Linux ALSA声卡驱动之二:声卡的创建

1.1.snd_card是什么

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，几乎所有与声卡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就是创建一个snd_card结构体。

1.2snd_card的定义 位于include/sound/core.h

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 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 */

	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 mutex user_ctl_lock;	/* protects user controls against
					   concurrent access */

	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 */
	int free_on_last_close;		/* free in context of file_release */
	wait_queue_head_t shutdown_sleep;
	atomic_t refcount;		/* refcount for disconnection */
	struct device *dev;		/* device assigned to this card */
	struct device *card_dev;	/* cardX object for sysfs */

};

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

struct list_head controls;    /* all controls for this card */ 记录该声卡下所有控制单元的链表

void *private_data;        /* private data for soundcard */ 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

2.声卡的建立流程

2.1.1第一步，创建snd_card的实例

int snd_card_create(int idx, const char *xid,
		    struct module *module, int extra_size,
		    struct snd_card **card_ret)

idx ：一个整数数值，该声卡的编号

xid：字符串，声卡的标示符。

extra_size:该参数决定在创建snd_card实例时，需要同时额外分配数据的大小，该数据的指针最终会赋值给snd_card的private_data数据成员。

rd_ret:返回所创建的snd_card实例指针。

2.1.2第二步，创建声卡芯片专用数据（个人感觉这一步可有可无）

2.1.3第三步，设置Driver的ID和名字

	strcpy(card->driver, "AMD7930");
	strcpy(card->shortname, "Sun AMD7930");
	sprintf(card->longname, "%s at 0x%02lx:0x%08Lx, irq %d",
		card->shortname,
		rp->flags & 0xffL,
		(unsigned long long)rp->start,
		irq);

snd_card的driver字段保存着芯片的ID字符串，user空间的alsa-lib会使用到该字符串，所以必须要保证该ID的唯一性，shortname字段更多的用于打印信息，longname字段会出现在/proc/asound/cards/中。

2.1.4第四步，创建声卡的功能部件（逻辑设备），例如PCM，mixer，MIDI等。

这时候可以创建声卡的各种功能部件了，还记得开头的snd_device结构体的device字段吗？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device链表中。

通常alsa-driver已经提供了一些常用的部件创建函数，因此不必直接调用snd_device_new();比如：

PCM ---- snd_pcm_new()

RAWMIDI ----- snd_rawmidi_new()

CONTROL ---- snd_ctl_create()

TIMER -- snd_timer_new()

..........

2.1.5第五步，注册声卡

if ((err = snd_card_register(card)) < 0)
		goto out_err;

2.2通过内核中一个实际的例子来看声卡的创建过程：

/sound/arm/pxa2xx-ac97.c

static int pxa2xx_ac97_probe(struct platform_device *dev)
{
	struct snd_card *card;


	pxa2xx_audio_ops_t *pdata = dev->dev.platform_data;

    //setp 1.
	ret = snd_card_create(SNDRV_DEFAULT_IDX1, SNDRV_DEFAULT_STR1,
			      THIS_MODULE, 0, &card);

    //step 3
	card->dev = &dev->dev;
	strncpy(card->driver, dev->dev.driver->name, sizeof(card->driver));

	ret = pxa2xx_pcm_new(card, &pxa2xx_ac97_pcm_client, &pxa2xx_ac97_pcm);
	
    //setp 2
	ret = pxa2xx_ac97_hw_probe(dev);
	if (ret)
		goto err;
    //step 4 
	ret = snd_ac97_bus(card, 0, &pxa2xx_ac97_ops, NULL, &ac97_bus);
	if (ret)
		goto err_remove;
	memset(&ac97_template, 0, sizeof(ac97_template));
	ret = snd_ac97_mixer(ac97_bus, &ac97_template, &pxa2xx_ac97_ac97);

    //step 3
	snprintf(card->shortname, sizeof(card->shortname),
		 "%s", snd_ac97_get_short_name(pxa2xx_ac97_ac97));
	snprintf(card->longname, sizeof(card->longname),
		 "%s (%s)", dev->dev.driver->name, card->mixername);

	if (pdata && pdata->codec_pdata[0])
		snd_ac97_dev_add_pdata(ac97_bus->codec[0], pdata->codec_pdata[0]);
	snd_card_set_dev(card, &dev->dev);
    //step 5
	ret = snd_card_register(card);
	if (ret == 0) {
		platform_set_drvdata(dev, card);
		return 0;
	}

	return ret;
}

经过以上的chua创建步骤zhih之后，声卡的逻辑结构如图所示：

下面zhu主要分析一下snd_card_create()和snd_card_register()这两个函数。

3.snd_card_create()

snd_card_create()在/sound/core/init.c中定义

int snd_card_create(int idx, const char *xid,
		    struct module *module, int extra_size,
		    struct snd_card **card_ret)
{
	struct snd_card *card;
	int err, idx2;

    //根据extra_size大小分配内存，该内存区可作为芯片专有数据使用
	card = kzalloc(sizeof(*card) + extra_size, GFP_KERNEL);
    //拷贝声卡ID字符串
	if (xid)
		strlcpy(card->id, xid, sizeof(card->id));
    //如果传入的声卡编号为-1，自动分配一个索引编号
	if (idx < 0) {
		for (idx2 = 0; idx2 < SNDRV_CARDS; idx2++)
			/* idx == -1 == 0xffff means: take any free slot */
			if (~snd_cards_lock & idx & 1<<idx2) {
				if (module_slot_match(module, idx2)) {
					idx = idx2;
					break;
				}
			}
	}
	if (idx < 0) {
		for (idx2 = 0; idx2 < SNDRV_CARDS; idx2++)
			/* idx == -1 == 0xffff means: take any free slot */
			if (~snd_cards_lock & idx & 1<<idx2) {
				if (!slots[idx2] || !*slots[idx2]) {
					idx = idx2;
					break;
				}
			}

    //初始化snd_card中必要的字段
	card->number = idx;
	card->module = module;
	INIT_LIST_HEAD(&card->devices);
	init_rwsem(&card->controls_rwsem);
	rwlock_init(&card->ctl_files_rwlock);
	mutex_init(&card->user_ctl_lock);
	INIT_LIST_HEAD(&card->controls);
	INIT_LIST_HEAD(&card->ctl_files);
	spin_lock_init(&card->files_lock);
	INIT_LIST_HEAD(&card->files_list);
	init_waitqueue_head(&card->shutdown_sleep);
	atomic_set(&card->refcount, 0);

	/* the control interface cannot be accessed from the user space until */
	/* snd_cards_bitmask and snd_cards are set with snd_card_register */
    //建立逻辑设备：control
	err = snd_ctl_create(card);
	if (err < 0) {
		snd_printk(KERN_ERR "unable to register control minors\n");
		goto __error;
	}
    //建立proc文件中的info节点，通常就是/proc/asound/card0/
	err = snd_info_card_create(card);
    //把第一步分配的内存指针放入private_data字段中：
	if (extra_size > 0)
		card->private_data = (char *)card + sizeof(struct snd_card);
	*card_ret = card;
	return 0;

}

4.snd_card_register()

int snd_card_register(struct snd_card *card)
{
	int err;
    
    //首先创建sysfs下的设备
	if (!card->card_dev) {
		card->card_dev = device_create(sound_class, card->dev,
					       MKDEV(0, 0), card,
					       "card%i", card->number);
		if (IS_ERR(card->card_dev))
			card->card_dev = NULL;
	}
    其中sound_class是在/sound/sound_core.c中创建的
-------------------------------------------------------------------------
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;
}
由此可见，声卡的class将会出现在文件系统的/sys/class/sound/下面，并且，sound_devnode()也决定了相应的设备节点也将会出现在/dev/snd/下面。
---------------------------------------------------------------------------------

通过snd_device_register_all()注册所有挂在该声卡下的逻辑设备，snd_device_register_all()实际上是通过snd_card的device链表，遍历所有snd_device，并且调用snd_device的ops->dev_register()来实现各自设备的注册的。
	if ((err = snd_device_register_all(card)) < 0)
		return err;


	snd_cards[card->number] = card;
	mutex_unlock(&snd_card_mutex);
	init_info_for_card(card);
    //最后建立一些相应的proc和sysfs下的文件或属性节点。到此声卡完成建立过程
	if (card->card_dev) {
		err = device_create_file(card->card_dev, &card_id_attrs);

		err = device_create_file(card->card_dev, &card_number_attrs);
		if (err < 0)
			return err;
	}

	return 0;
}