关于ALSA的小结

这一年来的一个重要工作是音频设备驱动方面，如驱动开发、调试debug、性能调优、音频路由、回声消除等。除此之外，熟悉了一下alsa-lib接口，Android音频系统，包括设备抽象层和音频策略。

遗憾的是一直没有什么机会接触蓝牙设备的集成和开发，可预知的未来无线设备会极大地普及。还有就是ffmpeg，以前经常用它来做一些兴致突来能力而及的小工具，年初时计划进一步在它的基础上学习H264或AVS的解码，但至今都未开始。问题一方面是我自身的懒惰，另一方面是公司不允许装虚拟机（PC装的是windows，毕竟办公都必须在windows环境下，开发登陆到Linux服务器）。试过在cygwin上面建立ffmpeg开发环境，以失败告终，ffplay无论如何都不能编译。

而值得自豪的是，alsa-driver算是消化得差不多了。sound/core目录里面的几大主体：control、hwdep和pcm，sound/soc目录的soc-core和soc-dapm都基本理顺了。还有info、jack、timer、rawmidi、usb-audio、ac97-bus，有空再研究。

关于control、hwdep、dapm、codec，写了不少文章记录我的学习成果，均在ALSA专栏。Card和pcm方面，由于内容太庞大，未整理出较完善的文档，暂借用前辈的经验，如下：

Linux音频驱动之二：声卡的创建：http://blog.csdn.net/droidphone/article/details/6289712

Linux音频驱动之三：PCM设备的创建：http://blog.csdn.net/droidphone/article/details/6308006

alsa驱动分析之一：http://blog.csdn.net/wylhistory/article/details/5114217

alsa驱动分析之二：http://blog.csdn.net/wylhistory/article/details/5114230

然后缪谈一些个人对于alsa开发的心得吧。

1、 清楚音频相关概念和原理，如采样率、采样精度、peroid、ADC/DAC、DAI、pops等等；明白音频数据流向，进而明白codec、i2s、dma各起到什么作用。

2、多看内核文档Documentation/sound/alsa，这些文档起到概述作用，配合源码可以让人更清晰音频概念和运作机理。

3、alsa-utils的几个工具，多用aplay和amixer进行调试。

4、熟悉alsa配置脚本asound.rc的写法：http://www.alsa-project.org/alsa-doc/alsa-lib/pcm_plugins.html，有时间可以练习写一个类似aplay的播放器工具。

其实这些都是老生常谈了，但很实用。

另外今天在网上找到一个wolfson音频处理芯片的PPT，写得很好，将音频开发的大致流程和关键点都写出来了。

Linux Audio for Android： http://download.csdn.net/detail/sepnic/3783997

hwdep模块简述

字符设备驱动中，ioctl是一个很常见的IO设备操作函数，可以自定义cmd命令字并实现对应的设备IO控制。

音频设备的控制有所不同：驱动层大部分控制操作定义各种snd_kcontrol_new，然后注册到SNDRV_DEV_CONTROL模块中（sound\core\control.c），详见snd_kcontrol探究；而上层调用alsa-lib的snd_ctl_open/snd_mixer_open来打开底层的SNDRV_DEV_CONTROL模块，详见DAPM之二：audio paths与dapm kcontrol。这方法常见于mixer-control，如音量调整、部件开关、通路连接等等。

除此之外，alsa还是可以实现类似于ioctl的函数的，只不过它封装成一个设备模块SNDRV_DEV_HWDEP，代码sound\core\ hwdep.c。该模块实现了read/write/ioctl/llseek/poll/mmap等接口。hwdep是Hardware Dependant Interface的简称。

题外话：如果想看自己板上的alsa有什么类型的设备可以cat /proc/asound/devices，如

~ # cat /proc/asound/devices
  0: [ 0]   : control
  4: [ 0- 0]: hardware dependent
 16: [ 0- 0]: digital audio playback
 24: [ 0- 0]: digital audio capture
 33:        : timer

~ #
 cat /proc/asound/devices
  0: [ 0]   : control
  4: [ 0- 0]: hardware dependent
 16: [ 0- 0]: digital audio playback
 24: [ 0- 0]: digital audio capture
 33:        : timer

~ # cat /proc/asound/devices 0: [ 0] : control 4: [ 0- 0]: hardware dependent 16: [ 0- 0]: digital audio playback 24: [ 0- 0]: digital audio capture 33: : timer设备节点号minor=0是control，=4是hwdep，=16是pcm- playback，=24是pcm-capture，=33是timer。

如下简单分析ioctl：

// 套接字接口函数集
static const struct file_operations snd_hwdep_f_ops =
{
    .owner =    THIS_MODULE,
    .llseek =   snd_hwdep_llseek,
    .read =     snd_hwdep_read,
    .write =    snd_hwdep_write,
    .open =     snd_hwdep_open,
    .release =  snd_hwdep_release,
    .poll =     snd_hwdep_poll,
    .unlocked_ioctl =   snd_hwdep_ioctl,
    .compat_ioctl = snd_hwdep_ioctl_compat,
    .mmap =     snd_hwdep_mmap,
};

static long snd_hwdep_ioctl(struct file * file, unsigned int cmd,
                unsigned long arg)
{
    struct snd_hwdep *hw = file->private_data;
    void __user *argp = (void __user *)arg;
    switch (cmd) {
    case SNDRV_HWDEP_IOCTL_PVERSION:
        return put_user(SNDRV_HWDEP_VERSION, (int __user *)argp);
    case SNDRV_HWDEP_IOCTL_INFO:
        return snd_hwdep_info(hw, argp);
    case SNDRV_HWDEP_IOCTL_DSP_STATUS:
        return snd_hwdep_dsp_status(hw, argp);
    case SNDRV_HWDEP_IOCTL_DSP_LOAD:
        return snd_hwdep_dsp_load(hw, argp);
    }
    if (hw->ops.ioctl)
        return hw->ops.ioctl(hw, file, cmd, arg);
    return -ENOTTY;
}

//套接字接口函数集
static const struct file_operations snd_hwdep_f_ops =
{
    .owner =    THIS_MODULE,
    .llseek =   snd_hwdep_llseek,
    .read =     snd_hwdep_read,
    .write =    snd_hwdep_write,
    .open =     snd_hwdep_open,
    .release =  snd_hwdep_release,
    .poll =     snd_hwdep_poll,
    .unlocked_ioctl =   snd_hwdep_ioctl,
    .compat_ioctl = snd_hwdep_ioctl_compat,
    .mmap =     snd_hwdep_mmap,
};

static long snd_hwdep_ioctl(struct file * file, unsigned int cmd,
                unsigned long arg)
{
    struct snd_hwdep *hw = file->private_data;
    void __user *argp = (void __user *)arg;
    switch (cmd) {
    case SNDRV_HWDEP_IOCTL_PVERSION:
        return put_user(SNDRV_HWDEP_VERSION, (int __user *)argp);
    case SNDRV_HWDEP_IOCTL_INFO:
        return snd_hwdep_info(hw, argp);
    case SNDRV_HWDEP_IOCTL_DSP_STATUS:
        return snd_hwdep_dsp_status(hw, argp);
    case SNDRV_HWDEP_IOCTL_DSP_LOAD:
        return snd_hwdep_dsp_load(hw, argp);
    }
    if (hw->ops.ioctl)
        return hw->ops.ioctl(hw, file, cmd, arg);
    return -ENOTTY;
}

//套接字接口函数集 static const struct file_operations snd_hwdep_f_ops = { .owner = THIS_MODULE, .llseek = snd_hwdep_llseek, .read = snd_hwdep_read, .write = snd_hwdep_write, .open = snd_hwdep_open, .release = snd_hwdep_release, .poll = snd_hwdep_poll, .unlocked_ioctl = snd_hwdep_ioctl, .compat_ioctl = snd_hwdep_ioctl_compat, .mmap = snd_hwdep_mmap, }; static long snd_hwdep_ioctl(struct file * file, unsigned int cmd, unsigned long arg) { struct snd_hwdep *hw = file->private_data; void __user *argp = (void __user *)arg; switch (cmd) { case SNDRV_HWDEP_IOCTL_PVERSION: return put_user(SNDRV_HWDEP_VERSION, (int __user *)argp); case SNDRV_HWDEP_IOCTL_INFO: return snd_hwdep_info(hw, argp); case SNDRV_HWDEP_IOCTL_DSP_STATUS: return snd_hwdep_dsp_status(hw, argp); case SNDRV_HWDEP_IOCTL_DSP_LOAD: return snd_hwdep_dsp_load(hw, argp); } if (hw->ops.ioctl) return hw->ops.ioctl(hw, file, cmd, arg); return -ENOTTY; }从snd_hwdep_ioctl可以看出，系统默认只有4个cmd，功能主要是download dsp image。从return hw->ops.ioctl(hw, file, cmd, arg)语句可以看出，我们可自定义cmd和ioctl函数。

实现自定义的hwdep操作函数

1、 首先实现需要的操作函数：

static int my_hwdep_open(struct snd_hwdep * hw, struct file *file)
{
    printk(KERN_INFO "my_hwdep_open\n");
    return 0;
}

static int my_hwdep_ioctl(struct snd_hwdep * hw, struct file *file, unsigned int cmd, unsigned long arg)
{
#define MY_SOC_IOCTL_SET_CALL_PATH   _IOWR('H', 0x10, int)

    switch (cmd) {
    case MY_SOC_IOCTL_SET_CALL_PATH:
        //设置电话语音通路
        return 0;
        break;
    //......
    }

    err("Not supported ioctl for MY-HWDEP");
    return -ENOIOCTLCMD;
}

static int
my_hwdep_open(struct snd_hwdep * hw, struct file *file)
{
    printk(KERN_INFO "my_hwdep_open\n");
    return 0;
}

static int my_hwdep_ioctl(struct snd_hwdep * hw, struct file *file,
unsigned int cmd, unsigned long arg)
{
#define MY_SOC_IOCTL_SET_CALL_PATH   _IOWR('H', 0x10, int)

    switch (cmd) {
    case MY_SOC_IOCTL_SET_CALL_PATH:
        //设置电话语音通路
        return 0;
        break;
    //......
    }

    err("Not supported ioctl for MY-HWDEP");
    return -ENOIOCTLCMD;
}

static int my_hwdep_open(struct snd_hwdep * hw, struct file *file) { printk(KERN_INFO "my_hwdep_open\n"); return 0; } static int my_hwdep_ioctl(struct snd_hwdep * hw, struct file *file, unsigned int cmd, unsigned long arg) { #define MY_SOC_IOCTL_SET_CALL_PATH _IOWR('H', 0x10, int) switch (cmd) { case MY_SOC_IOCTL_SET_CALL_PATH: //设置电话语音通路 return 0; break; //...... } err("Not supported ioctl for MY-HWDEP"); return -ENOIOCTLCMD; }
2、 注册操作函数到hwdep模块：

struct snd_hwdep *hwdep;

if (snd_hwdep_new(codec->card, "MY-HWDEP", 0, &hwdep) < 0) {
    printk(KERN_ERR "create MY-HWDEP fail");
    return;
}

sprintf(hwdep->name, "MY-HWDEP %d", 0);

hwdep->iface = SNDRV_HWDEP_IFACE_WMT;
hwdep->ops.open = wmt_hwdep_open;
hwdep->ops.ioctl = wmt_hwdep_ioctl;

struct
 snd_hwdep *hwdep;

if (snd_hwdep_new(codec->card, "MY-HWDEP", 0, &hwdep) < 0) {
    printk(KERN_ERR "create MY-HWDEP fail");
    return;
}

sprintf(hwdep->name, "MY-HWDEP %d", 0);

hwdep->iface = SNDRV_HWDEP_IFACE_WMT;
hwdep->ops.open = wmt_hwdep_open;
hwdep->ops.ioctl = wmt_hwdep_ioctl;

struct snd_hwdep *hwdep; if (snd_hwdep_new(codec->card, "MY-HWDEP", 0, &hwdep) < 0) { printk(KERN_ERR "create MY-HWDEP fail"); return; } sprintf(hwdep->name, "MY-HWDEP %d", 0); hwdep->iface = SNDRV_HWDEP_IFACE_WMT; hwdep->ops.open = wmt_hwdep_open; hwdep->ops.ioctl = wmt_hwdep_ioctl;这里摘录snd_hwdep_new的代码注释，让大家更明白上面的注册过程：

/**
 * snd_hwdep_new - create a new hwdep instance
 * @card: the card instance
 * @id: the id string
 * @device: the device index (zero-based)
 * @rhwdep: the pointer to store the new hwdep instance
 *
 * Creates a new hwdep instance with the given index on the card.
 * The callbacks (hwdep->ops) must be set on the returned instance
 * after this call manually by the caller.
 *
 * Returns zero if successful, or a negative error code on failure.
 */

/**
 * snd_hwdep_new - create a new hwdep instance
 * @card: the card instance
 * @id: the id string
 * @device: the device index (zero-based)
 * @rhwdep: the pointer to store the new hwdep instance
 *
 * Creates a new hwdep instance with the given index on the card.
 * The callbacks (hwdep->ops) must be set on the returned instance
 * after this call manually by the caller.
 *
 * Returns zero if successful, or a negative error code on failure.
 */

/** * snd_hwdep_new - create a new hwdep instance * @card: the card instance * @id: the id string * @device: the device index (zero-based) * @rhwdep: the pointer to store the new hwdep instance * * Creates a new hwdep instance with the given index on the card. * The callbacks (hwdep->ops) must be set on the returned instance * after this call manually by the caller. * * Returns zero if successful, or a negative error code on failure. */按照以上实现hwdep ioctl后，上层可以通过alsa-lib的相关接口来调用。

上层调用范例

#include <fcntl.h>
#include <sys/ioctl.h>
#include <alsa/hwdep.h>
#include <alsa/error.h>
#include <stdio.h>

#define MY_SOC_IOCTL_SET_CALL_PATH   _IOWR('H', 0x10, int)

int main()
{
    const char *devicename = "hw:0,0";
    snd_hwdep_t *hwdep;
    int err;
    int enable = 1;

    if ((err = snd_hwdep_open(&hwdep, devicename, O_RDWR)) < 0) {
        printf("hwdep interface open error: %s \n", snd_strerror(err));
        return -1;
    }

    if ((err = snd_hwdep_ioctl(hwdep, MY_SOC_IOCTL_SET_CALL_PATH, &enable)) < 0) {
        printf("hwdep ioctl error: %s \n", snd_strerror(err));
    }

    snd_hwdep_close(hwdep);

    return 0;
}

#include
<fcntl.h>
#include <sys/ioctl.h>
#include <alsa/hwdep.h>
#include <alsa/error.h>
#include <stdio.h>

#define MY_SOC_IOCTL_SET_CALL_PATH   _IOWR('H', 0x10, int)

int main()
{
    const char *devicename = "hw:0,0";
    snd_hwdep_t *hwdep;
    int err;
    int enable = 1;

    if ((err = snd_hwdep_open(&hwdep, devicename, O_RDWR)) < 0) {
        printf("hwdep interface open error: %s \n", snd_strerror(err));
        return -1;
    }

    if ((err = snd_hwdep_ioctl(hwdep, MY_SOC_IOCTL_SET_CALL_PATH,
&enable)) < 0) {
        printf("hwdep ioctl error: %s \n", snd_strerror(err));
    }

    snd_hwdep_close(hwdep);

    return 0;
}

#include <fcntl.h> #include <sys/ioctl.h> #include <alsa/hwdep.h> #include <alsa/error.h> #include <stdio.h> #define MY_SOC_IOCTL_SET_CALL_PATH _IOWR('H', 0x10, int) int main() { const char *devicename = "hw:0,0"; snd_hwdep_t *hwdep; int err; int enable = 1; if ((err = snd_hwdep_open(&hwdep, devicename, O_RDWR)) < 0) { printf("hwdep interface open error: %s \n", snd_strerror(err)); return -1; } if ((err = snd_hwdep_ioctl(hwdep, MY_SOC_IOCTL_SET_CALL_PATH, &enable)) < 0) { printf("hwdep ioctl error: %s \n", snd_strerror(err)); } snd_hwdep_close(hwdep); return 0; }

总结

可以看出hwdep的本意主要是用于download dsp image，但通过它也可实现类似于其他字符设备的ioctl。我说过音频大多控制是通过snd_kcontrol，但有些功能如果使用这种方式会比较繁琐且模块太过耦合。

举个例子：电话语音通路，它不同于音乐回放通路，通话时才需要打开。如果用snd_kcontrol，则上层需要调用多个control.set，并且更换CODEC芯片的话，上层也要跟着修改control name；如果使用hwdep ioctl的话，就没有这个问题，只需要保证命令字cmd一致，底层如何管理通话通路的一系列部件开关，上层都不需要关心。