications (onVIDIOC_S_AUDOUT) must set thisfield to zero.
__u32	reserved[2]	Reserved for future extensions. Drivers andapplications must set the array to zero.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

No audio outputs combine with the current videooutput, or the number of the selected audio output is out of bounds orit does not combine, or there are no audio outputs at all and theioctl is not supported.

EBUSY

I/O is in progress, the output cannot beswitched.

ioctl VIDIOC_G_CHIP_IDENT

Name

VIDIOC_G_CHIP_IDENT -- Identify the chips on a TV card

Synopsis

int ioctl(int fd, int request, struct v4l2_chip_ident*argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_CHIP_IDENT

argp

Description

Experimental: This is an experimental interface and may change inthe future.

For driver debugging purposes this ioctl allows testapplications to query the driver about the chips present on the TVcard. Regular applications should not use it. When you found a chipspecific bug, please contact the Video4Linux mailing list (https://listman.redhat.com/mailman/listinfo/video4linux-list)so it can be fixed.

To query the driver applications must initialize thematch_type andmatch_chip fields of a struct v4l2_chip_identand call VIDIOC_G_CHIP_IDENT with a pointer tothis structure. On success the driver stores information about theselected chip in theident andrevision fields. On failure the structureremains unchanged.

When match_type isV4L2_CHIP_MATCH_HOST,match_chip selects the nth non-I²C chipon the TV card. You can enumerate all chips by starting at zero andincrementingmatch_chip by one untilVIDIOC_G_CHIP_IDENT fails with anEINVAL error code.Drivers may also interpretmatch_chip as arandom ID, but we recommend against that. The number zero alwaysselects the host chip, e. g. the chip connected to the PCI bus.

When match_type isV4L2_CHIP_MATCH_I2C_DRIVER,match_chip contains a driver ID as definedin thelinux/i2c-id.h header file. For instanceI2C_DRIVERID_SAA7127 will match any chipsupported by the saa7127 driver, regardless of its I²C bus address.When multiple chips supported by the same driver are present, theioctl will returnV4L2_IDENT_AMBIGUOUS in theident field.

When match_type isV4L2_CHIP_MATCH_I2C_ADDR,match_chip selects a chip by its 7 bitI²C bus address.

On success, the ident field willcontain a chip ID from the Linuxmedia/v4l2-chip-ident.h header file, and therevision field will contain a driverspecific value, or zero if no particular revision is associated withthis chip.

When the driver could not identify the selected chip,ident will containV4L2_IDENT_UNKNOWN. When no chip matchedmatch_type andmatch_chip, the ioctl will succeed but theident field will containV4L2_IDENT_NONE. If multiple chips matched,ident will containV4L2_IDENT_AMBIGUOUS. In all these cases therevision field remains unchanged.

This ioctl is optional, not all drivers may support it. Itwas introduced in Linux 2.6.21.

We recommended the v4l2-dbgutility over calling this ioctl directly. It is available from theLinuxTV v4l-dvb repository; seehttp://linuxtv.org/repo/ foraccess instructions.

Table 1. struct v4l2_chip_ident

__u32	match_type	See Table 2 for a list ofpossible types.
__u32	match_chip	Match a chip by this number, interpreted accordingto the match_type field.
__u32	ident	A chip identifier as defined in the Linuxmedia/v4l2-chip-ident.hheader file, or one ofthe values fromTable 3.
__u32	revision	A chip revision, chip and driver specific.

Table 2. Chip Match Types

V4L2_CHIP_MATCH_HOST	0	Match the nth chip on the card, zero for the host chip. Does not match I2C chips.
V4L2_CHIP_MATCH_I2C_DRIVER	1	Match an I2C chip by its driver ID from thelinux/i2c-id.h header file.
V4L2_CHIP_MATCH_I2C_ADDR	2	Match a chip by its 7 bit I2C bus address.

Table 3. Chip Identifiers

V4L2_IDENT_NONE	0	No chip matched.
V4L2_IDENT_AMBIGUOUS	1	Multiple chips matched.
V4L2_IDENT_UNKNOWN	2	A chip is present at this address, but the drivercould not identify it.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The driver does not support this ioctl, or thematch_type is invalid.

ioctl VIDIOC_G_CROP, VIDIOC_S_CROP

Name

VIDIOC_G_CROP, VIDIOC_S_CROP -- Get or set the current cropping rectangle

Synopsis

int ioctl(int fd, int request, struct v4l2_crop *argp);

int ioctl(int fd, int request, const struct v4l2_crop *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_CROP, VIDIOC_S_CROP

argp

Description

To query the cropping rectangle size and positionapplications set the type field of av4l2_crop structure to the respective buffer(stream) type and call theVIDIOC_G_CROP ioctlwith a pointer to this structure. The driver fills the rest of thestructure or returns theEINVAL error code if cropping is not supported.

To change the cropping rectangle applications initialize thetype and struct v4l2_rect substructure namedc of a v4l2_crop structure and call theVIDIOC_S_CROP ioctl with a pointer to thisstructure.

The driver first adjusts the requested dimensions againsthardware limits, i. e. the bounds given by the capture/output window,and it rounds to the closest possible values of horizontal andvertical offset, width and height. In particular the driver must roundthe vertical offset of the cropping rectangle to frame lines modulotwo, such that the field order cannot be confused.

Second the driver adjusts the image size (the oppositerectangle of the scaling process, source or target depending on thedata direction) to the closest size possible while maintaining thecurrent horizontal and vertical scaling factor.

Finally the driver programs the hardware with the actualcropping and image parameters.VIDIOC_S_CROP is awrite-only ioctl, it does not return the actual parameters. To querythem applications must callVIDIOC_G_CROP andVIDIOC_G_FMT. When the parameters are unsuitable the application maymodify the cropping or image parameters and repeat the cycle untilsatisfactory parameters have been negotiated.

When cropping is not supported then no parameters arechanged and VIDIOC_S_CROP returns theEINVAL error code.

Table 1. struct v4l2_crop

enum v4l2_buf_type	type	Type of the data stream, set by the application.Only these types are valid here:V4L2_BUF_TYPE_VIDEO_CAPTURE,V4L2_BUF_TYPE_VIDEO_OUTPUT,V4L2_BUF_TYPE_VIDEO_OVERLAY, and custom (driverdefined) types with codeV4L2_BUF_TYPE_PRIVATEand higher.
struct v4l2_rect	c	Cropping rectangle. The same co-ordinate system asfor struct v4l2_cropcapbounds is used.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

Cropping is not supported.

ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL

Name

VIDIOC_G_CTRL, VIDIOC_S_CTRL -- Get or set the value of a control

Synopsis

int ioctl(int fd, int request, struct v4l2_control*argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_CTRL, VIDIOC_S_CTRL

argp

Description

To get the current value of a control applicationsinitialize the id field of a structv4l2_control and call theVIDIOC_G_CTRL ioctl with a pointer to thisstructure. To change the value of a control applications initializetheid and valuefields of a structv4l2_control and call theVIDIOC_S_CTRL ioctl.

When the id is invalid driversreturn an EINVAL error code. When the value is outof bounds drivers can choose to take the closest valid value or returnanERANGE error code, whatever seems more appropriate. However,VIDIOC_S_CTRL is a write-only ioctl, it does notreturn the actual new value.

These ioctls work only with user controls. For othercontrol classes the VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS orVIDIOC_TRY_EXT_CTRLSmust be used.

Table 1. struct v4l2_control

__u32	id	Identifies the control, set by theapplication.
__s32	value	New value or current value.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_controlid isinvalid.

ERANGE

The struct v4l2_controlvalueis out of bounds.

EBUSY

The control is temporarily not changeable, possiblybecause another applications took over control of the device functionthis control belongs to.

ioctl VIDIOC_G_ENC_INDEX

Name

VIDIOC_G_ENC_INDEX -- Get meta data about a compressed video stream

Synopsis

int ioctl(int fd, int request, struct v4l2_enc_idx *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_ENC_INDEX

argp

Description

Experimental: This is an experimentalinterface and may change in the future.

The VIDIOC_G_ENC_INDEX ioctl providesmeta data about a compressed video stream the same or anotherapplication currently reads from the driver, which is useful forrandom access into the stream without decoding it.

To read the data applications must callVIDIOC_G_ENC_INDEX with a pointer to astruct v4l2_enc_idx. On success the driver fills theentry array, stores the number of elementswritten in the entries field, andinitializes theentries_cap field.

Each element of the entry arraycontains meta data about one picture. AVIDIOC_G_ENC_INDEX call reads up toV4L2_ENC_IDX_ENTRIES entries from a driverbuffer, which can hold up to entries_capentries. This number can be lower or higher thanV4L2_ENC_IDX_ENTRIES, but not zero. When theapplication fails to read the meta data in time the oldest entrieswill be lost. When the buffer is empty or no capturing/encoding is inprogress, entries will be zero.

Currently this ioctl is only defined for MPEG-2 programstreams and video elementary streams.

Table 1. struct v4l2_enc_idx

__u32	entries	The number of entries the driver stored in theentry array.
__u32	entries_cap	The number of entries the driver canbuffer. Must be greater than zero.
__u32	reserved[4]	Reserved for future extensions.Drivers must set the array to zero.
struct v4l2_enc_idx_entry	entry[V4L2_ENC_IDX_ENTRIES]	Meta data about a compressed video stream. Eachelement of the array corresponds to one picture, sorted in ascendingorder by theiroffset.

Table 2. struct v4l2_enc_idx_entry

__u64	offset	The offset in bytes from the beginning of thecompressed video stream to the beginning of this picture, that is aPES packet header as defined inISO 13818-1 or apictureheader as defined in ISO 13818-2. Whenthe encoder is stopped, the driver resets the offset to zero.
__u64	pts	The 33 bit Presentation TimeStamp of this picture as defined inISO 13818-1.
__u32	length	The length of this picture in bytes.
__u32	flags	Flags containing the coding type of this picture, see Table 3.
__u32	reserved[2]	Reserved for future extensions.Drivers must set the array to zero.

Table 3. Index Entry Flags

V4L2_ENC_IDX_FRAME_I	0x00	This is an Intra-coded picture.
V4L2_ENC_IDX_FRAME_P	0x01	This is a Predictive-coded picture.
V4L2_ENC_IDX_FRAME_B	0x02	This is a Bidirectionally predictive-codedpicture.
V4L2_ENC_IDX_FRAME_MASK	0x0F	AND the flags field withthis mask to obtain the picture coding type.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The driver does not support this ioctl.

ioctl VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS,VIDIOC_TRY_EXT_CTRLS

Name

VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS, VIDIOC_TRY_EXT_CTRLS -- Get or set the value of several controls, try controlvalues

Synopsis

int ioctl(int fd, int request, struct v4l2_ext_controls*argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS,VIDIOC_TRY_EXT_CTRLS

argp

Description

These ioctls allow the caller to get or set multiplecontrols atomically. Control IDs are grouped into control classes (seeTable 3) and all controls in the control arraymust belong to the same control class.

Applications must always fill in thecount,ctrl_class,controls andreserved fields of struct v4l2_ext_controls, andinitialize the struct v4l2_ext_controlarray pointed to by thecontrols fields.

To get the current value of a set of controls applicationsinitialize the id field of eachstruct v4l2_ext_control and call theVIDIOC_G_EXT_CTRLS ioctl.

To change the value of a set of controls applicationsinitialize the id andvalue fields of a struct v4l2_ext_control andcall theVIDIOC_S_EXT_CTRLS ioctl. The controlswill only be set ifall control values arevalid.

To check if the a set of controls have correct valuesapplications initialize theid andvalue fields of a struct v4l2_ext_control andcall theVIDIOC_TRY_EXT_CTRLS ioctl. It is up tothe driver whether wrong values are automatically adjusted to a validvalue or if an error is returned.

When the id orctrl_class is invalid drivers return anEINVAL error code. When the value is out of bounds drivers can choose to takethe closest valid value or return an ERANGE error code, whatever seems moreappropriate. In the first case the new value is set instruct v4l2_ext_control.

The driver will only set/get these controls if all controlvalues are correct. This prevents the situation where only some of thecontrols were set/get. Only low-level errors (e. g. a failed i2ccommand) can still cause this situation.

Table 1. struct v4l2_ext_control

__u32	id		Identifies the control, set by theapplication.
__u32	reserved2[2]		Reserved for future extensions. Drivers andapplications must set the array to zero.
union	(anonymous)
	__s32	value	New value or current value.
	__s64	value64	New value or current value.
	void *	reserved	Reserved for future pointer-type controls. Currently unused.

Table 2. struct v4l2_ext_controls

__u32	ctrl_class	The control class to which all controls belong, seeTable 3.
__u32	count	The number of controls in the controls array. Mayalso be zero.
__u32	error_idx	Set by the driver in case of an error. It is theindex of the control causing the error or equal to 'count' when theerror is not associated with a particular control. Undefined when theioctl returns 0 (success).
__u32	reserved[2]	Reserved for future extensions. Drivers andapplications must set the array to zero.
struct v4l2_ext_control *	controls	Pointer to an array ofcount v4l2_ext_control structures. Ignoredifcountequals zero.

Table 3. Control classes

V4L2_CTRL_CLASS_USER	0x980000	The class containing user controls. These controlsare described in Section 1.8. All controls that can be setusing the VIDIOC_S_CTRL andVIDIOC_G_CTRL ioctl belong to thisclass.
V4L2_CTRL_CLASS_MPEG	0x990000	The class containing MPEG compression controls.These controls are described in sectionSection 1.9.5.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_ext_controlidis invalid or the struct v4l2_ext_controlsctrl_class is invalid. This error code isalso returned by theVIDIOC_S_EXT_CTRLS andVIDIOC_TRY_EXT_CTRLS ioctls if two or morecontrol values are in conflict.

ERANGE

The struct v4l2_ext_controlvalueis out of bounds.

EBUSY

The control is temporarily not changeable, possiblybecause another applications took over control of the device functionthis control belongs to.

ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF

Name

VIDIOC_G_FBUF, VIDIOC_S_FBUF -- Get or set frame buffer overlay parameters

Synopsis

int ioctl(int fd, int request, struct v4l2_framebuffer *argp);

int ioctl(int fd, int request, const struct v4l2_framebuffer *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_FBUF, VIDIOC_S_FBUF

argp

Description

Applications can use the VIDIOC_G_FBUF andVIDIOC_S_FBUF ioctl to get and set theframebuffer parameters for aVideoOverlay orVideo Output Overlay(OSD). The type of overlay is implied by the device type (capture oroutput device) and can be determined with theVIDIOC_QUERYCAPioctl.One/dev/videoN device must not support bothkinds of overlay.

The V4L2 API distinguishes destructive and non-destructiveoverlays. A destructive overlay copies captured video images into thevideo memory of a graphics card. A non-destructive overlay blendsvideo images into a VGA signal or graphics into a video signal.Video Output Overlays are alwaysnon-destructive.

To get the current parameters applications call theVIDIOC_G_FBUF ioctl with a pointer to av4l2_framebuffer structure. The driver fillsall fields of the structure or returns anEINVAL error code when overlays arenot supported.

To set the parameters for a Video OutputOverlay, applications must initialize theflags field of a structv4l2_framebuffer. Since the framebuffer isimplemented on the TV card all other parameters are determined by thedriver. When an application callsVIDIOC_S_FBUFwith a pointer to this structure, the driver prepares for the overlayand returns the framebuffer parameters asVIDIOC_G_FBUF does, or it returns an errorcode.

To set the parameters for a non-destructiveVideo Overlay, applications must initialize theflags field, thefmt substructure, and callVIDIOC_S_FBUF. Again the driver prepares for theoverlay and returns the framebuffer parameters asVIDIOC_G_FBUF does, or it returns an errorcode.

For a destructive Video Overlayapplications must additionally provide abase address. Setting up a DMA to arandom memory location can jeopardize the system security, itsstability or even damage the hardware, therefore only the superusercan set the parameters for a destructive video overlay.

Table 1. struct v4l2_framebuffer

__u32	capability		Overlay capability flags set by the driver, seeTable 2.
__u32	flags		Overlay control flags set by application anddriver, see Table 3
void *	base		Physical base address of the framebuffer,that is the address of the pixel in the top left corner of theframebuffer.a This field is irrelevant tonon-destructive Video Overlays. Fordestructive Video Overlays applications mustprovide a base address. The driver may accept only base addresseswhich are a multiple of two, four or eight bytes. ForVideo Output Overlays the driver must returna valid base address, so applications can find the corresponding Linuxframebuffer device (seeSection 4.4).
struct v4l2_pix_format	fmt		Layout of the frame buffer. Thev4l2_pix_format structure is defined inChapter 2, for clarification the fields and acceptable values are listed below:
	__u32	width	Width of the frame buffer in pixels.
	__u32	height	Height of the frame buffer in pixels.
	__u32	pixelformat	The pixel format of theframebuffer. For non-destructive VideoOverlays this field only defines a format for thestruct v4l2_windowchromakeyfield. For destructive VideoOverlays applications must initialize this field. ForVideo Output Overlays the driver must returna valid format. Usually this is an RGB format (for exampleV4L2_PIX_FMT_RGB565) but YUV formats (only packed YUV formats when chroma keying is used,not includingV4L2_PIX_FMT_YUYV andV4L2_PIX_FMT_UYVY) and theV4L2_PIX_FMT_PAL8 format are also permitted. Thebehavior of the driver when an application requests a compressedformat is undefined. SeeChapter 2 for information onpixel formats.
	enum v4l2_field	field	Drivers and applications shall ignore this field.If applicable, the field order is selected with theVIDIOC_S_FMTioctl, using thefield field ofstruct v4l2_window.
	__u32	bytesperline	Distance in bytes between the leftmost pixels intwo adjacent lines.
This field is irrelevant tonon-destructive VideoOverlays. For destructive VideoOverlays both applications and drivers can set this fieldto request padding bytes at the end of each line. Drivers however mayignore the requested value, returningwidthtimes bytes-per-pixel or a larger value required by the hardware. Thatimplies applications can just set this field to zero to get areasonable default. For Video OutputOverlays the driver must return a validvalue. Video hardware may access padding bytes, thereforethey must reside in accessible memory. Consider for example the casewhere padding bytes after the last line of an image cross a systempage boundary. Capture devices may write padding bytes, the value isundefined. Output devices ignore the contents of paddingbytes. When the image format is planar thebytesperline value applies to the largestplane and is divided by the same factor as thewidth field for any smaller planes. Forexample the Cb and Cr planes of a YUV 4:2:0 image have half as manypadding bytes following each line as the Y plane. To avoid ambiguitiesdrivers must return abytesperline valuerounded up to a multiple of the scale factor.
	__u32	sizeimage	This field is irrelevant tonon-destructive Video Overlays. Fordestructive Video Overlays applications mustinitialize this field. ForVideo OutputOverlays the driver must return a validformat. Together with base itdefines the framebuffer memory accessible by thedriver.
	enum v4l2_colorspace	colorspace	This information supplements thepixelformat and must be set by the driver,seeSection 2.2.
	__u32	priv	Reserved for additional information about custom(driver defined) formats. When not used drivers and applications mustset this field to zero.
Notes: a. A physical base address may not suit allplatforms. GK notes in theory we should pass something like PCI device+ memory region + offset instead. If you encounter problems pleasediscuss on the Video4Linux mailing list:https://listman.redhat.com/mailman/listinfo/video4linux-list.

Table 2. Frame Buffer Capability Flags

V4L2_FBUF_CAP_EXTERNOVERLAY	0x0001	The device is capable of non-destructive overlays.When the driver clears this flag, only destructive overlays aresupported. There are no drivers yet which support both destructive andnon-destructive overlays.
V4L2_FBUF_CAP_CHROMAKEY	0x0002	The device supports clipping by chroma-keying theimages. That is, image pixels replace pixels in the VGA or videosignal only where the latter assume a certain color. Chroma-keyingmakes no sense for destructive overlays.
V4L2_FBUF_CAP_LIST_CLIPPING	0x0004	The device supports clipping using a list of cliprectangles.
V4L2_FBUF_CAP_BITMAP_CLIPPING	0x0008	The device supports clipping using a bit mask.
V4L2_FBUF_CAP_LOCAL_ALPHA	0x0010	The device supports clipping/blending using thealpha channel of the framebuffer or VGA signal. Alpha blending makesno sense for destructive overlays.
V4L2_FBUF_CAP_GLOBAL_ALPHA	0x0020	The device supports alpha blending using a globalalpha value. Alpha blending makes no sense for destructive overlays.
V4L2_FBUF_CAP_LOCAL_INV_ALPHA	0x0040	The device supports clipping/blending using theinverted alpha channel of the framebuffer or VGA signal. Alphablending makes no sense for destructive overlays.

Table 3. Frame Buffer Flags

V4L2_FBUF_FLAG_PRIMARY	0x0001	The framebuffer is the primary graphics surface.In other words, the overlay is destructive. [?]
V4L2_FBUF_FLAG_OVERLAY	0x0002	The frame buffer is an overlay surface the samesize as the capture. [?]
The purpose ofV4L2_FBUF_FLAG_PRIMARY andV4L2_FBUF_FLAG_OVERLAY was never quite clear.Most drivers seem to ignore these flags. For compatibility with thebttv driver applications should set theV4L2_FBUF_FLAG_OVERLAY flag.
V4L2_FBUF_FLAG_CHROMAKEY	0x0004	Use chroma-keying. The chroma-key color isdetermined by the chromakeyfield ofstruct v4l2_window and negotiated with theVIDIOC_S_FMT ioctl, seeSection 4.2andSection 4.4.
There are no flags to enableclipping using a list of clip rectangles or a bitmap. These methodsare negotiated with theVIDIOC_S_FMT ioctl, seeSection 4.2andSection 4.4.
V4L2_FBUF_FLAG_LOCAL_ALPHA	0x0008	Use the alpha channel of the framebuffer to clip orblend framebuffer pixels with video images. The blendfunction is: output = framebuffer pixel * alpha + video pixel * (1 -alpha). The actual alpha depth depends on the framebuffer pixelformat.
V4L2_FBUF_FLAG_GLOBAL_ALPHA	0x0010	Use a global alpha value to blend the framebufferwith video images. The blend function is: output = (framebuffer pixel* alpha + video pixel * (255 - alpha)) / 255. The alpha value isdetermined by theglobal_alpha field ofstruct v4l2_window and negotiated with theVIDIOC_S_FMT ioctl, seeSection 4.2andSection 4.4.
V4L2_FBUF_FLAG_LOCAL_INV_ALPHA	0x0020	LikeV4L2_FBUF_FLAG_LOCAL_ALPHA, use the alpha channelof the framebuffer to clip or blend framebuffer pixels with videoimages, but with an inverted alpha value. The blend function is:output = framebuffer pixel * (1 - alpha) + video pixel * alpha. Theactual alpha depth depends on the framebuffer pixel format.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EPERM

VIDIOC_S_FBUF can only be calledby a privileged user to negotiate the parameters for a destructiveoverlay.

EBUSY

The framebuffer parameters cannot be changed at thistime because overlay is already enabled, or capturing is enabledand the hardware cannot capture and overlay simultaneously.

EINVAL

The ioctl is not supported or theVIDIOC_S_FBUF parameters are unsuitable.

ioctl VIDIOC_G_FMT, VIDIOC_S_FMT,VIDIOC_TRY_FMT

Name

VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT -- Get or set the data format, try a format

Synopsis

int ioctl(int fd, int request, struct v4l2_format*argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT

argp

Description

These ioctls are used to negotiate the format of data(typically image format) exchanged between driver andapplication.

To query the current parameters applications set thetype field of a structv4l2_format to the respective buffer (stream)type. For example video capture devices useV4L2_BUF_TYPE_VIDEO_CAPTURE. When the applicationcalls the VIDIOC_G_FMT ioctl with a pointer tothis structure the driver fills the respective member of thefmt union. In case of video capture devicesthat is the struct v4l2_pix_formatpix member.When the requested buffer type is not supported drivers return anEINVAL error code.

To change the current format parameters applicationsinitialize the type field and allfields of the respective fmtunion member. For details see the documentation of the various devicestypes inChapter 4. Good practice is to query thecurrent parameters first, and tomodify only those parameters not suitable for the application. Whenthe application calls theVIDIOC_S_FMT ioctlwith a pointer to av4l2_format structurethe driver checksand adjusts the parameters against hardware abilities. Driversshould not return an error code unless the input is ambiguous, this isa mechanism to fathom device capabilities and to approach parametersacceptable for both the application and driver. On success the drivermay program the hardware, allocate resources and generally prepare fordata exchange.Finally theVIDIOC_S_FMT ioctl returns thecurrent format parameters asVIDIOC_G_FMT does.Very simple, inflexible devices may even ignore all input and alwaysreturn the default parameters. However all V4L2 devices exchangingdata with the application must implement theVIDIOC_G_FMT andVIDIOC_S_FMT ioctl. When the requested buffertype is not supported drivers return anEINVAL error code on aVIDIOC_S_FMT attempt. When I/O is already inprogress or the resource is not available for other reasons driversreturn theEBUSY error code.

The VIDIOC_TRY_FMT ioctl is equivalentto VIDIOC_S_FMT with one exception: it does notchange driver state. It can also be called at any time, neverreturningEBUSY. This function is provided tonegotiate parameters, to learn about hardware limitations, withoutdisabling I/O or possibly time consuming hardware preparations.Although strongly recommended drivers are not required to implementthis ioctl.

Table 1. struct v4l2_format

enum v4l2_buf_type	type		Type of the data stream, see Table 3-2.
union	fmt
	struct v4l2_pix_format	pix	Definition of an image format, seeChapter 2, used by video capture and outputdevices.
	struct v4l2_window	win	Definition of an overlaid image, seeSection 4.2, used by video overlay devices.
	struct v4l2_vbi_format	vbi	Raw VBI capture or output parameters. This isdiscussed in more detail in Section 4.7. Used by raw VBIcapture and output devices.
	struct v4l2_sliced_vbi_format	sliced	Sliced VBI capture or output parameters. SeeSection 4.8 for details. Used by sliced VBIcapture and output devices.
	__u8	raw_data[200]	Place holder for future extensions and custom(driver defined) formats withtypeV4L2_BUF_TYPE_PRIVATEand higher.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EBUSY

The data format cannot be changed at thistime, for example because I/O is already in progress.

EINVAL

The struct v4l2_formattypefield is invalid, the requested buffer type not supported, orVIDIOC_TRY_FMT was called and is notsupported with this buffer type.

ioctl VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY

Name

VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY -- Get or set tuner or modulator radiofrequency

Synopsis

int ioctl(int fd, int request, struct v4l2_frequency*argp);

int ioctl(int fd, int request, const struct v4l2_frequency*argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY

argp

Description

To get the current tuner or modulator radio frequencyapplications set the tuner field of astruct v4l2_frequency to the respective tuner or modulator number (onlyinput devices have tuners, only output devices have modulators), zeroout thereserved array andcall the VIDIOC_G_FREQUENCY ioctl with a pointerto this structure. The driver stores the current frequency in thefrequency field.

To change the current tuner or modulator radio frequencyapplications initialize thetuner,type andfrequency fields, and thereserved array of a struct v4l2_frequency andcall theVIDIOC_S_FREQUENCY ioctl with a pointerto this structure. When the requested frequency is not possible thedriver assumes the closest possible value. HoweverVIDIOC_S_FREQUENCY is a write-only ioctl, it doesnot return the actual new frequency.

Table 1. struct v4l2_frequency

__u32	tuner	The tuner or modulator index number. This is thesame value as in the struct v4l2_inputtunerfield and the struct v4l2_tunerindex field, orthe struct v4l2_outputmodulator field and thestruct v4l2_modulatorindex field.
enum v4l2_tuner_type	type	The tuner type. This is the same value as in thestruct v4l2_tunertype field. The field is notapplicable to modulators, i. e. ignored by drivers.
__u32	frequency	Tuning frequency in units of 62.5 kHz, or if thestruct v4l2_tuner or struct v4l2_modulatorcapabilities flagV4L2_TUNER_CAP_LOW is set, in units of 62.5Hz.
__u32	reserved[8];	Reserved for future extensions. Drivers and applications must set the array to zero.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The tuner index is out ofbounds or the value in thetype field iswrong.

ioctl VIDIOC_G_INPUT, VIDIOC_S_INPUT

Name

VIDIOC_G_INPUT, VIDIOC_S_INPUT -- Query or select the current video input

Synopsis

int ioctl(int fd, int request, int *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_INPUT, VIDIOC_S_INPUT

argp

Description

To query the current video input applications call theVIDIOC_G_INPUT ioctl with a pointer to an integerwhere the driver stores the number of the input, as in thestruct v4l2_inputindex field. This ioctl willfail only when there are no video inputs, returningEINVAL.

To select a video input applications store the number of thedesired input in an integer and call theVIDIOC_S_INPUT ioctl with a pointer to thisinteger. Side effects are possible. For example inputs may supportdifferent video standards, so the driver may implicitly switch thecurrent standard. It is good practice to select an input beforequerying or negotiating any other parameters.

Information about video inputs is available using theVIDIOC_ENUMINPUT ioctl.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The number of the video input is out of bounds, orthere are no video inputs at all and this ioctl is notsupported.

EBUSY

I/O is in progress, the input cannot beswitched.

ioctl VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP

Name

VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP -- 

Synopsis

int ioctl(int fd, int request, v4l2_jpegcompression *argp);

int ioctl(int fd, int request, const v4l2_jpegcompression *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP

argp

Description

[to do]

Ronald Bultje elaborates:

APP is some application-specific information. Theapplication can set it itself, and it'll be stored in the JPEG-encodedfields (eg; interlacing information for in an AVI or so). COM is thesame, but it's comments, like 'encoded by me' or so.

jpeg_markers describes whether the huffman tables,quantization tables and the restart interval information (allJPEG-specific stuff) should be stored in the JPEG-encoded fields.These define how the JPEG field is encoded. If you omit them,applications assume you've used standard encoding. You usually do wantto add them.

Table 1. struct v4l2_jpegcompression

int	quality
int	APPn
int	APP_len
char	APP_data[60]
int	COM_len
char	COM_data[60]
__u32	jpeg_markers	See Table 2.

Table 2. JPEG Markers Flags

V4L2_JPEG_MARKER_DHT	(1<<3)	Define Huffman Tables
V4L2_JPEG_MARKER_DQT	(1<<4)	Define Quantization Tables
V4L2_JPEG_MARKER_DRI	(1<<5)	Define Restart Interval
V4L2_JPEG_MARKER_COM	(1<<6)	Comment segment
V4L2_JPEG_MARKER_APP	(1<<7)	App segment, driver will always use APP0

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

This ioctl is not supported.

ioctl VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR

Name

VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR -- Get or set modulator attributes

Synopsis

int ioctl(int fd, int request, struct v4l2_modulator*argp);

int ioctl(int fd, int request, const struct v4l2_modulator*argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR

argp

Description

To query the attributes of a modulator applications initializethe index field and zero out thereserved array of a struct v4l2_modulator andcall theVIDIOC_G_MODULATOR ioctl with a pointerto this structure. Drivers fill the rest of the structure or return anEINVAL error code when the index is out of bounds. To enumerate all modulatorsapplications shall begin at index zero, incrementing by one until thedriver returns EINVAL.

Modulators have two writable properties, an audiomodulation set and the radio frequency. To change the modulated audiosubprograms, applications initialize theindex andtxsubchans fields and thereserved array and call theVIDIOC_S_MODULATOR ioctl. Drivers may choose adifferent audio modulation if the request cannot be satisfied. Howeverthis is a write-only ioctl, it does not return the actual audiomodulation selected.

To change the radio frequency the VIDIOC_S_FREQUENCY ioctlis available.

Table 1. struct v4l2_modulator

__u32	index	Identifies the modulator, set by theapplication.
__u8	name[32]	Name of the modulator, a NUL-terminated ASCIIstring. This information is intended for the user.
__u32	capability	Modulator capability flags. No flags are definedfor this field, the tuner flags in struct v4l2_tunerare used accordingly. The audio flags indicate the abilityto encode audio subprograms. They will notchange for example with the current video standard.
__u32	rangelow	The lowest tunable frequency in units of 62.5KHz, or if the capabilityflagV4L2_TUNER_CAP_LOW is set, in units of 62.5Hz.
__u32	rangehigh	The highest tunable frequency in units of 62.5KHz, or if the capabilityflagV4L2_TUNER_CAP_LOW is set, in units of 62.5Hz.
__u32	txsubchans	With this field applications can determine howaudio sub-carriers shall be modulated. It contains a set of flags asdefined inTable 2. Note the tunerrxsubchans flags are reused, but thesemantics are different. Video output devices are assumed to have ananalog or PCM audio input with 1-3 channels. Thetxsubchans flags select one or morechannels for modulation, together with some audio subprogramindicator, for example a stereo pilot tone.
__u32	reserved[4]	Reserved for future extensions. Drivers andapplications must set the array to zero.

Table 2. Modulator Audio Transmission Flags

V4L2_TUNER_SUB_MONO	0x0001	Modulate channel 1 as mono audio, when the inputhas more channels, a down-mix of channel 1 and 2. This flag does notcombine withV4L2_TUNER_SUB_STEREO orV4L2_TUNER_SUB_LANG1.
V4L2_TUNER_SUB_STEREO	0x0002	Modulate channel 1 and 2 as left and rightchannel of a stereo audio signal. When the input has only one channelor two channels andV4L2_TUNER_SUB_SAP is alsoset, channel 1 is encoded as left and right channel. This flag doesnot combine withV4L2_TUNER_SUB_MONOorV4L2_TUNER_SUB_LANG1. When the driver does notsupport stereo audio it shall fall back to mono.
V4L2_TUNER_SUB_LANG1	0x0008	Modulate channel 1 and 2 as primary and secondarylanguage of a bilingual audio signal. When the input has only onechannel it is used for both languages. It is not possible to encodethe primary or secondary language only. This flag does not combinewithV4L2_TUNER_SUB_MONOorV4L2_TUNER_SUB_STEREO. If the hardware does notsupport the respective audio matrix, or the current video standarddoes not permit bilingual audio theVIDIOC_S_MODULATOR ioctl shall return anEINVAL error codeand the driver shall fall back to mono or stereo mode.
V4L2_TUNER_SUB_LANG2	0x0004	Same effect asV4L2_TUNER_SUB_LANG1.
V4L2_TUNER_SUB_SAP	0x0004	When combined with V4L2_TUNER_SUB_MONO the first channel is encoded as mono audio, the lastchannel as Second Audio Program. When the input has only one channelit is used for both audio tracks. When the input has three channelsthe mono track is a down-mix of channel 1 and 2. When combined withV4L2_TUNER_SUB_STEREOchannel 1 and 2 areencoded as left and right stereo audio, channel 3 as Second AudioProgram. When the input has only two channels, the first is encoded asleft and right channel and the second as SAP. When the input has onlyone channel it is used for all audio tracks. It is not possible toencode a Second Audio Program only. This flag must combine withV4L2_TUNER_SUB_MONOorV4L2_TUNER_SUB_STEREO. If the hardware does notsupport the respective audio matrix, or the current video standarddoes not permit SAP theVIDIOC_S_MODULATOR ioctlshall return anEINVAL error code and driver shall fall back to mono or stereomode.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_modulatorindex is out of bounds.

ioctl VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT

Name

VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT -- Query or select the current video output

Synopsis

int ioctl(int fd, int request, int *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT

argp

Description

To query the current video output applications call theVIDIOC_G_OUTPUT ioctl with a pointer to an integerwhere the driver stores the number of the output, as in thestruct v4l2_outputindex field. This ioctlwill fail only when there are no video outputs, returning theEINVAL error code.

To select a video output applications store the number of thedesired output in an integer and call theVIDIOC_S_OUTPUT ioctl with a pointer to this integer.Side effects are possible. For example outputs may support differentvideo standards, so the driver may implicitly switch the currentstandard. It is good practice to select an output before querying ornegotiating any other parameters.

Information about video outputs is available using theVIDIOC_ENUMOUTPUT ioctl.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The number of the video output is out of bounds, orthere are no video outputs at all and this ioctl is notsupported.

EBUSY

I/O is in progress, the output cannot beswitched.

ioctl VIDIOC_G_PARM, VIDIOC_S_PARM

Name

VIDIOC_G_PARM, VIDIOC_S_PARM -- Get or set streaming parameters

Synopsis

int ioctl(int fd, int request, v4l2_streamparm *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_PARM, VIDIOC_S_PARM

argp

Description

The current video standard determines a nominal number offrames per second. If less than this number of frames is to becaptured or output, applications can request frame skipping orduplicating on the driver side. This is especially useful when usingtheread() or write(), whichare not augmented by timestamps or sequence counters, and to avoidunneccessary data copying.

Further these ioctls can be used to determine the number ofbuffers used internally by a driver in read/write mode. Forimplications see the section discussing theread()function.

To get and set the streaming parameters applications callthe VIDIOC_G_PARM andVIDIOC_S_PARM ioctl, respectively. They take apointer to a structv4l2_streamparm whichcontains a union holding separate parameters for input and outputdevices.

Table 1. struct v4l2_streamparm

enum v4l2_buf_type	type		The buffer (stream) type, same as struct v4l2_formattype, set by the application.
union	parm
	struct v4l2_captureparm	capture	Parameters for capture devices, used whentypeisV4L2_BUF_TYPE_VIDEO_CAPTURE.
	struct v4l2_outputparm	output	Parameters for output devices, used whentypeisV4L2_BUF_TYPE_VIDEO_OUTPUT.
	__u8	raw_data[200]	A place holder for future extensions and custom(driver defined) buffer types V4L2_BUF_TYPE_PRIVATEandhigher.

Table 2. struct v4l2_captureparm

__u32	capability	See Table 4.
__u32	capturemode	Set by drivers and applications, see Table 5.
struct v4l2_fract	timeperframe	This is is the desired period betweensuccessive frames captured by the driver, in seconds. Thefield is intended to skip frames on the driver side, saving I/Obandwidth. Applications store here the desired frameperiod, drivers return the actual frame period, which must be greateror equal to the nominal frame period determined by the current videostandard (struct v4l2_standardframeperiodfield). Changing the video standard (also implicitly by switching thevideo input) may reset this parameter to the nominal frame period. Toreset manually applications can just set this field tozero. Drivers support this function only when they set theV4L2_CAP_TIMEPERFRAME flag in thecapability field.
__u32	extendedmode	Custom (driver specific) streaming parameters. Whenunused, applications and drivers must set this field to zero.Applications using this field should check the driver name andversion, seeSection 1.2.
__u32	readbuffers	Applications set this field to the desired numberof buffers used internally by the driver inread() mode. Driversreturn the actual number of buffers. When an application requests zerobuffers, drivers should just return the current setting rather thanthe minimum or an error code. For details seeSection 3.1.
__u32	reserved[4]	Reserved for future extensions. Drivers andapplications must set the array to zero.

Table 3. struct v4l2_outputparm

__u32	capability	See Table 4.
__u32	outputmode	Set by drivers and applications, see Table 5.
struct v4l2_fract	timeperframe	This is is the desired period betweensuccessive frames output by the driver, in seconds.
The field is intended torepeat frames on the driver side in write() mode (in streamingmode timestamps can be used to throttle the output), saving I/Obandwidth. Applications store here the desired frameperiod, drivers return the actual frame period, which must be greateror equal to the nominal frame period determined by the current videostandard (struct v4l2_standardframeperiodfield). Changing the video standard (also implicitly by switching thevideo output) may reset this parameter to the nominal frame period. Toreset manually applications can just set this field tozero. Drivers support this function only when they set theV4L2_CAP_TIMEPERFRAME flag in thecapability field.
__u32	extendedmode	Custom (driver specific) streaming parameters. Whenunused, applications and drivers must set this field to zero.Applications using this field should check the driver name andversion, seeSection 1.2.
__u32	writebuffers	Applications set this field to the desired numberof buffers used internally by the driver inwrite() mode. Drivers return the actual number ofbuffers. When an application requests zero buffers, drivers shouldjust return the current setting rather than the minimum or an errorcode. For details seeSection 3.1.
__u32	reserved[4]	Reserved for future extensions. Drivers andapplications must set the array to zero.

Table 4. Streaming Parameters Capabilites

V4L2_CAP_TIMEPERFRAME

0x1000

The frame skipping/repeating controlled by thetimeperframe field is supported.

Table 5. Capture Parameters Flags

V4L2_MODE_HIGHQUALITY

0x0001

High quality imaging mode. High quality modeis intended for still imaging applications. The idea is to get thebest possible image quality that the hardware can deliver. It is notdefined how the driver writer may achieve that; it will depend on thehardware and the ingenuity of the driver writer. High quality mode isa different mode from the the regular motion video capture modes. Inhigh quality mode: The driver may be able to capture higherresolutions than for motion capture. The driver may support fewer pixel formatsthan motion capture (eg; true color). The driver may capture and arithmeticallycombine multiple successive fields or frames to remove color edgeartifacts and reduce the noise in the video data. The driver may capture images in slices likea scanner in order to handle larger format images than would otherwisebe possible. An image capture operation may besignificantly slower than motion capture. Moving objects in the image might haveexcessive motion blur. Capture might only work through theread() call.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

This ioctl is not supported.

ioctl VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY

Name

VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY -- Query or request the access priority associated with afile descriptor

Synopsis

int ioctl(int fd, int request, enum v4l2_priority *argp);

int ioctl(int fd, int request, const enum v4l2_priority *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY

argp

Pointer to an enum v4l2_priority type.

Description

To query the current access priorityapplications call the VIDIOC_G_PRIORITY ioctlwith a pointer to an enum v4l2_priority variable where the driver storesthe current priority.

To request an access priority applications store thedesired priority in an enum v4l2_priority variable and callVIDIOC_S_PRIORITY ioctl with a pointer to thisvariable.

Table 1. enum v4l2_priority

V4L2_PRIORITY_UNSET	0
V4L2_PRIORITY_BACKGROUND	1	Lowest priority, usually applications running inbackground, for example monitoring VBI transmissions. A proxyapplication running in user space will be necessary if multipleapplications want to read from a device at this priority.
V4L2_PRIORITY_INTERACTIVE	2
V4L2_PRIORITY_DEFAULT	2	Medium priority, usually applications started andinteractively controlled by the user. For example TV viewers, Teletextbrowsers, or just "panel" applications to change the channel or videocontrols. This is the default priority unless an application requestsanother.
V4L2_PRIORITY_RECORD	3	Highest priority. Only one file descriptor can havethis priority, it blocks any other fd from changing device properties.Usually applications which must not be interrupted, like videorecording.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The requested priority value is invalid, or thedriver does not support access priorities.

EBUSY

Another application already requested higherpriority.

ioctl VIDIOC_G_SLICED_VBI_CAP

Name

VIDIOC_G_SLICED_VBI_CAP -- Query sliced VBI capabilities

Synopsis

int ioctl(int fd, int request, struct v4l2_sliced_vbi_cap *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_SLICED_VBI_CAP

argp

Description

To find out which data services are supported by a slicedVBI capture or output device, applications initialize thetype field of a struct v4l2_sliced_vbi_cap,clear the reserved array andcall the VIDIOC_G_SLICED_VBI_CAP ioctl. Thedriver fills in the remaining fields or returns anEINVAL error code if thesliced VBI API is unsupported ortypeis invalid.

Note the type field was added,and the ioctl changed from read-only to write-read, in Linux 2.6.19.

Table 1. struct v4l2_sliced_vbi_cap

__u16	service_set	A set of all data servicessupported by the driver. Equal to the union of all elements of theservice_linesarray.
__u16	service_lines[2][24]	Each element of this arraycontains a set of data services the hardware can look for or insertinto a particular scan line. Data services are defined inTable 2. Array indices map to ITU-Rline numbers (see alsoFigure 4-2 andFigure 4-3) as follows:
		Element	525 line systems	625 line systems
		service_lines[0][1]	1	1
		service_lines[0][23]	23	23
		service_lines[1][1]	264	314
		service_lines[1][23]	286	336
		The number of VBI lines thehardware can capture or output per frame, or the number of services itcan identify on a given line may be limited. For example on PAL line16 the hardware may be able to look for a VPS or Teletext signal, butnot both at the same time. Applications can learn about these limitsusing the VIDIOC_S_FMT ioctl as described inSection 4.8.
		Drivers must setservice_lines[0][0] andservice_lines[1][0] to zero.
enum v4l2_buf_type	type	Type of the data stream, see Table 3-2. Should beV4L2_BUF_TYPE_SLICED_VBI_CAPTUREorV4L2_BUF_TYPE_SLICED_VBI_OUTPUT.
__u32	reserved[3]	This array is reserved for futureextensions. Applications and drivers must set it to zero.

Table 2. Sliced VBI services

Symbol	Value	Reference	Lines, usually	Payload
V4L2_SLICED_TELETEXT_B(TeletextSystem B)	0x0001	ETS 300 706,ITU BT.653	PAL/SECAM line 7-22, 320-335 (second field 7-22)	Last 42 of the 45 byte Teletext packet, that iswithout clock run-in and framing code, lsb first transmitted.
V4L2_SLICED_VPS	0x0400	ETS 300 231	PAL line 16	Byte number 3 to 15 according to Figure 9 ofETS 300 231, lsb first transmitted.
V4L2_SLICED_CAPTION_525	0x1000	EIA 608-B	NTSC line 21, 284 (second field 21)	Two bytes in transmission order, including paritybit, lsb first transmitted.
V4L2_SLICED_WSS_625	0x4000	EN 300 294,ITU BT.1119	PAL/SECAM line 23	Byte 0 1 msb lsb msb lsb Bit 7 6 5 4 3 2 1 0 x x 13 12 11 10 9
V4L2_SLICED_VBI_525	0x1000	Set of services applicable to 525line systems.
V4L2_SLICED_VBI_625	0x4401	Set of services applicable to 625line systems.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The device does not support sliced VBI capturing oroutput, or the value in thetype field iswrong.

ioctl VIDIOC_G_STD, VIDIOC_S_STD

Name

VIDIOC_G_STD, VIDIOC_S_STD -- Query or select the video standard of the current input

Synopsis

int ioctl(int fd, int request, v4l2_std_id*argp);

int ioctl(int fd, int request, const v4l2_std_id*argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_STD, VIDIOC_S_STD

argp

Description

To query and select the current video standard applicationsuse the VIDIOC_G_STD and VIDIOC_S_STD ioctls which take a pointer to av4l2_std_id type as argument.VIDIOC_G_STD canreturn a single flag or a set of flags as in struct v4l2_standard fieldid. The flags must be unambiguous suchthat they appear in only one enumeratedv4l2_standard structure.

VIDIOC_S_STD accepts one or moreflags, being a write-only ioctl it does not return the actual new standard asVIDIOC_G_STD does. When no flags are given orthe current input does not support the requested standard the driverreturns an EINVAL error code. When the standard set is ambiguous drivers mayreturnEINVAL or choose any of the requestedstandards.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

This ioctl is not supported, or theVIDIOC_S_STD parameter was unsuitable.

ioctl VIDIOC_G_TUNER, VIDIOC_S_TUNER

Name

VIDIOC_G_TUNER, VIDIOC_S_TUNER -- Get or set tuner attributes

Synopsis

int ioctl(int fd, int request, struct v4l2_tuner*argp);

int ioctl(int fd, int request, const struct v4l2_tuner*argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_TUNER, VIDIOC_S_TUNER

argp

Description

To query the attributes of a tuner applications initialize theindex field and zero out thereserved array of a struct v4l2_tuner and call theVIDIOC_G_TUNER ioctl with a pointer to thisstructure. Drivers fill the rest of the structure or return anEINVAL error code when the index is out of bounds. To enumerate all tunersapplications shall begin at index zero, incrementing by one until thedriver returns EINVAL.

Tuners have two writable properties, the audio mode andthe radio frequency. To change the audio mode, applications initializetheindex,audmodeandreserved fields and call theVIDIOC_S_TUNER ioctl. This willnot change the current tuner, which is determinedby the current video input. Drivers may choose a different audio modeif the requested mode is invalid or unsupported. Since this is awrite-only ioctl, it does not return the actuallyselected audio mode.

To change the radio frequency the VIDIOC_S_FREQUENCY ioctlis available.

Table 1. struct v4l2_tuner

__u32	index	Identifies the tuner, set by theapplication.
__u8	name[32]	Name of the tuner, aNUL-terminated ASCII string. This information is intended for theuser.
enum v4l2_tuner_type	type	Type of the tuner, see Table 2.
__u32	capability	Tuner capability flags, seeTable 3. Audio flags indicate the abilityto decode audio subprograms. They willnotchange, for example with the current video standard. Whenthe structure refers to a radio tuner only theV4L2_TUNER_CAP_LOWandV4L2_TUNER_CAP_STEREO flags can beset.
__u32	rangelow	The lowest tunable frequency inunits of 62.5 kHz, or if the capabilityflagV4L2_TUNER_CAP_LOW is set, in units of 62.5Hz.
__u32	rangehigh	The highest tunable frequency inunits of 62.5 kHz, or if the capabilityflagV4L2_TUNER_CAP_LOW is set, in units of 62.5Hz.
__u32	rxsubchans	Some tuners or audiodecoders can determine the received audio subprograms by analyzingaudio carriers, pilot tones or other indicators. To pass thisinformation drivers set flags defined inTable 4 in this field. Forexample:
		V4L2_TUNER_SUB_MONO	receiving mono audio
		STEREO | SAP	receiving stereo audio and a secondary audioprogram
		MONO | STEREO	receiving mono or stereo audio, the hardware cannotdistinguish
		LANG1 | LANG2	receiving bilingual audio
		MONO | STEREO | LANG1 | LANG2	receiving mono, stereo or bilingualaudio
		When theV4L2_TUNER_CAP_STEREO,_LANG1,_LANG2 or_SAP flag is cleared in thecapability field, the correspondingV4L2_TUNER_SUB_ flag must not be sethere. This field is valid only if this is the tuner of thecurrent video input, or when the structure refers to a radiotuner.
__u32	audmode	The selected audio mode, seeTable 5 for valid values. The audio mode doesnot affect audio subprogram detection, and like acontrol it does not automatically changeunless the requested mode is invalid or unsupported. SeeTable 6 for possible results whenthe selected and received audio programs do notmatch. Currently this is the only field of structv4l2_tuner applications canchange.
__u32	signal	The signal strength if known, rangingfrom 0 to 65535. Higher values indicate a better signal.
__s32	afc	Automatic frequency control: When theafc value is negative, the frequency is toolow, when positive too high.
__u32	reserved[4]	Reserved for future extensions. Drivers andapplications must set the array to zero.

Table 2. enum v4l2_tuner_type

V4L2_TUNER_RADIO	1
V4L2_TUNER_ANALOG_TV	2

Table 3. Tuner and Modulator Capability Flags

V4L2_TUNER_CAP_LOW	0x0001	When set, tuning frequencies are expressed in units of62.5 Hz, otherwise in units of 62.5 kHz.
V4L2_TUNER_CAP_NORM	0x0002	This is a multi-standard tuner; the video standardcan or must be switched. (B/G PAL tuners for example are typically not considered multi-standard because the video standard is automatically determined from the frequency band.) The set of supported video standards is available from the struct v4l2_input pointing to this tuner, see the description of ioctlVIDIOC_ENUMINPUT for details. OnlyV4L2_TUNER_ANALOG_TV tuners can have this capability.
V4L2_TUNER_CAP_STEREO	0x0010	Stereo audio reception is supported.
V4L2_TUNER_CAP_LANG1	0x0040	Reception of the primary language of a bilingualaudio program is supported. Bilingual audio is a feature oftwo-channel systems, transmitting the primary language monaural on themain audio carrier and a secondary language monaural on a secondcarrier. OnlyV4L2_TUNER_ANALOG_TV tuners can have this capability.
V4L2_TUNER_CAP_LANG2	0x0020	Reception of the secondary language of a bilingualaudio program is supported. OnlyV4L2_TUNER_ANALOG_TV tuners can have this capability.
V4L2_TUNER_CAP_SAP	0x0020	Reception of a secondary audio program issupported. This is a feature of the BTSC system which accompanies theNTSC video standard. Two audio carriers are available for mono orstereo transmissions of a primary language, and an independent thirdcarrier for a monaural secondary language. Only V4L2_TUNER_ANALOG_TV tuners can have this capability. Note theV4L2_TUNER_CAP_LANG2 andV4L2_TUNER_CAP_SAP flags are synonyms.V4L2_TUNER_CAP_SAP applies when the tunersupports theV4L2_STD_NTSC_M videostandard.

Table 4. Tuner Audio Reception Flags

V4L2_TUNER_SUB_MONO	0x0001	The tuner receives a mono audio signal.
V4L2_TUNER_SUB_STEREO	0x0002	The tuner receives a stereo audio signal.
V4L2_TUNER_SUB_LANG1	0x0008	The tuner receives the primary language of abilingual audio signal. Drivers must clear this flag when the currentvideo standard isV4L2_STD_NTSC_M.
V4L2_TUNER_SUB_LANG2	0x0004	The tuner receives the secondary language of abilingual audio signal (or a second audio program).
V4L2_TUNER_SUB_SAP	0x0004	The tuner receives a Second Audio Program. Note theV4L2_TUNER_SUB_LANG2 andV4L2_TUNER_SUB_SAP flags are synonyms. TheV4L2_TUNER_SUB_SAP flag applies when thecurrent video standard is V4L2_STD_NTSC_M.

Table 5. Tuner Audio Modes

V4L2_TUNER_MODE_MONO	0	Play mono audio. When the tuner receives a stereosignal this a down-mix of the left and right channel. When the tunerreceives a bilingual or SAP signal this mode selects the primarylanguage.
V4L2_TUNER_MODE_STEREO	1	Play stereo audio. When the tuner receivesbilingual audio it may play different languages on the left and rightchannel or the primary language on both channels. behave as in monomode. Playing different languages in this mode isdeprecated. New drivers should do this only inMODE_LANG1_LANG2. When the tunerreceives no stereo signal or does not support stereo reception thedriver shall fall back toMODE_MONO.
V4L2_TUNER_MODE_LANG1	3	Play the primary language, mono or stereo. OnlyV4L2_TUNER_ANALOG_TVtuners support thismode.
V4L2_TUNER_MODE_LANG2	2	Play the secondary language, mono. When the tunerreceives no bilingual audio or SAP, or their reception is notsupported the driver shall fall back to mono or stereo mode. OnlyV4L2_TUNER_ANALOG_TV tuners support thismode.
V4L2_TUNER_MODE_SAP	2	Play the Second Audio Program. When the tunerreceives no bilingual audio or SAP, or their reception is notsupported the driver shall fall back to mono or stereo mode. OnlyV4L2_TUNER_ANALOG_TV tuners support this mode.Note theV4L2_TUNER_MODE_LANG2 andV4L2_TUNER_MODE_SAPare synonyms.
V4L2_TUNER_MODE_LANG1_LANG2	4	Play the primary language on the left channel, thesecondary language on the right channel. When the tuner receives nobilingual audio or SAP, it shall fall back toMODE_LANG1 orMODE_MONO.OnlyV4L2_TUNER_ANALOG_TV tuners support thismode.

Table 6. Tuner Audio Matrix

	SelectedV4L2_TUNER_MODE_
ReceivedV4L2_TUNER_SUB_	MONO	STEREO	LANG1	LANG2 = SAP	LANG1_LANG2[a]
MONO	Mono	Mono/Mono	Mono	Mono	Mono/Mono
MONO | SAP	Mono	Mono/Mono	Mono	SAP	Mono/SAP (preferred) or Mono/Mono
STEREO	L+R	L/R	Stereo L/R (preferred) or Mono L+R	Stereo L/R (preferred) or Mono L+R	L/R (preferred) or L+R/L+R
STEREO | SAP	L+R	L/R	Stereo L/R (preferred) or Mono L+R	SAP	L+R/SAP (preferred) or L/R or L+R/L+R
LANG1 | LANG2	Language 1	Lang1/Lang2 (deprecated[b]) orLang1/Lang1	Language 1	Language 2	Lang1/Lang2 (preferred) or Lang1/Lang1
Notes: a. This mode has been added in Linux 2.6.17 and may not be supported by olderdrivers. b. Playback ofboth languages in MODE_STEREO is deprecated. Inthe future drivers should produce only the primary language in thismode. Applications should requestMODE_LANG1_LANG2 to record both languages or astereo signal.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_tunerindex isout of bounds.

ioctl VIDIOC_LOG_STATUS

Name

VIDIOC_LOG_STATUS -- Log driver status information

Synopsis

int ioctl(int fd, int request);

Description

As the video/audio devices become more complicated itbecomes harder to debug problems. When this ioctl is called the driverwill output the current device status to the kernel log. This isparticular useful when dealing with problems like no sound, no videoand incorrectly tuned channels. Also many modern devices autodetectvideo and audio standards and this ioctl will report what the devicethinks what the standard is. Mismatches may give an indication wherethe problem is.

This ioctl is optional and not all drivers support it. Itwas introduced in Linux 2.6.15.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The driver does not support this ioctl.

ioctl VIDIOC_OVERLAY

Name

VIDIOC_OVERLAY -- Start or stop video overlay

Synopsis

int ioctl(int fd, int request, const int *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_OVERLAY

argp

Description

This ioctl is part of the video overlay I/O method. Applications call VIDIOC_OVERLAY to start or stop the overlay. It takes a pointer to an integer which must be set to zero by the application to stop overlay, to one to start.

Drivers do not support VIDIOC_STREAMON orVIDIOC_STREAMOFF withV4L2_BUF_TYPE_VIDEO_OVERLAY.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

Video overlay is not supported, or theparameters have not been set up. See Section 4.2 for the necessary steps.

ioctl VIDIOC_QBUF, VIDIOC_DQBUF

Name

VIDIOC_QBUF, VIDIOC_DQBUF -- Exchange a buffer with the driver

Synopsis

int ioctl(int fd, int request, struct v4l2_buffer *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_QBUF, VIDIOC_DQBUF

argp

Description

Applications call the VIDIOC_QBUF ioctlto enqueue an empty (capturing) or filled (output) buffer in thedriver's incoming queue. The semantics depend on the selected I/Omethod.

To enqueue a memory mappedbuffer applications set the type field of astruct v4l2_buffer to the same buffer type as previously struct v4l2_formattype and struct v4l2_requestbufferstype, thememoryfield toV4L2_MEMORY_MMAP and theindex field. Valid index numbers range fromzero to the number of buffers allocated withVIDIOC_REQBUFS(struct v4l2_requestbufferscount) minus one. Thecontents of the struct v4l2_buffer returnedby aVIDIOC_QUERYBUFioctl will do as well. When the buffer isintended for output (type isV4L2_BUF_TYPE_VIDEO_OUTPUT orV4L2_BUF_TYPE_VBI_OUTPUT) applications must alsoinitialize thebytesused,field andtimestamp fields. SeeSection 3.5 for details. WhenVIDIOC_QBUF is called with a pointer to thisstructure the driver sets theV4L2_BUF_FLAG_MAPPED andV4L2_BUF_FLAG_QUEUED flags and clears theV4L2_BUF_FLAG_DONE flag in theflags field, or it returns anEINVAL error code.

To enqueue a user pointerbuffer applications set the type field of astruct v4l2_buffer to the same buffer type as previously struct v4l2_formattype and struct v4l2_requestbufferstype, thememoryfield toV4L2_MEMORY_USERPTR and them.userptr field to the address of thebuffer andlength to its size. When thebuffer is intended for output additional fields must be set as above.WhenVIDIOC_QBUF is called with a pointer to thisstructure the driver sets theV4L2_BUF_FLAG_QUEUEDflag and clears theV4L2_BUF_FLAG_MAPPED andV4L2_BUF_FLAG_DONE flags in theflags field, or it returns an error code.This ioctl locks the memory pages of the buffer in physical memory,they cannot be swapped out to disk. Buffers remain locked untildequeued, until theVIDIOC_STREAMOFF orVIDIOC_REQBUFS ioctl arecalled, or until the device is closed.

Applications call the VIDIOC_DQBUFioctl to dequeue a filled (capturing) or displayed (output) bufferfrom the driver's outgoing queue. They just set thetypeandmemoryfields of a struct v4l2_buffer as above, whenVIDIOC_DQBUFis called with a pointer to this structure the driver fills theremaining fields or returns an error code.

By default VIDIOC_DQBUF blocks when nobuffer is in the outgoing queue. When theO_NONBLOCK flag was given to theopen()function,VIDIOC_DQBUFreturns immediatelywith an EAGAIN error code when no buffer is available.

The v4l2_buffer structure isspecified in Section 3.5.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EAGAIN

Non-blocking I/O has been selected usingO_NONBLOCK and no buffer was in the outgoingqueue.

EINVAL

The buffer type is notsupported, or the index is out of bounds,or no buffers have been allocated yet, or theuserptr orlength are invalid.

ENOMEM

Not enough physical or virtual memory was available toenqueue a user pointer buffer.

EIO

VIDIOC_DQBUF failed due to aninternal error. Can also indicate temporary problems like signalloss. Note the driver might dequeue an (empty) buffer despitereturning an error, or even stop capturing.

ioctl VIDIOC_QUERYBUF

Name

VIDIOC_QUERYBUF -- Query the status of a buffer

Synopsis

int ioctl(int fd, int request, struct v4l2_buffer *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_QUERYBUF

argp

Description

This ioctl is part of the memorymapping I/O method. It can be used to query the status of abuffer at any time after buffers have been allocated with theVIDIOC_REQBUFS ioctl.

Applications set the type field of a struct v4l2_buffer to the same buffer type as previouslystruct v4l2_formattype and struct v4l2_requestbufferstype, and theindex field. Valid index numbers range from zeroto the number of buffers allocated withVIDIOC_REQBUFS (struct v4l2_requestbufferscount) minus one.After calling VIDIOC_QUERYBUF with a pointer to this structure drivers return an error code or fill the rest ofthe structure.

In the flags field theV4L2_BUF_FLAG_MAPPED,V4L2_BUF_FLAG_QUEUED andV4L2_BUF_FLAG_DONE flags will be valid. Thememory field will be set toV4L2_MEMORY_MMAP, the m.offsetcontains the offset of the buffer from the start of the device memory,thelength field its size. The driver mayor may not set the remaining fields and flags, they are meaningless inthis context.

The v4l2_buffer structure is specified in Section 3.5.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The buffer type is notsupported, or the index is out of bounds.

ioctl VIDIOC_QUERYCAP

Name

VIDIOC_QUERYCAP -- Query device capabilities

Synopsis

int ioctl(int fd, int request, struct v4l2_capability *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_QUERYCAP

argp

Description

All V4L2 devices support theVIDIOC_QUERYCAP ioctl. It is used to identifykernel devices compatible with this specification and to obtaininformation about driver and hardware capabilities. The ioctl takes apointer to a struct v4l2_capability which is filled by the driver. When thedriver is not compatible with this specification the ioctl returns anEINVAL error code.

Table 1. struct v4l2_capability

__u8	driver[16]	Name of the driver, a unique NUL-terminatedASCII string. For example: "bttv". Driver specific applications canuse this information to verify the driver identity. It is also usefulto work around known bugs, or to identify drivers in error reports.The driver version is stored in the versionfield. Storing strings in fixed sized arrays is badpractice but unavoidable here. Drivers and applications should takeprecautions to never read or write beyond the end of the array and tomake sure the strings are properly NUL-terminated.
__u8	card[32]	Name of the device, a NUL-terminated ASCII string.For example: "Yoyodyne TV/FM". One driver may support different brandsor models of video hardware. This information is intended for users,for example in a menu of available devices. Since multiple TV cards ofthe same brand may be installed which are supported by the samedriver, this name should be combined with the character device filename (e. g./dev/video2) or thebus_info string to avoidambiguities.
__u8	bus_info[32]	Location of the device in the system, aNUL-terminated ASCII string. For example: "PCI Slot 4". Thisinformation is intended for users, to distinguish multipleidentical devices. If no such information is available the field maysimply count the devices controlled by the driver, or contain theempty string (bus_info[0] = 0).
__u32	version	Version number of the driver. Together withthe driver field this identifies aparticular driver. The version number is formatted using theKERNEL_VERSION() macro:
#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + (c)) __u32 version = KERNEL_VERSION(0, 8, 1); printf ("Version: %u.%u.%u\n", (version >> 16) & 0xFF, (version >> 8) & 0xFF, version & 0xFF);
__u32	capabilities	Device capabilities, see Table 2.
__u32	reserved[4]	Reserved for future extensions. Drivers must setthis array to zero.

Table 2. Device Capabilities Flags

V4L2_CAP_VIDEO_CAPTURE	0x00000001	The device supports the Video Capture interface.
V4L2_CAP_VIDEO_OUTPUT	0x00000002	The device supports the Video Output interface.
V4L2_CAP_VIDEO_OVERLAY	0x00000004	The device supports the Video Overlay interface. A video overlay devicetypically stores captured images directly in the video memory of agraphics card, with hardware clipping and scaling.
V4L2_CAP_VBI_CAPTURE	0x00000010	The device supports the RawVBI Capture interface, providing Teletext and Closed Captiondata.
V4L2_CAP_VBI_OUTPUT	0x00000020	The device supports the Raw VBI Output interface.
V4L2_CAP_SLICED_VBI_CAPTURE	0x00000040	The device supports the Sliced VBI Capture interface.
V4L2_CAP_SLICED_VBI_OUTPUT	0x00000080	The device supports the Sliced VBI Output interface.
V4L2_CAP_RDS_CAPTURE	0x00000100	[to be defined]
V4L2_CAP_VIDEO_OUTPUT_OVERLAY	0x00000200	The device supports the VideoOutput Overlay (OSD) interface. Unlike theVideoOverlay interface, this is a secondary function of videooutput devices and overlays an image onto an outgoing video signal.When the driver sets this flag, it must clear theV4L2_CAP_VIDEO_OVERLAY flag and viceversa.[a]
V4L2_CAP_TUNER	0x00010000	The device has some sort of tuner or modulator toreceive or emit RF-modulated video signals. For more information abouttuner and modulator programming seeSection 1.6.
V4L2_CAP_AUDIO	0x00020000	The device has audio inputs or outputs. It may ormay not support audio recording or playback, in PCM or compressedformats. PCM audio support must be implemented as ALSA or OSSinterface. For more information on audio inputs and outputs seeSection 1.5.
V4L2_CAP_RADIO	0x00040000	This is a radio receiver.
V4L2_CAP_READWRITE	0x01000000	The device supports the read() and/or write()I/O methods.
V4L2_CAP_ASYNCIO	0x02000000	The device supports the asynchronous I/O methods.
V4L2_CAP_STREAMING	0x04000000	The device supports the streaming I/O method.
Notes: a. The struct v4l2_framebuffer lacks anenum v4l2_buf_type field, therefore the type of overlay is implied by thedriver capabilities.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The device is not compatible with thisspecification.

ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU

Name

VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU -- Enumerate controls and menu control items

Synopsis

int ioctl(int fd, int request, struct v4l2_queryctrl *argp);

int ioctl(int fd, int request, struct v4l2_querymenu *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU

argp

Description

To query the attributes of a control applications set theid field of a struct v4l2_queryctrl and call theVIDIOC_QUERYCTRL ioctl with a pointer to thisstructure. The driver fills the rest of the structure or returns anEINVAL error code when theid is invalid.

It is possible to enumerate controls by callingVIDIOC_QUERYCTRL with successiveid values starting fromV4L2_CID_BASE up to and exclusiveV4L2_CID_BASE_LASTP1. Drivers may returnEINVAL if a control in this range is notsupported. Further applications can enumerate private controls, whichare not defined in this specification, by starting atV4L2_CID_PRIVATE_BASE and incrementingid until the driver returnsEINVAL.

In both cases, when the driver sets theV4L2_CTRL_FLAG_DISABLED flag in theflags field this control is permanentlydisabled and should be ignored by the application.[24]

When the application ORs id withV4L2_CTRL_FLAG_NEXT_CTRL the driver returns thenext supported control, orEINVAL if there isnone. Drivers which do not support this flag yet always returnEINVAL.

Additional information is required for menu controls, thename of menu items. To query them applications set theid andindexfields of struct v4l2_querymenu and call theVIDIOC_QUERYMENU ioctl with a pointer to thisstructure. The driver fills the rest of the structure or returns anEINVALerror code when theid orindex is invalid. Menu items are enumeratedby callingVIDIOC_QUERYMENU with successiveindex values from struct v4l2_queryctrlminimum (0) tomaximum, inclusive.

See also the examples in Section 1.8.

Table 1. struct v4l2_queryctrl

__u32	id	Identifies the control, set by the application. SeeTable 1-1 for predefined IDs. When the ID is ORedwith V4L2_CTRL_FLAG_NEXT_CTRL the driver clears the flag and returnsthe first control with a higher ID. Drivers which do not support thisflag yet always return anEINVAL error code.
enum v4l2_ctrl_type	type	Type of control, see Table 3.
__u8	name[32]	Name of the control, a NUL-terminated ASCIIstring. This information is intended for the user.
__s32	minimum	Minimum value, inclusive. This field gives a lowerbound forV4L2_CTRL_TYPE_INTEGER controls. It maynot be valid for any other type of control, includingV4L2_CTRL_TYPE_INTEGER64 controls. Note this is asigned value.
__s32	maximum	Maximum value, inclusive. This field gives an upperbound forV4L2_CTRL_TYPE_INTEGER controls and thehighest valid index forV4L2_CTRL_TYPE_MENUcontrols. It may not be valid for any other type of control, includingV4L2_CTRL_TYPE_INTEGER64 controls. Note this is asigned value.
__s32	step	This field gives a step size forV4L2_CTRL_TYPE_INTEGER controls. It may not bevalid for any other type of control, includingV4L2_CTRL_TYPE_INTEGER64controls. Generally drivers should not scale hardwarecontrol values. It may be necessary for example when thename orid implya particular unit and the hardware actually accepts only multiples ofsaid unit. If so, drivers must take care values are properly roundedwhen scaling, such that errors will not accumulate on repeatedread-write cycles. This field gives the smallest change ofan integer control actually affecting hardware. Often the informationis needed when the user can change controls by keyboard or GUIbuttons, rather than a slider. When for example a hardware registeraccepts values 0-511 and the driver reports 0-65535, step should be128. Note although signed, the step value is supposed tobe always positive.
__s32	default_value	The default value of aV4L2_CTRL_TYPE_INTEGER,_BOOLEAN or_MENUcontrol.Not valid for other types of controls. Drivers reset controls onlywhen the driver is loaded, not later, in particular not when thefunc-open; is called.
__u32	flags	Control flags, see Table 4.
__u32	reserved[2]	Reserved for future extensions. Drivers must setthe array to zero.

Table 2. struct v4l2_querymenu

__u32	id	Identifies the control, set by the applicationfrom the respective struct v4l2_queryctrlid.
__u32	index	Index of the menu item, starting at zero, set by the application.
__u8	name[32]	Name of the menu item, a NUL-terminated ASCIIstring. This information is intended for the user.
__u32	reserved	Reserved for future extensions. Drivers must setthe array to zero.

Table 3. enum v4l2_ctrl_type

Type	minimum	step	maximum	Description
V4L2_CTRL_TYPE_INTEGER	any	any	any	An integer-valued control ranging from minimum tomaximum inclusive. The step value indicates the increment betweenvalues which are actually different on the hardware.
V4L2_CTRL_TYPE_BOOLEAN	0	1	1	A boolean-valued control. Zero corresponds to"disabled", and one means "enabled".
V4L2_CTRL_TYPE_MENU	0	1	N-1	The control has a menu of N choices. The names ofthe menu items can be enumerated with theVIDIOC_QUERYMENU ioctl.
V4L2_CTRL_TYPE_BUTTON	0	0	0	A control which performs an action when set.Drivers must ignore the value passed withVIDIOC_S_CTRL and return anEINVAL error code on aVIDIOC_G_CTRL attempt.
V4L2_CTRL_TYPE_INTEGER64	n/a	n/a	n/a	A 64-bit integer valued control. Minimum, maximumand step size cannot be queried.
V4L2_CTRL_TYPE_CTRL_CLASS	n/a	n/a	n/a	This is not a control. WhenVIDIOC_QUERYCTRL is called with a control IDequal to a control class code (seeTable 3), theioctl returns the name of the control class and this control type.Older drivers which do not support this feature return anEINVAL error code.

Table 4. Control Flags

V4L2_CTRL_FLAG_DISABLED	0x0001	This control is permanently disabled and should beignored by the application. Any attempt to change the control willresult in anEINVAL error code.
V4L2_CTRL_FLAG_GRABBED	0x0002	This control is temporarily unchangeable, forexample because another application took over control of therespective resource. Such controls may be displayed specially in auser interface. Attempts to change the control may result in anEBUSY error code.
V4L2_CTRL_FLAG_READ_ONLY	0x0004	This control is permanently readable only. Anyattempt to change the control will result in anEINVAL error code.
V4L2_CTRL_FLAG_UPDATE	0x0008	A hint that changing this control may affect thevalue of other controls within the same control class. Applicationsshould update their user interface accordingly.
V4L2_CTRL_FLAG_INACTIVE	0x0010	This control is not applicable to the currentconfiguration and should be displayed accordingly in a user interface.For example the flag may be set on a MPEG audio level 2 bitratecontrol when MPEG audio encoding level 1 was selected with anothercontrol.
V4L2_CTRL_FLAG_SLIDER	0x0020	A hint that this control is best represented as aslider-like element in a user interface.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_queryctrlidis invalid. The struct v4l2_querymenuid orindex is invalid.

ioctl VIDIOC_QUERYSTD

Name

VIDIOC_QUERYSTD -- Sense the video standard received by the currentinput

Synopsis

int ioctl(int fd, int request, v4l2_std_id *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_QUERYSTD

argp

Description

The hardware may be able to detect the current videostandard automatically. To do so, applications callVIDIOC_QUERYSTD with a pointer to av4l2_std_idtype. Thedriver stores here a set of candidates, this can be a single flag or aset of supported standards if for example the hardware can onlydistinguish between 50 and 60 Hz systems. When detection is notpossible or fails, the set must contain all standards supported by thecurrent video input or output.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

This ioctl is not supported.

ioctl VIDIOC_REQBUFS

Name

VIDIOC_REQBUFS -- Initiate Memory Mapping or User Pointer I/O

Synopsis

int ioctl(int fd, int request, struct v4l2_requestbuffers *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_REQBUFS

argp

Description

This ioctl is used to initiate memorymapped or user pointerI/O. Memory mapped buffers are located in device memory and must beallocated with this ioctl before they can be mapped into theapplication's address space. User buffers are allocated byapplications themselves, and this ioctl is merely used to switch thedriver into user pointer I/O mode.

To allocate device buffers applications initialize threefields of a v4l2_requestbuffers structure.They set the type field to the respectivestream or buffer type, thecount field tothe desired number of buffers, andmemorymust be set to V4L2_MEMORY_MMAP. When the ioctlis called with a pointer to this structure the driver attempts toallocate the requested number of buffers and stores the actual numberallocated in thecount field. It can besmaller than the number requested, even zero, when the driver runs outof free memory. A larger number is possible when the driver requiresmore buffers to function correctly.[25] When memory mapping I/O is not supported the ioctlreturns an EINVAL error code.

Applications can call VIDIOC_REQBUFSagain to change the number of buffers, however this cannot succeedwhen any buffers are still mapped. Acountvalue of zero frees all buffers, after aborting or finishing any DMAin progress, an implicitVIDIOC_STREAMOFF.

To negotiate user pointer I/O, applications initialize onlythe type field and setmemory toV4L2_MEMORY_USERPTR. When the ioctl is calledwith a pointer to this structure the driver prepares for user pointerI/O, when this I/O method is not supported the ioctl returns anEINVAL error code.

Table 1. struct v4l2_requestbuffers

__u32	count	The number of buffers requested or granted. Thisfield is only used whenmemory is set toV4L2_MEMORY_MMAP.
enum v4l2_buf_type	type	Type of the stream or buffers, this is the sameas the struct v4l2_formattypefield. SeeTable 3-2 for valid values.
enum v4l2_memory	memory	Applications set this field toV4L2_MEMORY_MMAPorV4L2_MEMORY_USERPTR.
__u32	reserved[2]	A place holder for future extensions and custom(driver defined) buffer typesV4L2_BUF_TYPE_PRIVATE andhigher.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EBUSY

The driver supports multiple opens and I/O is alreadyin progress, or reallocation of buffers was attempted although one ormore are still mapped.

EINVAL

The buffer type (type field) or therequested I/O method (memory) is notsupported.

ioctl VIDIOC_STREAMON, VIDIOC_STREAMOFF

Name

VIDIOC_STREAMON, VIDIOC_STREAMOFF -- Start or stop streaming I/O

Synopsis

int ioctl(int fd, int request, const int *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_STREAMON, VIDIOC_STREAMOFF

argp

Description

The VIDIOC_STREAMON andVIDIOC_STREAMOFF ioctl start and stop the captureor output process during streaming (memorymapping oruser pointer) I/O.

Specifically the capture hardware is disabled and no inputbuffers are filled (if there are any empty buffers in the incomingqueue) untilVIDIOC_STREAMONhas been called.Accordingly the output hardware is disabled, no video signal isproduced untilVIDIOC_STREAMON has been called.The ioctl will succeed only when at least one output buffer is in theincoming queue.

The VIDIOC_STREAMOFF ioctl, apart ofaborting or finishing any DMA in progress, unlocks any user pointerbuffers locked in physical memory, and it removes all buffers from theincoming and outgoing queues. That means all images captured but notdequeued yet will be lost, likewise all images enqueued for output butnot transmitted yet. I/O returns to the same state as after callingVIDIOC_REQBUFS and can be restarted accordingly.

Both ioctls take a pointer to an integer, the desired buffer orstream type. This is the same as struct v4l2_requestbufferstype.

Note applications can be preempted for unknown periods rightbefore or after theVIDIOC_STREAMON orVIDIOC_STREAMOFF calls, there is no notion ofstarting or stopping "now". Buffer timestamps can be used tosynchronize with other events.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

Streaming I/O is not supported, the buffertype is not supported, or no buffers havebeen allocated (memory mapping) or enqueued (output) yet.

V4L2 mmap()

Name

v4l2-mmap -- Map device memory into application address space

Synopsis

#include 
#include 

void *mmap(void *start, size_t length, int prot, int flags, int fd, off_t offset);

Arguments

start

Map the buffer to this address in the application's address space. When the MAP_FIXEDflag is specified, start must be a multiple of thepagesize and mmap will fail when the specified addresscannot be used. Use of this option is discouraged; applications shouldjust specify aNULL pointer here.

length

Length of the memory area to map. This must be thesame value as returned by the driver in the struct v4l2_bufferlength field.

prot

The prot argument describes thedesired memory protection. Regardless of the device type and thedirection of data exchange it should be set toPROT_READ |PROT_WRITE,permitting read and write access to image buffers. Drivers shouldsupport at least this combination of flags. Note the Linuxvideo-buf kernel module, which is used by thebttv, saa7134, saa7146, cx88 and vivi driver supports onlyPROT_READ | PROT_WRITE. Whenthe driver does not support the desired protection themmap() function fails.

Note device memory accesses (e. g. the memory on agraphics card with video capturing hardware) may incur a performancepenalty compared to main memory accesses, or reads may besignificantly slower than writes or vice versa. Other I/O methods maybe more efficient in this case.

flags

The flags parameterspecifies the type of the mapped object, mapping options and whethermodifications made to the mapped copy of the page are private to theprocess or are to be shared with other references.

MAP_FIXED requests that thedriver selects no other address than the one specified. If thespecified address cannot be used,mmap() will fail. IfMAP_FIXED is specified,start must be a multiple of the pagesize. Useof this option is discouraged.

One of the MAP_SHARED orMAP_PRIVATE flags must be set.MAP_SHARED allows applications to share themapped memory with other (e. g. child-) processes. Note the Linuxvideo-buf module which is used by the bttv,saa7134, saa7146, cx88 and vivi driver supports onlyMAP_SHARED.MAP_PRIVATErequests copy-on-write semantics. V4L2 applications should not set theMAP_PRIVATE,MAP_DENYWRITE,MAP_EXECUTABLE orMAP_ANONflag.

fd

File descriptor returned by open().

offset

Offset of the buffer in device memory. This must be thesame value as returned by the driver in the struct v4l2_bufferm unionoffset field.

Description

The mmap() function asks to maplength bytes starting atoffset in the memory of the device specified byfd into the application address space,preferably at address start. This latteraddress is a hint only, and is usually specified as 0.

Suitable length and offset parameters are queried with theVIDIOC_QUERYBUF ioctl. Buffers must be allocated with theVIDIOC_REQBUFS ioctl before they can be queried.

To unmap buffers the munmap() function is used.

Return Value

On success mmap() returns a pointer tothe mapped buffer. On errorMAP_FAILED (-1) isreturned, and theerrno variable is setappropriately. Possible error codes are:

EBADF

fd is not a valid filedescriptor.

EACCES

fd isnot open for reading and writing.

EINVAL

The start orlength oroffset are notsuitable. (E. g. they are too large, or not aligned on aPAGESIZE boundary.)

The flags orprot value is not supported.

No buffers have been allocated with theVIDIOC_REQBUFS ioctl.

ENOMEM

Not enough physical or virtual memory was available tocomplete the request.

V4L2 munmap()

Name

v4l2-munmap -- Unmap device memory

Synopsis

#include 
#include 

int munmap(void *start, size_t length);

Arguments

start

Address of the mapped buffer as returned by themmap() function.

length

Length of the mapped buffer. This must be the samevalue as given to mmap() and returned by thedriver in the struct v4l2_bufferlengthfield.

Description

Unmaps a previously with the mmap() function mappedbuffer and frees it, if possible.

Return Value

On success munmap() returns 0, onfailure -1 and theerrno variable is setappropriately:

EINVAL

The start orlength is incorrect, or no buffers have beenmapped yet.

V4L2 open()

Name

v4l2-open -- Open a V4L2 device

Synopsis

#include 

int open(const char *device_name, int flags);

Arguments

device_name

Device to be opened.

flags

Open flags. Access mode must beO_RDWR. This is just a technicality, input devicesstill support only reading and output devices only writing.

When the O_NONBLOCK flag isgiven, the read() function and theVIDIOC_DQBUF ioctl will returntheEAGAIN error code when no data is available or no buffer is in the driveroutgoing queue, otherwise these functions block until data becomesavailable. All V4L2 drivers exchanging data with applications mustsupport theO_NONBLOCK flag.

Other flags have no effect.

Description

To open a V4L2 device applications callopen() with the desired device name. Thisfunction has no side effects; all data format parameters, currentinput or output, control values or other properties remain unchanged.At the firstopen() call after loading the driverthey will be reset to default values, drivers are never in anundefined state.

Return Value

On success open returns the new filedescriptor. On error -1 is returned, and theerrnovariable is set appropriately. Possible error codes are:

EACCES

The caller has no permission to access thedevice.

EBUSY

The driver does not support multiple opens and thedevice is already in use.

ENXIO

No device corresponding to this device special fileexists.

ENOMEM

Not enough kernel memory was available to complete therequest.

EMFILE

The process already has the maximum number offiles open.

ENFILE

The limit on the total number of files open on thesystem has been reached.

V4L2 poll()

Name

v4l2-poll -- Wait for some event on a file descriptor

Synopsis

#include 

int poll(struct pollfd *ufds, unsigned int nfds, int timeout);

Description

With the poll() function applicationscan suspend execution until the driver has captured data or is readyto accept data for output.

When streaming I/O has been negotiated this function waitsuntil a buffer has been filled or displayed and can be dequeued withtheVIDIOC_DQBUF ioctl. When buffers are already in the outgoingqueue of the driver the function returns immediately.

On success poll() returns the number offile descriptors that have been selected (that is, file descriptorsfor which therevents field of therespectivepollfd structure is non-zero).Capture devices set thePOLLIN andPOLLRDNORM flags in therevents field, output devices thePOLLOUTandPOLLWRNORMflags. When the function timed out it returns a value of zero, onfailure it returns-1 and theerrno variable is set appropriately. When theapplication did not callVIDIOC_QBUF orVIDIOC_STREAMON yet thepoll() function succeeds, but sets thePOLLERR flag in therevents field.

When use of the read() function hasbeen negotiated and the driver does not capture yet, thepoll function starts capturing. When that failsit returns aPOLLERR as above. Otherwise it waitsuntil data has been captured and can be read. When the driver capturescontinuously (as opposed to, for example, still images) the functionmay return immediately.

When use of the write() function hasbeen negotiated thepoll function just waitsuntil the driver is ready for a non-blockingwrite() call.

All drivers implementing the read() orwrite() function or streaming I/O must alsosupport thepoll() function.

For more details see thepoll() manual page.

Return Value

On success, poll() returns the numberstructures which have non-zeroreventsfields, or zero if the call timed out. On error-1 is returned, and theerrnovariable is set appropriately:

EBADF

One or more of the ufds membersspecify an invalid file descriptor.

EBUSY

The driver does not support multiple read or writestreams and the device is already in use.

EFAULT

ufds references an inaccessiblememory area.

EINTR

The call was interrupted by a signal.

EINVAL

The nfds argument is greaterthan OPEN_MAX.

V4L2 read()

Name

v4l2-read -- Read from a V4L2 device

Synopsis

#include 

ssize_t read(int fd, void *buf, size_t count);

Arguments

fd

File descriptor returned by open().

buf

count

Description

read() attempts to read up tocount bytes from file descriptorfd into the buffer starting atbuf. The layout of the data in the buffer isdiscussed in the respective device interface section, see ##. If count is zero,read() returns zero and has no other results. Ifcount