Linux内核4.14版本——alsa框架分析(4)-声卡的创建

      card是整个声卡的最底层结构，用于管理该声卡下的所有设备包括control, pcm,timer等。在linux系统中使用struct snd_card结构代表一个card。

/* main structure for soundcard */

struct snd_card {
	int number;			/* number of soundcard (index to
								snd_cards) */

	char id[16];			/* id string of this card */
	char driver[16];		/* driver name */
	char shortname[32];		/* short name of this soundcard */
	char longname[80];		/* name of this soundcard */
	char irq_descr[32];		/* Interrupt description */
	char mixername[80];		/* mixer name */
	char components[128];		/* card components delimited with
								space */
	struct module *module;		/* top-level module */

	void *private_data;		/* private data for soundcard */
	void (*private_free) (struct snd_card *card); /* callback for freeing of
								private data */
	struct list_head devices;	/* devices */

	struct device ctl_dev;		/* control device */
	unsigned int last_numid;	/* last used numeric ID */
	struct rw_semaphore controls_rwsem;	/* controls list lock */
	rwlock_t ctl_files_rwlock;	/* ctl_files list lock */
	int controls_count;		/* count of all controls */
	int user_ctl_count;		/* count of all user controls */
	struct list_head controls;	/* all controls for this card */
	struct list_head ctl_files;	/* active control files */

	struct snd_info_entry *proc_root;	/* root for soundcard specific files */
	struct snd_info_entry *proc_id;	/* the card id */
	struct proc_dir_entry *proc_root_link;	/* number link to real id */

	struct list_head files_list;	/* all files associated to this card */
	struct snd_shutdown_f_ops *s_f_ops; /* file operations in the shutdown
								state */
	spinlock_t files_lock;		/* lock the files for this card */
	int shutdown;			/* this card is going down */
	struct completion *release_completion;
	struct device *dev;		/* device assigned to this card */
	struct device card_dev;		/* cardX object for sysfs */
	const struct attribute_group *dev_groups[4]; /* assigned sysfs attr */
	bool registered;		/* card_dev is registered? */

#ifdef CONFIG_PM
	unsigned int power_state;	/* power state */
	wait_queue_head_t power_sleep;
#endif

#if IS_ENABLED(CONFIG_SND_MIXER_OSS)
	struct snd_mixer_oss *mixer_oss;
	int mixer_oss_change_count;
#endif
};

此结构注释写的很详细，在此只需要关注重点字段，用到的时候再说。

.number:    soundcard的序号，通常为0。

.id:              card的标识符，通常是字符串形式。

.driver/shortname/longname:  会在具体驱动中设置，主要反映在/proc/asound/cards中。

.private_data:   card的私有数据。

.devices:     用于管理该card下所有的设备。

.controls:    用于管理该card下的所有control设备。 

.ctl_files:     用于管理该card下的active的control设备。

.dev:             和card相关的设备。

.card_dev:    card用于在sys中显示，用于代表该card。

registered:    代表是否在系统中注册了card。

创建并且初始化card

几乎所有的音频驱动都会在刚驱动的开始创建card，通常会使用snd_card_new函数。

/**
 *  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;
	int err;

	if (snd_BUG_ON(!card_ret))
		return -EINVAL;
	*card_ret = NULL;

	if (extra_size < 0)
		extra_size = 0;
	card = kzalloc(sizeof(*card) + extra_size, GFP_KERNEL);
	if (!card)
		return -ENOMEM;
	if (extra_size > 0)
		card->private_data = (char *)card + sizeof(struct snd_card);
	if (xid)
		strlcpy(card->id, xid, sizeof(card->id));
	err = 0;
	mutex_lock(&snd_card_mutex);
	if (idx < 0) /* first check the matching module-name slot */
		idx = get_slot_from_bitmask(idx, module_slot_match, module);
	if (idx < 0) /* if not matched, assign an empty slot */
		idx = get_slot_from_bitmask(idx, check_empty_slot, module);
	if (idx < 0)
		err = -ENODEV;
	else if (idx < snd_ecards_limit) {
		if (test_bit(idx, snd_cards_lock))
			err = -EBUSY;	/* invalid */
	} else if (idx >= SNDRV_CARDS)
		err = -ENODEV;
	if (err < 0) {
		mutex_unlock(&snd_card_mutex);
		dev_err(parent, "cannot find the slot for index %d (range 0-%i), error: %d\n",
			 idx, snd_ecards_limit - 1, err);
		kfree(card);
		return err;
	}
	set_bit(idx, snd_cards_lock);		/* lock it */
	if (idx >= snd_ecards_limit)
		snd_ecards_limit = idx + 1; /* increase the limit */
	mutex_unlock(&snd_card_mutex);
	card->dev = parent;
	card->number = idx;
	card->module = module;
	INIT_LIST_HEAD(&card->devices);
	init_rwsem(&card->controls_rwsem);
	rwlock_init(&card->ctl_files_rwlock);
	INIT_LIST_HEAD(&card->controls);
	INIT_LIST_HEAD(&card->ctl_files);
	spin_lock_init(&card->files_lock);
	INIT_LIST_HEAD(&card->files_list);
#ifdef CONFIG_PM
	init_waitqueue_head(&card->power_sleep);
#endif

	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;
}

1.   通过kzalloc分配一个snd_card结构。根据是否有extra_size，设置card的私有数据private_data。

2.   根据card id赋值card->id成员，此成员通话是ASCII String。

3.   如果idx= -1，系统会自动分配一个索引编号。

4.   card变量的一系列初始化，包括devices, controls等。

5.   初始化card_dev设备，设置parent, class, 设备属性，以及name。

sound_class会在init_soundcore中做初始化操作。

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;
}

6.   创建card的control设备。根据注释control接口在snd_card_register之后，用户空间才可以访问。control设备在control小节详解。

7.   调用snd_info_card_create函数在proc下创建card0目录。

int snd_info_card_create(struct snd_card *card)  
{  
    char str[8];  
    struct snd_info_entry *entry;  
  
    if (snd_BUG_ON(!card))  
        return -ENXIO;  
  
    sprintf(str, "card%i", card->number);  
    if ((entry = snd_info_create_module_entry(card->module, str, NULL)) == NULL)  
        return -ENOMEM;  
    entry->mode = S_IFDIR | S_IRUGO | S_IXUGO;  
    if (snd_info_register(entry) < 0) {  
        snd_info_free_entry(entry);  
        return -ENOMEM;  
    }  
    card->proc_root = entry;  
    return 0;  
}  

最终会根据entry的mode，创建目录。

声卡的注册

在声卡初始化，会在驱动程序中设置card，完后会调用snd_card_register注册此card到系统中去。

/**
 *  snd_card_register - register the soundcard
 *  @card: soundcard structure
 *
 *  This function registers all the devices assigned to the soundcard.
 *  Until calling this, the ALSA control interface is blocked from the
 *  external accesses.  Thus, you should call this function at the end
 *  of the initialization of the card.
 *
 *  Return: Zero otherwise a negative error code if the registration failed.
 */
int snd_card_register(struct snd_card *card)
{
	int err;

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

	if (!card->registered) {
		err = device_add(&card->card_dev);
		if (err < 0)
			return err;
		card->registered = true;
	}

	if ((err = snd_device_register_all(card)) < 0)
		return err;
	mutex_lock(&snd_card_mutex);
	if (snd_cards[card->number]) {
		/* already registered */
		mutex_unlock(&snd_card_mutex);
		return snd_info_card_register(card); /* register pending info */
	}
	if (*card->id) {
		/* make a unique id name from the given string */
		char tmpid[sizeof(card->id)];
		memcpy(tmpid, card->id, sizeof(card->id));
		snd_card_set_id_no_lock(card, tmpid, tmpid);
	} else {
		/* create an id from either shortname or longname */
		const char *src;
		src = *card->shortname ? card->shortname : card->longname;
		snd_card_set_id_no_lock(card, src,
					retrieve_id_from_card_name(src));
	}
	snd_cards[card->number] = card;
	mutex_unlock(&snd_card_mutex);
	init_info_for_card(card);
#if IS_ENABLED(CONFIG_SND_MIXER_OSS)
	if (snd_mixer_oss_notify_callback)
		snd_mixer_oss_notify_callback(card, SND_MIXER_OSS_NOTIFY_REGISTER);
#endif
	return 0;
}

1.   合法性判断，如果此处card不存在，panic。

2.   根据card的registered判断是否已经注册，如果注册继续。否则调用device_add添加设备，设置registered标志。

3.   调用snd_device_register_all注册所有card的设备，包括pcm, control等

/*
 * register all the devices on the card.
 * called from init.c
 */
int snd_device_register_all(struct snd_card *card)
{
	struct snd_device *dev;
	int err;
	
	if (snd_BUG_ON(!card))
		return -ENXIO;
	list_for_each_entry(dev, &card->devices, list) {
		err = __snd_device_register(dev);
		if (err < 0)
			return err;
	}
	return 0;
}

从card的devices中依次取出一个devices,调用__snd_device_register函数注册。

static int __snd_device_register(struct snd_device *dev)
{
	if (dev->state == SNDRV_DEV_BUILD) {
		if (dev->ops->dev_register) {
			int err = dev->ops->dev_register(dev);
			if (err < 0)
				return err;
		}
		dev->state = SNDRV_DEV_REGISTERED;
	}
	return 0;
}

此函数最终会调用各个devices的snd_device_ops中的dev_register函数。

4.  会在snd_cards结构体数组中查找是否已经存在。

5.  通过给定的string, 设置一个独一无二的id name，先后会通过id,shortname, longname设置此 unique id。

6.  在snd_cards结构体数组中设置此card，根据number为下标。

7.  调用init_info_for_card函数在card0下创建id。

至此，整个声卡就完成了创建过程。