Linux ALSA驱动之二：声卡的创建流程

1、struct snd_card

1.1、snd_card是啥

        snd_card可以说是整个ALSA音频驱动最顶层的一个结构，整个声卡的软件逻辑结构开始于该结构，几乎所有与声音相关的逻辑设备都是在snd_card的管理之下，声卡驱动的第一个动作通常就是创建一个snd_card结构体。因此我们也从 struct snd_card的讲解开始。

1.2、snd_card定义

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 */
	size_t user_ctl_alloc_size;	// current memory allocation by 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 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? */
	bool managed;			        /* managed via devres */
	bool releasing;			        /* during card free process */
	int sync_irq;			        /* assigned irq, used for PCM sync */
	wait_queue_head_t remove_sleep;

	size_t total_pcm_alloc_bytes;	/* total amount of allocated buffers */
	struct mutex memory_mutex;	    /* protection for the above */
#ifdef CONFIG_SND_DEBUG
	struct dentry *debugfs_root;    /* debugfs root for card */
#endif

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

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

        对于每个声卡，都需要有一个snd_card结构体来描述。它记录着声卡的信息并管理声卡的所有设备。其中几个比较重要的成员：
        int number                          声卡的序号，通常为0。

        struct list_head devices     记录该声卡下所有逻辑设备的链表。

        struct list_head controls    记录该声卡下所有的控制单元的链表。

        void *private_data             声卡的私有数据，可以在创建声卡时通过参数指定数据的大小。

        bool registered                  声卡是否在系统中注册了。

2、声卡创建流程

2.1、创建一个card实例

struct snd_card *card;
snd_card_new(dev, SNDRV_DEFAULT_IDX1, SNDRV_DEFAULT_STR1,
			      THIS_MODULE, sizeof(struct snd_dw_hdmi), &card);

2.2、创建声卡的芯片专用数据

        设置该声卡的一些资源信息，例如：中断、IO、DMA等。有两种方式进行创建。

1）、作为声卡的private_data

        在创建声卡的时候传入外部数据长度。

struct mychip {
          struct snd_card *card;
          ....
};

err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
                     sizeof(struct mychip), &card);

struct mychip *chip = card->private_data;
chip->card = card;

2）、作为声卡的一个子设备

        在snd_device_new中指定extra_size为0。

static int snd_mychip_dev_free(struct snd_device *device)
{
    return snd_mychip_free(device->device_data);
}

struct snd_card *card;
struct mychip *chip;
err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
                    0, &card);
.....
chip = kzalloc(sizeof(*chip), GFP_KERNEL);

chip->card = card;

static struct snd_device_ops ops = {
    .dev_free = snd_mychip_dev_free,
};
....

snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);

        声卡注销时，会调用snd_mychip_dev_free, 自动释放内存。

        snd_device_new不会给芯片专用数据device_data分配空间，因此在调用之前，必须为芯片专用分配空间，在ops的dev_free中定义析构函数对芯片专用数据进行析构。dev_free会在调用snd_card_free时自动调用。对于用户自定义的 device、type可以使用SNDRV_DEV_LOWLEVEL。

        snd_mychip_dev_free() 是用来free前面kzmalloc的空间。

2.3、设置驱动ID和名字

    strcpy(card->driver, “My Chip”);
    strcpy(card->shortname, “My Own Chip 123”);
    sprintf(card->longname, “%s at 0x%lx irq %i”,
        card->shortname, chip->ioport, chip->irq)

或
	strncpy(card->driver, shortname, sizeof(card->driver));
	strncpy(card->shortname, shortname, sizeof(card->shortname));
	strncpy(card->longname, longname, sizeof(card->longname));

2.4、创建声卡功能逻辑部件，如PCM,mixer, MIDI

        每一种部件的创建最终会调用snd_device_new()来生成一个snd_device实例，并把该实例链接到snd_card的devices链表中。通常，alsa-driver的已经提供了一些常用的部件的创建函数，而不必直接调用snd_device_new()，比如： snd_pcm_new()。

2.5、注册声卡

/* register it */
err = snd_card_register(card);
if (err < 0) {
    pk_error("failed to register pc-midi sound card: error %d\n",
        err);
	goto fail_register;
}

3、snd_card_new函数详解

        用于创建并初始化一个声卡的结构体

/**
 *  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
 *
 *  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;
    /* 1. 分配snd_card和private_data的空间
	   在snd_card后面的空间分配，card->private_data指向该空间
    */
	card = kzalloc(sizeof(*card) + extra_size, GFP_KERNEL);
	if (!card)
		return -ENOMEM;

	err = snd_card_init(card, parent, idx, xid, module, extra_size);
	if (err < 0) {
		kfree(card);
		return err;
	}

	*card_ret = card;
	return 0;
}

static int snd_card_init(struct snd_card *card, struct device *parent,
			 int idx, const char *xid, struct module *module,
			 size_t extra_size)
{
	int err;
#ifdef CONFIG_SND_DEBUG
	char name[8];
#endif
    /* 1、根据传入的参数赋值xid, idx, module, parent */
    /* （1）. 为 card->private_datad 赋值 */
	if (extra_size > 0)
		card->private_data = (char *)card + sizeof(struct snd_card);
    /* （2）. 为 card->id 赋值 */
	if (xid)
		strscpy(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);
		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);
	/* （3）. 赋值parent */
    card->dev = parent;
    /* （4）. 分配snd_card的序号 */
	card->number = idx;
#ifdef MODULE
	WARN_ON(!module);
	/* （5）. 赋值module */
	card->module = module;
#endif
    /* 2、初始化结构体和变量 */
	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);
	mutex_init(&card->memory_mutex);
#ifdef CONFIG_PM
	init_waitqueue_head(&card->power_sleep);
	init_waitqueue_head(&card->power_ref_sleep);
	atomic_set(&card->power_ref, 0);
#endif
	init_waitqueue_head(&card->remove_sleep);
	card->sync_irq = -1;
    
    /* 设置设备文件节点的名字 */
	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 */
    /* 创建一个control设备 */
	err = snd_ctl_create(card);
	if (err < 0) {
		dev_err(parent, "unable to register control minors\n");
		goto __error;
	}
    /* 生成声卡的proc文件 */
	err = snd_info_card_create(card);
	if (err < 0) {
		dev_err(parent, "unable to create card info\n");
		goto __error_ctl;
	}

#ifdef CONFIG_SND_DEBUG
	sprintf(name, "card%d", idx);
	card->debugfs_root = debugfs_create_dir(name, sound_debugfs_root);
#endif
	return 0;

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

        这函数会执行如下操作：

                1、分配snd_card和private_data的空间。

                2、初始化结构体、变量、创建control设备、生成声卡的proc文件等全部都在snd_card_init函数中完成。

                3、获取private_data的地址等。

                4、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;
}

                5、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。

                6、调用snd_info_card_create函数在proc下创建card0目录，最終会根据entry的mode，创建目录。

/*
 * create a card proc file
 * called from init.c
 */
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);
	entry = create_subdir(card->module, str);
	if (!entry)
		return -ENOMEM;
	card->proc_root = entry;

	return snd_card_ro_proc_new(card, "id", card, snd_card_id_read);
}

4、snd_card_register函数详解

        用来注册声卡，主要完成了如下操作：

                1、创建声卡的设备节点。

                2、注册所有的逻辑设备。

                3、添加当前的声卡到声卡数组。

                4、注册声卡的proc文件

/**
 *  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;
    
    /* 合法性判断，如果此处card不存在，panic。 */
	if (snd_BUG_ON(!card))
		return -EINVAL;

    /* 1、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。创建声卡的sysfs设备节点。其中card->card_dev在创建声卡结构体的时候被赋值。
       card->card_dev.class = sound_class;
       sound_class在sound模块被加载的时候创建
       设备节点：/dev/snd/cartd%i
       
    */
	if (!card->registered) {
		err = device_add(&card->card_dev);
		if (err < 0)
			return err;
		card->registered = true;
	} else {
		if (card->managed)
			devm_remove_action(card->dev, trigger_card_free, card);
	}

	if (card->managed) {
		err = devm_add_action(card->dev, trigger_card_free, card);
		if (err < 0)
			return err;
	}
    
    /* 2、调用snd_device_register_all注册所有card的设备，包括pcm, control等 */
	err = snd_device_register_all(card);
	if (err < 0)
		return err;
	mutex_lock(&snd_card_mutex);
    /* 3、添加当前的声卡到声卡数组 */
	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);

    /* 4、注册声卡的proc文件 */
	err = snd_info_card_register(card);
	if (err < 0)
		return err;

#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;
}

/*
 * 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;
    /* 遍历注册所有的snd_device，调用__snd_device_register函数完成注册 */
	list_for_each_entry(dev, &card->devices, list) {
		err = __snd_device_register(dev);
		if (err < 0)
			return err;
	}
	return 0;
}

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。

        声卡注册完成之后，声卡的软件逻辑结果如下：