Linux内核4.14版本——alsa框架分析(2)-sound.c分析（应用层如何调用到内核层的）

1.   alsa_sound_init

1.1 snd_minors

1.2 snd_register_device

2 以pcm和clt的调用关系来分析如何注册snd_minors

2.1 pcm注册snd_minors

2.1.1 snd_pcm_new

2.1.2 _snd_pcm_new

2.1.3 snd_pcm_dev_register

2.2 clt注册snd_minors

2.2.1 snd_ctl_create

2.2.2 snd_ctl_dev_register

3. sound成功编入内核后的现象

3.1 查看cat /proc/devices

3.2 ls /dev/snd/ -l  

---

1. alsa_sound_init

alsa的核心入口函数是sound\core\sound.c中的alsa_sound_init

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

static void __exit alsa_sound_exit(void)
{
	snd_info_done();
	unregister_chrdev(major, "alsa");
}

subsys_initcall(alsa_sound_init);
module_exit(alsa_sound_exit);

注册了一个主设备号为CONFIG_SND_MAJOR（116），名字为“alsa”的字符设备，其操作函数为snd_fops。

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

在应用程序中调用open，最终在内核中会调用snd_open。

static struct snd_minor *snd_minors[SNDRV_OS_MINORS];
static int snd_open(struct inode *inode, struct file *file)
{
	unsigned int minor = iminor(inode);
	struct snd_minor *mptr = NULL;
	const struct file_operations *new_fops;
	int err = 0;

	if (minor >= ARRAY_SIZE(snd_minors))
		return -ENODEV;
	mutex_lock(&sound_mutex);
	mptr = snd_minors[minor];
    .......
	new_fops = fops_get(mptr->f_ops);
    .....

	if (file->f_op->open)
		err = file->f_op->open(inode, file);
	return err;
}

1.1 snd_minors

最终会通过次设备号来匹配snd_minors，并且调用它的open函数。我们看看全局变量snd_minors在哪注册的，它是静态变量，所以在sound.c文件中搜索，发现其注册为snd_register_device。

/**
 * snd_register_device - Register the ALSA device file for the card
 * @type: the device type, SNDRV_DEVICE_TYPE_XXX
 * @card: the card instance
 * @dev: the device index
 * @f_ops: the file operations
 * @private_data: user pointer for f_ops->open()
 * @device: the device to register
 *
 * Registers an ALSA device file for the given card.
 * The operators have to be set in reg parameter.
 *
 * Return: Zero if successful, or a negative error code on failure.
 */
int snd_register_device(int type, struct snd_card *card, int dev,
			const struct file_operations *f_ops,
			void *private_data, struct device *device)
{
	int minor;
	int err = 0;
	struct snd_minor *preg;

	if (snd_BUG_ON(!device))
		return -EINVAL;

	preg = kmalloc(sizeof *preg, GFP_KERNEL);
	if (preg == NULL)
		return -ENOMEM;
	preg->type = type;
	preg->card = card ? card->number : -1;
	preg->device = dev;
	preg->f_ops = f_ops;
	preg->private_data = private_data;
	preg->card_ptr = card;
	mutex_lock(&sound_mutex);
	minor = snd_find_free_minor(type, card, dev);
	if (minor < 0) {
		err = minor;
		goto error;
	}

	preg->dev = device;
	device->devt = MKDEV(major, minor);
	err = device_add(device);
	if (err < 0)
		goto error;

	snd_minors[minor] = preg;
 error:
	mutex_unlock(&sound_mutex);
	if (err < 0)
		kfree(preg);
	return err;
}

1.2 snd_register_device

搜索snd_register_device，看谁调用了该函数，发现有如下函数调用了。

我们以红框中的两个函数分析他。

2 以pcm和clt的调用关系来分析如何注册snd_minors

2.1 pcm注册snd_minors

snd_pcm_new->_snd_pcm_new->snd_pcm_dev_register->snd_register_device

2.1.1 snd_pcm_new

/**
 * snd_pcm_new - create a new PCM instance
 * @card: the card instance
 * @id: the id string
 * @device: the device index (zero based)
 * @playback_count: the number of substreams for playback
 * @capture_count: the number of substreams for capture
 * @rpcm: the pointer to store the new pcm instance
 *
 * Creates a new PCM instance.
 *
 * The pcm operators have to be set afterwards to the new instance
 * via snd_pcm_set_ops().
 *
 * Return: Zero if successful, or a negative error code on failure.
 */
int snd_pcm_new(struct snd_card *card, const char *id, int device,
		int playback_count, int capture_count, struct snd_pcm **rpcm)
{
	return _snd_pcm_new(card, id, device, playback_count, capture_count,
			false, rpcm);
}

2.1.2 _snd_pcm_new

static int _snd_pcm_new(struct snd_card *card, const char *id, int device,
		int playback_count, int capture_count, bool internal,
		struct snd_pcm **rpcm)
{
	struct snd_pcm *pcm;
	int err;
	static struct snd_device_ops ops = {
		.dev_free = snd_pcm_dev_free,
		.dev_register =	snd_pcm_dev_register,
		.dev_disconnect = snd_pcm_dev_disconnect,
	};

........
	if (id)
		strlcpy(pcm->id, id, sizeof(pcm->id));

	err = snd_pcm_new_stream(pcm, SNDRV_PCM_STREAM_PLAYBACK,
				 playback_count);
	if (err < 0)
		goto free_pcm;

	err = snd_pcm_new_stream(pcm, SNDRV_PCM_STREAM_CAPTURE, capture_count);
	if (err < 0)
		goto free_pcm;

	err = snd_device_new(card, SNDRV_DEV_PCM, pcm, &ops);
	if (err < 0)
		goto free_pcm;

	if (rpcm)
		*rpcm = pcm;
	return 0;

free_pcm:
	snd_pcm_free(pcm);
	return err;
}

先创建playback和capture两个substream，最终创建pcm设备，ops最终为pcm的ops。

2.1.3 snd_pcm_dev_register

static int snd_pcm_dev_register(struct snd_device *device)
{
..............
	err = snd_pcm_add(pcm);
	if (err)
		goto unlock;
	for (cidx = 0; cidx < 2; cidx++) {
		int devtype = -1;
		if (pcm->streams[cidx].substream == NULL)
			continue;
		switch (cidx) {
		case SNDRV_PCM_STREAM_PLAYBACK:
			devtype = SNDRV_DEVICE_TYPE_PCM_PLAYBACK;
			break;
		case SNDRV_PCM_STREAM_CAPTURE:
			devtype = SNDRV_DEVICE_TYPE_PCM_CAPTURE;
			break;
		}
		/* register pcm */
		err = snd_register_device(devtype, pcm->card, pcm->device,
					  &snd_pcm_f_ops[cidx], pcm,
        .............
	}

	pcm_call_notify(pcm, n_register);

 unlock:
	mutex_unlock(®ister_mutex);
	return err;
}

这里注意snd_pcm_f_ops结构体，如下所示。sound\core\pcm_native.c

/*
 *  Register section
 */

const struct file_operations snd_pcm_f_ops[2] = {
	{
		.owner =		THIS_MODULE,
		.write =		snd_pcm_write,
		.write_iter =		snd_pcm_writev,
		.open =			snd_pcm_playback_open,
		.release =		snd_pcm_release,
		.llseek =		no_llseek,
		.poll =			snd_pcm_playback_poll,
		.unlocked_ioctl =	snd_pcm_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,
		.read_iter =		snd_pcm_readv,
		.open =			snd_pcm_capture_open,
		.release =		snd_pcm_release,
		.llseek =		no_llseek,
		.poll =			snd_pcm_capture_poll,
		.unlocked_ioctl =	snd_pcm_ioctl,
		.compat_ioctl = 	snd_pcm_ioctl_compat,
		.mmap =			snd_pcm_mmap,
		.fasync =		snd_pcm_fasync,
		.get_unmapped_area =	snd_pcm_get_unmapped_area,
	}
};

该操作函数集被注册在snd_minors结构体的操作函数集中，所以上层应用最终的open,compat_ioctl等函数，最终都会调用到这里。

int snd_register_device(int type, struct snd_card *card, int dev,
			const struct file_operations *f_ops,
			void *private_data, struct device *device)
{
    ..........
	preg->f_ops = f_ops;
	preg->private_data = private_data;
	preg->card_ptr = card;
	mutex_lock(&sound_mutex);
	minor = snd_find_free_minor(type, card, dev);
	if (minor < 0) {
		err = minor;
		goto error;
	}
    .............
	snd_minors[minor] = preg;
 error:
	mutex_unlock(&sound_mutex);
	if (err < 0)
		kfree(preg);
	return err;
}

2.2 clt注册snd_minors

snd_card_new->snd_ctl_create->snd_ctl_dev_register->snd_register_device

2.2.1 snd_ctl_create

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,
		.dev_disconnect = snd_ctl_dev_disconnect,
	};
	int err;

	if (snd_BUG_ON(!card))
		return -ENXIO;
	if (snd_BUG_ON(card->number < 0 || card->number >= SNDRV_CARDS))
		return -ENXIO;

	snd_device_initialize(&card->ctl_dev, card);
	dev_set_name(&card->ctl_dev, "controlC%d", card->number);

	err = snd_device_new(card, SNDRV_DEV_CONTROL, card, &ops);
	if (err < 0)
		put_device(&card->ctl_dev);
	return err;
}

最终会调用snd_device_new把control设备加入到snd_device链表里面，ops也为control设备的ops。

2.2.2 snd_ctl_dev_register

/*
 * registration of the control device
 */
static int snd_ctl_dev_register(struct snd_device *device)
{
	struct snd_card *card = device->device_data;

	return snd_register_device(SNDRV_DEVICE_TYPE_CONTROL, card, -1,
				   &snd_ctl_f_ops, card, &card->ctl_dev);
}

注册control设备，snd_ctl_f_ops为改设备的操作函数集。

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成功编入内核后的现象

3.1 查看cat /proc/devices

主设备号为116的，名称为alsa的设备。

3.2 ls /dev/snd/ -l

主设备号为116，次设备号为33，名称为timer的字符设备。