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