Linux Alsa声卡驱动（2）：代码分析

一：初始化/注册声卡设备

（1）注册ALSA

kernel\sound\core：sound.c

int __init alsa_sound_init(void)
{
    ... ...
	if (register_chrdev(major, "alsa", &snd_fops)) 
    ... ...
}

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

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;

	mptr = snd_minors[minor];
    ... ...
	new_fops = fops_get(mptr->f_ops);
    ... ...
}

static struct snd_minor *snd_minors[SNDRV_OS_MINORS];

struct snd_minor {
	int type;			/* SNDRV_DEVICE_TYPE_XXX */
	int card;			/* card number */
	int device;			/* device number */
	const struct file_operations *f_ops;	/* file operations */
	void *private_data;		/* private data for f_ops->open */
	struct device *dev;		/* device for sysfs */
	struct snd_card *card_ptr;	/* assigned card instance */
};

通过追踪代码可以发现声卡初始化中最终调用的file_operations对象是通过snd_minors[minor]结构成员获取。继续追踪snd_minors[minor]来源。

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;
}

可以看到在snd_register_device声卡注册接口中对snd_minor对象preg的成员分别进行了赋值（设备类型，设备号，file_operations等等）。最后将其赋值给需要用到的snd_minors[minor]。后续我们就需要用snd_register_device接口来分别对不同设备进行注册，最终会通过总线将它们匹配起来。

（2）ALSA接口信息

kernel\sound\core：sound.c

int __init alsa_sound_init(void)
{
    ... ...
	if (snd_info_init() < 0) { 
    ... ...
}


kernel\sound\core：info.c

int __init snd_info_init(void)
{
	snd_proc_root = snd_info_create_entry("asound", NULL);
	if (!snd_proc_root)
		return -ENOMEM;
	snd_proc_root->mode = S_IFDIR | 0555;
	snd_proc_root->p = proc_mkdir("asound", NULL);
	if (!snd_proc_root->p)
		goto error;
    ... ...
	if (snd_info_version_init() < 0 ||
	    snd_minor_info_init() < 0 ||
	    snd_minor_info_oss_init() < 0 ||
	    snd_card_info_init() < 0 ||
	    snd_info_minor_register() < 0)
		goto error;
	return 0;

 error:
	snd_info_free_entry(snd_proc_root);
	return -ENOMEM;
}

同样在声卡入口函数中对ALSA的接口信息进行了初始化，并通过“proc_mkdir”内核函数在根目录proc下创建了“asound”目录作为存储路径，见下图：

 card0/card7：其中0和7代表的是声卡号。其子目录包含了pcmp，pcmc

该节点是由snd_card_new接口调用创建，详细见后续对于snd_card_new的分析。

cards：列出系统中可用的，注册的声卡。

devices：列出系统card下所有注册的device，包括control，pcm，timer，seq等等。

pcm：系统的pcm设备，包括capture和playback。

timers:    描述一些ALSA相关的定时器信息。

version:  描述ALSA版本信息，详细可追踪接口snd_info_version_init

 

二：注册声卡功能部件

（1）control

kernel\sound\core：control.c

/*
 * 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);
}

/*
 * 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,
		.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;
}

可以看到ctl部件通过snd_ctl_create函数中的snd_device_ops成员对象间接调用snd_ctl_dev_register-->>snd_register_device接口来注册相应接口。

下面逐一分析snd_ctl_create的内容。

（1.1）snd_device_initialize

kernel\sound\core：init.c

/**
 * snd_device_initialize - Initialize struct device for sound devices
 * @dev: device to initialize
 * @card: card to assign, optional
 */
void snd_device_initialize(struct device *dev, struct snd_card *card)
{
	device_initialize(dev);
	if (card)
		dev->parent = &card->card_dev;
	dev->class = sound_class;
	dev->release = default_release;
}
EXPORT_SYMBOL_GPL(snd_device_initialize);

device_initialize(dev)接口用来准备后续用到的device数据，重点在于sound_class

kernel\sound：sound_core.c

struct class *sound_class;
EXPORT_SYMBOL(sound_class);

static char *sound_devnode(struct device *dev, umode_t *mode)
{
	if (MAJOR(dev->devt) == SOUND_MAJOR)
		return NULL;
	return kasprintf(GFP_KERNEL, "snd/%s", dev_name(dev));
}

static int __init init_soundcore(void)
{
	int rc;

	rc = init_oss_soundcore();
	if (rc)
		return rc;

	sound_class = class_create(THIS_MODULE, "sound");
	if (IS_ERR(sound_class)) {
		cleanup_oss_soundcore();
		return PTR_ERR(sound_class);
	}

	sound_class->devnode = sound_devnode;

	return 0;
}

subsys_initcall(init_soundcore);

分析sound_core.c文件可知最终会创建“snd/%s”在“/dev”目录下，“%s”为要注册的功能部件，后续会讲到。

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

kernel\drivers\base: core.c

/**
 * dev_set_name - set a device name
 * @dev: device
 * @fmt: format string for the device's name
 */
int dev_set_name(struct device *dev, const char *fmt, ...)
{
	va_list vargs;
	int err;

	va_start(vargs, fmt);
	err = kobject_set_name_vargs(&dev->kobj, fmt, vargs);
	va_end(vargs);
	return err;
}
EXPORT_SYMBOL_GPL(dev_set_name);

分析dev_set_name可了解到该函数用来设置声卡功能部件的名称“controlC%d”

（1.3）snd_device_new(card, SNDRV_DEV_CONTROL, card, &ops)

kernel\sound\core：device.c

/**
 * snd_device_new - create an ALSA device component
 * @card: the card instance
 * @type: the device type, SNDRV_DEV_XXX
 * @device_data: the data pointer of this device
 * @ops: the operator table
 *
 * Creates a new device component for the given data pointer.
 * The device will be assigned to the card and managed together
 * by the card.
 *
 * The data pointer plays a role as the identifier, too, so the
 * pointer address must be unique and unchanged.
 *
 * Return: Zero if successful, or a negative error code on failure.
 */
int snd_device_new(struct snd_card *card, enum snd_device_type type,
		   void *device_data, struct snd_device_ops *ops)
{
	struct snd_device *dev;
	struct list_head *p;

	if (snd_BUG_ON(!card || !device_data || !ops))
		return -ENXIO;
	dev = kzalloc(sizeof(*dev), GFP_KERNEL);
	if (!dev)
		return -ENOMEM;
	INIT_LIST_HEAD(&dev->list);
	dev->card = card;
	dev->type = type;
	dev->state = SNDRV_DEV_BUILD;
	dev->device_data = device_data;
	dev->ops = ops;

	/* insert the entry in an incrementally sorted list */
	list_for_each_prev(p, &card->devices) {
		struct snd_device *pdev = list_entry(p, struct snd_device, list);
		if ((unsigned int)pdev->type <= (unsigned int)type)
			break;
	}

	list_add(&dev->list, p);
	return 0;
}
EXPORT_SYMBOL(snd_device_new);

最终通过该接口完成声卡功能部件ctl节点的创建，如下图所示：

 继续追踪会发现snd_ctl_create接口是由snd_card_new调用，其中还涉及开篇讲到的card info的创建snd_info_card_create。

kernel\sound\core：init.c

/**
 *  snd_card_new - create and initialize a soundcard structure
 *  @parent: the parent device object
 *  @idx: card index (address) [0 ... (SNDRV_CARDS-1)]
 *  @xid: card identification (ASCII string)
 *  @module: top level module for locking
 *  @extra_size: allocate this extra size after the main soundcard structure
 *  @card_ret: the pointer to store the created card instance
 *
 *  Creates and initializes a soundcard structure.
 *
 *  The function allocates snd_card instance via kzalloc with the given
 *  space for the driver to use freely.  The allocated struct is stored
 *  in the given card_ret pointer.
 *
 *  Return: Zero if successful or a negative error code.
 */
int snd_card_new(struct device *parent, int idx, const char *xid,
		    struct module *module, int extra_size,
		    struct snd_card **card_ret)
{
	struct snd_card *card;
    ... ...

	device_initialize(&card->card_dev);
	card->card_dev.parent = parent;
	card->card_dev.class = sound_class;
	card->card_dev.release = release_card_device;
	card->card_dev.groups = card->dev_groups;
	card->dev_groups[0] = &card_dev_attr_group;
	err = kobject_set_name(&card->card_dev.kobj, "card%d", idx);
	if (err < 0)
		goto __error;

	snprintf(card->irq_descr, sizeof(card->irq_descr), "%s:%s",
		 dev_driver_string(card->dev), dev_name(&card->card_dev));

	/* the control interface cannot be accessed from the user space until */
	/* snd_cards_bitmask and snd_cards are set with snd_card_register */
	err = snd_ctl_create(card);
	if (err < 0) {
		dev_err(parent, "unable to register control minors\n");
		goto __error;
	}
	err = snd_info_card_create(card);
	if (err < 0) {
		dev_err(parent, "unable to create card info\n");
		goto __error_ctl;
	}
	*card_ret = card;
	return 0;

      __error_ctl:
	snd_device_free_all(card);
      __error:
	put_device(&card->card_dev);
  	return err;
}
EXPORT_SYMBOL(snd_card_new);

综上，后续对于ctl声卡部件的创建，我们直接调用snd_card_new即可。

（2）pcm

kernel\sound\core：pcm.c

static int snd_pcm_dev_register(struct snd_device *device)
{
	struct snd_pcm *pcm;

	pcm = device->device_data;

	mutex_lock(®ister_mutex);
	err = snd_pcm_add(pcm);
	if (err)
		goto unlock;
	for (cidx = 0; cidx < 2; cidx++) {
        ... ...
		/* register pcm */
		err = snd_register_device(devtype, pcm->card, pcm->device,
					  &snd_pcm_f_ops[cidx], pcm,
					  &pcm->streams[cidx].dev);
		if (err < 0) {
			list_del_init(&pcm->list);
			goto unlock;
		}

		for (substream = pcm->streams[cidx].substream; substream; substream = substream->next)
			snd_pcm_timer_init(substream);
	}

	pcm_call_notify(pcm, n_register);

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


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,
	};
	static struct snd_device_ops internal_ops = {
		.dev_free = snd_pcm_dev_free,
	};

	if (snd_BUG_ON(!card))
		return -ENXIO;
	if (rpcm)
		*rpcm = NULL;
	pcm = kzalloc(sizeof(*pcm), GFP_KERNEL);
	if (!pcm)
		return -ENOMEM;
	pcm->card = card;
	pcm->device = device;
	pcm->internal = internal;
	mutex_init(&pcm->open_mutex);
	init_waitqueue_head(&pcm->open_wait);
	INIT_LIST_HEAD(&pcm->list);
	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,
			     internal ? &internal_ops : &ops);
	if (err < 0)
		goto free_pcm;

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

free_pcm:
	snd_pcm_free(pcm);
	return err;
}

可以看到与ctl部件一致，pcm也是通过“_snd_pcm_new”接口函数的成员对象snd_device_ops来间接调用。阅读代码可知道设备的节点"pcmC%iD%i%c"是在snd_pcm_new_stream中创建。

/**
 * snd_pcm_new_stream - create a new PCM stream
 * @pcm: the pcm instance
 * @stream: the stream direction, SNDRV_PCM_STREAM_XXX
 * @substream_count: the number of substreams
 *
 * Creates a new stream for the pcm.
 * The corresponding stream on the pcm must have been empty before
 * calling this, i.e. zero must be given to the argument of
 * snd_pcm_new().
 *
 * Return: Zero if successful, or a negative error code on failure.
 */
int snd_pcm_new_stream(struct snd_pcm *pcm, int stream, int substream_count)
{
	int idx, err;
	struct snd_pcm_str *pstr = &pcm->streams[stream];
	struct snd_pcm_substream *substream, *prev;

    ... ...
	snd_device_initialize(&pstr->dev, pcm->card);
	pstr->dev.groups = pcm_dev_attr_groups;
	dev_set_name(&pstr->dev, "pcmC%iD%i%c", pcm->card->number, pcm->device,
		     stream == SNDRV_PCM_STREAM_PLAYBACK ? 'p' : 'c');

    ... ...
}				
EXPORT_SYMBOL(snd_pcm_new_stream);

 最终声卡pcm部件的创建是由接口snd_pcm_new来完成。

/**
 * 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);
}
EXPORT_SYMBOL(snd_pcm_new);

（3）其他

timer, seq于ctl和pmc类似不再缀叙。