ALSA代码分析

android 下的声卡设备节点

ALSA由以下文件提供了接口规范，ASOC框架依赖ALSA框架，创建soundcard 设备

1、sound/core/sound.c //实现了snd_fops 它起中转作用

2、sound/core/control.c //实现了snd_ctl_f_ops 控制接口

3、sound/core/pcm_native.c //实现了snd_pcm_f_ops pcm中的 playback capture接口

alsa框架属于分层结构，如下图

--------------------------struct file_operations snd_fops（sound.c）----------------------------------
|
control （control.c snd_ctl_f_ops） device.(pcm c0d0p pcm c0d0c)（pcm_naive.c &snd_pcm_f_ops）.......file_operations
-----------------------------------------------------------------------------------------------------

硬件相关代码

-----------------------------------------------------------------------------------------------------

上层定义了一套file_operations供用户调用，其实只是起一个中转作用。

1、sound/core/sound.c //实现了snd_fops 它起中转作用

static int __init alsa_sound_init(void)
{
	snd_major = major;
	snd_ecards_limit = cards_limit;
	if (register_chrdev(major, "alsa", &snd_fops)) {
		snd_printk(KERN_ERR "unable to register native major device number %d\n", major);
		return -EIO;
	}
	if (snd_info_init() < 0) {
		unregister_chrdev(major, "alsa");
		return -ENOMEM;
	}
	snd_info_minor_register();
#ifndef MODULE
	printk(KERN_INFO "Advanced Linux Sound Architecture Driver Initialized.\n");
#endif
	return 0;
}

static const struct file_operations snd_fops =
{
	.owner =	THIS_MODULE,
	.open =		snd_open,
	.llseek =	noop_llseek,
};

上面代码初始化的时候向内核注册了一个alsa的设备节点，对应ops为snd_fops，内部只有一个open函数，user空间的用户调用时，将会通过次设备号，转到对应的设备的open函数。

2、sound/core/control.c //实现了snd_ctl_f_ops 控制接口

在sound/core/init.c 中，snd_ctl_create函数创建control节点，

调用snd_ctl_dev_register->snd_ctl_f_ops(sound/core/contrlo.c)

int snd_card_create(int idx, const char *xid,
		    struct module *module, int extra_size,
		    struct snd_card **card_ret)
{
    ........
	err = snd_ctl_create(card);  /*此处创建control节点*/
    ........
	
}


/*
 * create control core:
 * called from init.c
 */
int snd_ctl_create(struct snd_card *card)
{
	static struct snd_device_ops ops = {
		.dev_free = snd_ctl_dev_free,
		.dev_register =	snd_ctl_dev_register,/*当snd_device_new调用后会回调register函数*/
		.dev_disconnect = snd_ctl_dev_disconnect,
	};

	if (snd_BUG_ON(!card))
		return -ENXIO;
	return snd_device_new(card, SNDRV_DEV_CONTROL, card, &ops);
}


/*
 * registration of the control device
 */
static int snd_ctl_dev_register(struct snd_device *device)
{
	.......

	if ((err = snd_register_device(SNDRV_DEVICE_TYPE_CONTROL, card, -1,
				       &snd_ctl_f_ops, card, name)) < 0)
		return err;
	return 0;
}


/*
 *  INIT PART
 */

static const struct file_operations snd_ctl_f_ops =
{
	.owner =	THIS_MODULE,
	.read =		snd_ctl_read,
	.open =		snd_ctl_open,
	.release =	snd_ctl_release,
	.llseek =	no_llseek,
	.poll =		snd_ctl_poll,
	.unlocked_ioctl =	snd_ctl_ioctl,
	.compat_ioctl =	snd_ctl_ioctl_compat,
	.fasync =	snd_ctl_fasync,
};

3、sound/core/pcm_native.c //实现了snd_pcm_f_ops pcm中的 playback capture接口

snd_pcm_new
snd_pcm_dev_register

snd_pcm_f_ops


/*
 *  Register section
 */

const struct file_operations snd_pcm_f_ops[2] = {
	{
		.owner =		THIS_MODULE,
		.write =		snd_pcm_write,
		.aio_write =		snd_pcm_aio_write,
		.open =			snd_pcm_playback_open,
		.release =		snd_pcm_release,
		.llseek =		no_llseek,
		.poll =			snd_pcm_playback_poll,
		.unlocked_ioctl =	snd_pcm_playback_ioctl,
		.compat_ioctl = 	snd_pcm_ioctl_compat,
		.mmap =			snd_pcm_mmap,
		.fasync =		snd_pcm_fasync,
		.get_unmapped_area =	snd_pcm_get_unmapped_area,
	},
	{
		.owner =		THIS_MODULE,
		.read =			snd_pcm_read,
		.aio_read =		snd_pcm_aio_read,
		.open =			snd_pcm_capture_open,
		.release =		snd_pcm_release,
		.llseek =		no_llseek,
		.poll =			snd_pcm_capture_poll,
		.unlocked_ioctl =	snd_pcm_capture_ioctl,
		.compat_ioctl = 	snd_pcm_ioctl_compat,
		.mmap =			snd_pcm_mmap,
		.fasync =		snd_pcm_fasync,
		.get_unmapped_area =	snd_pcm_get_unmapped_area,
	}
};

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