USB Audio Class (UAC) 分析
一个UAC设备插入到Ubuntu 14.04电脑上dmesg中打印的信息如下：
[ 2367.490491] usb 3-3.2: new full-speed USB device number 9 using xhci_hcd
[ 2367.580010] usb 3-3.2: New USB device found, idVendor=0d8c, idProduct=0132
[ 2367.580018] usb 3-3.2: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[ 2367.580023] usb 3-3.2: Product: USB PnP Audio Device
[ 2367.580027] usb 3-3.2: Manufacturer: C-Media Electronics Inc.
[ 2367.581679] input: C-Media Electronics Inc. USB PnP Audio Device as /devices/pci0000:00/0000:00:14.0/usb3/3-3/3-3.2/3-3.2:1.2/0003:0D8C:0132.0004/input/input18
[ 2367.581999] hid-generic 0003:0D8C:0132.0004: input,hidraw3: USB HID v1.11 Device [C-Media Electronics Inc. USB PnP Audio Device] on usb-0000:00:14.0-3.2/input2
[ 2367.913280] usbcore: registered new interface driver snd-usb-audio这里可以看到其驱动程序为snd-usb-audio，依据这个关键词在内核中查找到如下内容：
tony@tony-E431:~/linux-3.4.y$ grep "registered new interface driver" ./ -rn
Binary file ./vmlinux.o matches
Binary file ./.tmp_vmlinux1 matches
Binary file ./.tmp_vmlinux2 matches
Binary file ./drivers/built-in.o matches
Binary file ./drivers/usb/built-in.o matches
Binary file ./drivers/usb/core/built-in.o matches
./drivers/usb/core/driver.c:865: pr_info("%s: registered new interface driver %s\n",
Binary file ./drivers/usb/core/driver.o matches
Binary file ./drivers/usb/core/usbcore.o matches
Binary file ./arch/arm/boot/Image matches
Binary file ./vmlinux matches
tony@tony-E431:~/linux-3.4.y$ 以及
// file: linux-3.4.y/sound/usb/card.c
/*
* entry point for linux usb interface
*/
static struct usb_driver usb_audio_driver = {
.name = "snd-usb-audio",
.probe = usb_audio_probe,
.disconnect = usb_audio_disconnect,
.suspend = usb_audio_suspend,
.resume = usb_audio_resume,
.id_table = usb_audio_ids,
.supports_autosuspend = 1,
};根据probe方法snd_usb_audio_probe调用了snd_usb_apply_boot_quirk和snd_card_register，查到了
// linux-3.4.y/sound/usb/card.c
/*
* new 2.5 USB kernel API
*/
static int usb_audio_probe(struct usb_interface *intf,
const struct usb_device_id *id)
{
struct snd_usb_audio *chip;
chip = snd_usb_audio_probe(interface_to_usbdev(intf), intf, id);
if (chip) {
usb_set_intfdata(intf, chip);
return 0;
} else
return -EIO;
}
// linux-3.4.y/sound/usb/quirks.c
int snd_usb_apply_boot_quirk(struct usb_device *dev,
struct usb_interface *intf,
const struct snd_usb_audio_quirk *quirk)
{
u32 id = USB_ID(le16_to_cpu(dev->descriptor.idVendor),
le16_to_cpu(dev->descriptor.idProduct));
switch (id) {
case USB_ID(0x041e, 0x3000):
/* SB Extigy needs special boot-up sequence */
/* if more models come, this will go to the quirk list. */
return snd_usb_extigy_boot_quirk(dev, intf);
case USB_ID(0x041e, 0x3020):
/* SB Audigy 2 NX needs its own boot-up magic, too */
return snd_usb_audigy2nx_boot_quirk(dev);
case USB_ID(0x10f5, 0x0200):
/* C-Media CM106 / Turtle Beach Audio Advantage Roadie */
return snd_usb_cm106_boot_quirk(dev);
case USB_ID(0x0d8c, 0x0102):
/* C-Media CM6206 / CM106-Like Sound Device */
case USB_ID(0x0ccd, 0x00b1): /* Terratec Aureon 7.1 USB */
return snd_usb_cm6206_boot_quirk(dev);
case USB_ID(0x133e, 0x0815):
/* Access Music VirusTI Desktop */
return snd_usb_accessmusic_boot_quirk(dev);
case USB_ID(0x17cc, 0x1000): /* Komplete Audio 6 */
case USB_ID(0x17cc, 0x1010): /* Traktor Audio 6 */
case USB_ID(0x17cc, 0x1020): /* Traktor Audio 10 */
return snd_usb_nativeinstruments_boot_quirk(dev);
case USB_ID(0x0763, 0x2012): /* M-Audio Fast Track Pro USB */
return snd_usb_fasttrackpro_boot_quirk(dev);
}
return 0;
}snd_usb_audio_probe
/*
* probe the active usb device
*
* note that this can be called multiple times per a device, when it
* includes multiple audio control interfaces.
*
* thus we check the usb device pointer and creates the card instance
* only at the first time. the successive calls of this function will
* append the pcm interface to the corresponding card.
*/
static struct snd_usb_audio *
snd_usb_audio_probe(struct usb_device *dev,
struct usb_interface *intf,
const struct usb_device_id *usb_id)
{
const struct snd_usb_audio_quirk *quirk = (const struct snd_usb_audio_quirk *)usb_id->driver_info;
int i, err;
struct snd_usb_audio *chip;
struct usb_host_interface *alts;
int ifnum;
u32 id;
alts = &intf->altsetting[0];
ifnum = get_iface_desc(alts)->bInterfaceNumber;
id = USB_ID(le16_to_cpu(dev->descriptor.idVendor),
le16_to_cpu(dev->descriptor.idProduct));
if (quirk && quirk->ifnum >= 0 && ifnum != quirk->ifnum)
goto __err_val;
if (snd_usb_apply_boot_quirk(dev, intf, quirk) < 0)
goto __err_val;
/*
* found a config. now register to ALSA
*/
/* check whether it's already registered */
chip = NULL;
mutex_lock(®ister_mutex);
for (i = 0; i < SNDRV_CARDS; i++) {
if (usb_chip[i] && usb_chip[i]->dev == dev) {
if (usb_chip[i]->shutdown) {
snd_printk(KERN_ERR "USB device is in the shutdown state, cannot create a card instance\n");
goto __error;
}
chip = usb_chip[i];
chip->probing = 1;
break;
}
}
if (! chip) {
/* it's a fresh one.
* now look for an empty slot and create a new card instance
*/
for (i = 0; i < SNDRV_CARDS; i++)
if (enable[i] && ! usb_chip[i] &&
(vid[i] == -1 || vid[i] == USB_ID_VENDOR(id)) &&
(pid[i] == -1 || pid[i] == USB_ID_PRODUCT(id))) {
if (snd_usb_audio_create(dev, i, quirk, &chip) < 0) {
goto __error;
}
snd_card_set_dev(chip->card, &intf->dev);
chip->pm_intf = intf;
break;
}
if (!chip) {
printk(KERN_ERR "no available usb audio device\n");
goto __error;
}
}
/*
* For devices with more than one control interface, we assume the
* first contains the audio controls. We might need a more specific
* check here in the future.
*/
if (!chip->ctrl_intf)
chip->ctrl_intf = alts;
chip->txfr_quirk = 0;
err = 1; /* continue */
if (quirk && quirk->ifnum != QUIRK_NO_INTERFACE) {
/* need some special handlings */
if ((err = snd_usb_create_quirk(chip, intf, &usb_audio_driver, quirk)) < 0)
goto __error;
}
if (err > 0) {
/* create normal USB audio interfaces */
if (snd_usb_create_streams(chip, ifnum) < 0 ||
snd_usb_create_mixer(chip, ifnum, ignore_ctl_error) < 0) {
goto __error;
}
}
/* we are allowed to call snd_card_register() many times */
if (snd_card_register(chip->card) < 0) {
goto __error;
}
usb_chip[chip->index] = chip;
chip->num_interfaces++;
chip->probing = 0;
mutex_unlock(®ister_mutex);
return chip;
__error:
if (chip) {
if (!chip->num_interfaces)
snd_card_free(chip->card);
chip->probing = 0;
}
mutex_unlock(®ister_mutex);
__err_val:
return NULL;
}probe依次调用了：
snd_usb_apply_boot_quirk
snd_usb_audio_create
snd_usb_create_quirk
snd_usb_create_streams
snd_usb_create_stream
snd_usb_parse_audio_interface 这里根据usb的信息解析成pcm参数（如声道数量，采样率等等）
snd_usb_create_mixer
snd_card_register// linux-3.4.y/sound/core/init.c
snd_card_register调用了init_info_for_card以及snd_device_register_all。打开DEBUG后
<6>[ 58.704000] usb 1-1.1: new full-speed USB device number 5 using nxp-ehci
<6>[ 58.804000] usb 1-1.1: New USB device found, idVendor=0d8c, idProduct=0132
<6>[ 58.812000] usb 1-1.1: New USB device strings: Mfr=1, Product=2, SerialNumber=0
<6>[ 58.820000] usb 1-1.1: Product: USB PnP Audio Device
<6>[ 58.824000] usb 1-1.1: Manufacturer: C-Media Electronics Inc.
<6>[ 58.832000] ALSA sound/usb/stream.c:438 5:1:1: add audio endpoint 0x82
<6>[ 58.840000] ALSA sound/usb/stream.c:438 5:1:2: add audio endpoint 0x82
<6>[ 58.848000] ALSA sound/usb/mixer.c:1214 [10] FU [Mic Capture Switch] ch = 1, val = 0/1/1
<3>[ 58.856000] ALSA sound/usb/mixer.c:868 10:0: cannot get min/max values for control 2 (id 10)
<6>[ 58.864000] ALSA sound/usb/mixer.c:1214 [10] FU [Mic Capture Volume] ch = 2, val = 0/1/1
<3>[ 58.876000] ALSA sound/usb/mixer.c:868 10:0: cannot get min/max values for control 2 (id 10)
<6>[ 58.884000] input: C-Media Electronics Inc. USB PnP Audio Device as /devices/platform/nxp-ehci/usb1/1-1/1-1.1/1-1.1:1.2/input/input6
<6>[ 58.896000] generic-usb 0003:0D8C:0132.0003: input,hidraw0: USB HID v1.11 Device [C-Media Electronics Inc. USB PnP Audio Device] on usb-nxp-ehci-1.1/input2Bus 003 Device 019: ID 0d8c:0132 C-Media Electronics, Inc.
Device Descriptor:
bLength 18
bDescriptorType 1
bcdUSB 1.10
bDeviceClass 0 (Defined at Interface level)
bDeviceSubClass 0
bDeviceProtocol 0
bMaxPacketSize0 16
idVendor 0x0d8c C-Media Electronics, Inc.
idProduct 0x0132
bcdDevice 1.00
iManufacturer 1 C-Media Electronics Inc.
iProduct 2 USB PnP Audio Device
iSerial 0
bNumConfigurations 1
Configuration Descriptor:
bLength 9
bDescriptorType 2
wTotalLength 221
bNumInterfaces 3
bConfigurationValue 1
iConfiguration 0
bmAttributes 0x80
(Bus Powered)
MaxPower 100mA
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 0
bAlternateSetting 0
bNumEndpoints 0
bInterfaceClass 1 Audio
bInterfaceSubClass 1 Control Device
bInterfaceProtocol 0
iInterface 0
AudioControl Interface Descriptor:
bLength 9
bDescriptorType 36
bDescriptorSubtype 1 (HEADER)
bcdADC 1.00
wTotalLength 47
bInCollection 1
baInterfaceNr( 0) 1
AudioControl Interface Descriptor:
bLength 12
bDescriptorType 36
bDescriptorSubtype 2 (INPUT_TERMINAL)
bTerminalID 2
wTerminalType 0x0201 Microphone
bAssocTerminal 0
bNrChannels 2
wChannelConfig 0x0003
Left Front (L)
Right Front (R)
iChannelNames 0
iTerminal 0
AudioControl Interface Descriptor:
bLength 9
bDescriptorType 36
bDescriptorSubtype 3 (OUTPUT_TERMINAL)
bTerminalID 7
wTerminalType 0x0101 USB Streaming
bAssocTerminal 0
bSourceID 8
iTerminal 0
AudioControl Interface Descriptor:
bLength 7
bDescriptorType 36
bDescriptorSubtype 5 (SELECTOR_UNIT)
bUnitID 8
bNrInPins 1
baSource( 0) 10
iSelector 0
AudioControl Interface Descriptor:
bLength 10
bDescriptorType 36
bDescriptorSubtype 6 (FEATURE_UNIT)
bUnitID 10
bSourceID 2
bControlSize 1
bmaControls( 0) 0x01
Mute Control
bmaControls( 1) 0x02
Volume Control
bmaControls( 2) 0x02
Volume Control
iFeature 0
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 1
bAlternateSetting 0
bNumEndpoints 0
bInterfaceClass 1 Audio
bInterfaceSubClass 2 Streaming
bInterfaceProtocol 0
iInterface 0
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 1
bAlternateSetting 1
bNumEndpoints 1
bInterfaceClass 1 Audio
bInterfaceSubClass 2 Streaming
bInterfaceProtocol 0
iInterface 0
AudioStreaming Interface Descriptor:
bLength 7
bDescriptorType 36
bDescriptorSubtype 1 (AS_GENERAL)
bTerminalLink 7
bDelay 1 frames
wFormatTag 1 PCM
AudioStreaming Interface Descriptor:
bLength 29
bDescriptorType 36
bDescriptorSubtype 2 (FORMAT_TYPE)
bFormatType 1 (FORMAT_TYPE_I)
bNrChannels 1
bSubframeSize 2
bBitResolution 16
bSamFreqType 7 Discrete
tSamFreq[ 0] 8000
tSamFreq[ 1] 11025
tSamFreq[ 2] 16000
tSamFreq[ 3] 22050
tSamFreq[ 4] 32000
tSamFreq[ 5] 44100
tSamFreq[ 6] 48000
Endpoint Descriptor:
bLength 9
bDescriptorType 5
bEndpointAddress 0x82 EP 2 IN
bmAttributes 5
Transfer Type Isochronous
Synch Type Asynchronous
Usage Type Data
wMaxPacketSize 0x0064 1x 100 bytes
bInterval 1
bRefresh 0
bSynchAddress 0
AudioControl Endpoint Descriptor:
bLength 7
bDescriptorType 37
bDescriptorSubtype 1 (EP_GENERAL)
bmAttributes 0x01
Sampling Frequency
bLockDelayUnits 0 Undefined
wLockDelay 0 Undefined
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 1
bAlternateSetting 2
bNumEndpoints 1
bInterfaceClass 1 Audio
bInterfaceSubClass 2 Streaming
bInterfaceProtocol 0
iInterface 0
AudioStreaming Interface Descriptor:
bLength 7
bDescriptorType 36
bDescriptorSubtype 1 (AS_GENERAL)
bTerminalLink 7
bDelay 1 frames
wFormatTag 1 PCM
AudioStreaming Interface Descriptor:
bLength 29
bDescriptorType 36
bDescriptorSubtype 2 (FORMAT_TYPE)
bFormatType 1 (FORMAT_TYPE_I)
bNrChannels 2
bSubframeSize 2
bBitResolution 16
bSamFreqType 7 Discrete
tSamFreq[ 0] 8000
tSamFreq[ 1] 11025
tSamFreq[ 2] 16000
tSamFreq[ 3] 22050
tSamFreq[ 4] 32000
tSamFreq[ 5] 44100
tSamFreq[ 6] 48000
Endpoint Descriptor:
bLength 9
bDescriptorType 5
bEndpointAddress 0x82 EP 2 IN
bmAttributes 5
Transfer Type Isochronous
Synch Type Asynchronous
Usage Type Data
wMaxPacketSize 0x00c8 1x 200 bytes
bInterval 1
bRefresh 0
bSynchAddress 0
AudioControl Endpoint Descriptor:
bLength 7
bDescriptorType 37
bDescriptorSubtype 1 (EP_GENERAL)
bmAttributes 0x01
Sampling Frequency
bLockDelayUnits 0 Undefined
wLockDelay 0 Undefined
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 2
bAlternateSetting 0
bNumEndpoints 1
bInterfaceClass 3 Human Interface Device
bInterfaceSubClass 0 No Subclass
bInterfaceProtocol 0 None
iInterface 0
HID Device Descriptor:
bLength 9
bDescriptorType 33
bcdHID 1.11
bCountryCode 0 Not supported
bNumDescriptors 1
bDescriptorType 34 Report
wDescriptorLength 26
Report Descriptors:
** UNAVAILABLE **
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x87 EP 7 IN
bmAttributes 3
Transfer Type Interrupt
Synch Type None
Usage Type Data
wMaxPacketSize 0x0010 1x 16 bytes
bInterval 1
Device Status: 0x0000
(Bus Powered)总结：
UAC设备的参数是通过USB描述符确定的。比如声道是bNrChannels，位深是bBitResolution，采样率是bSamFreqType。截取其中一段说明：
bNrChannels 1
bSubframeSize 2
bBitResolution 16
bSamFreqType 7 Discrete
tSamFreq[ 0] 8000
tSamFreq[ 1] 11025
tSamFreq[ 2] 16000
tSamFreq[ 3] 22050
tSamFreq[ 4] 32000
tSamFreq[ 5] 44100
tSamFreq[ 6] 48000声道1，位深16，波特率8000~48000。