Linux ALSA声卡驱动之二：声卡的创建

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1. struct snd_card

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。正因为如此，本节中，我们也从 struct cnd_card开始吧。

1.2. snd_card的定义

snd_card的定义位于改头文件中：include/sound/core.h

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/* 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 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&
24000
nbsp;*/

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 */

int free_on_last_close; /* free in context of file_release */

wait_queue_head_t shutdown_sleep;

struct device *dev; /* device assigned to this card */

#ifndef CONFIG_SYSFS_DEPRECATED

struct device *card_dev; /* cardX object for sysfs */

#endif

#ifdef CONFIG_PM

unsigned int power_state; /* power state */

struct mutex power_lock; /* power lock */

wait_queue_head_t power_sleep;

#endif

#if defined(CONFIG_SND_MIXER_OSS) || defined(CONFIG_SND_MIXER_OSS_MODULE)

struct snd_mixer_oss *mixer_oss;

int mixer_oss_change_count;

#endif

};

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

2. 声卡的建立流程

2.1.1. 第一步，创建snd_card的一个实例

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struct snd_card *card;

int err;

....

err = snd_card_create(index, id, THIS_MODULE, 0, &card);

index 一个整数值，该声卡的编号
id 字符串，声卡的标识符
第四个参数 该参数决定在创建snd_card实例时，需要同时额外分配的私有数据的大小，该数据的指针最终会赋值给snd_card的private_data数据成员
card 返回所创建的snd_card实例的指针

2.1.2. 第二步，创建声卡的芯片专用数据

声卡的专用数据主要用于存放该声卡的一些资源信息，例如中断资源、io资源、dma资源等。可以有两种创建方法：

通过上一步中snd_card_create()中的第四个参数，让snd_card_create自己创建

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// struct mychip 用于保存专用数据

err = snd_card_create(index, id, THIS_MODULE,

sizeof(struct mychip), &card);

// 从private_data中取出

struct mychip *chip = card->private_data;

自己创建：

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struct mychip {

struct snd_card *card;

....

};

struct snd_card *card;

struct mychip *chip;

err = snd_card_create(index[dev], id[dev], THIS_MODULE, 0, &card);

// 专用数据记录snd_card实例

chip->card = card;

.....

chip = kzalloc(sizeof(*chip), GFP_KERNEL);

然后，把芯片的专有数据注册为声卡的一个低阶设备：

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static int snd_mychip_dev_free(struct snd_device *device)

{

return snd_mychip_free(device->device_data);

}

static struct snd_device_ops ops = {

.dev_free = snd_mychip_dev_free,

};

....

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

2.1.3. 第三步，设置Driver的ID和名字

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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);

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字段更多地用于打印信息，longname字段则会出现在/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_card结构体的devices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：

PCM ---- snd_pcm_new()

RAWMIDI -- snd_rawmidi_new()

CONTROL -- snd_ctl_create()

TIMER -- snd_timer_new()

INFO -- snd_card_proc_new()

JACK -- snd_jack_new()

2.1.5. 第五步，注册声卡

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err = snd_card_register(card);

if (err < 0) {

snd_card_free(card);

return err;

}

2.2. 一个实际的例子

我把/sound/arm/pxa2xx-ac97.c的部分代码贴上来：

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static int __devinit pxa2xx_ac97_probe(struct platform_device *dev)

{

struct snd_card *card;

struct snd_ac97_bus *ac97_bus;

struct snd_ac97_template ac97_template;

int ret;

pxa2xx_audio_ops_t *pdata = dev->dev.platform_data;

if (dev->id >= 0) {

dev_err(&dev->dev, "PXA2xx has only one AC97 port./n");

ret = -ENXIO;

goto err_dev;

}

////(1)////

ret = snd_card_create(SNDRV_DEFAULT_IDX1, SNDRV_DEFAULT_STR1,

THIS_MODULE, 0, &card);

if (ret < 0)

goto err;

card->dev = &dev->dev;

////(3)////

strncpy(card->driver, dev->dev.driver->name, sizeof(card->driver));

////(4)////

ret = pxa2xx_pcm_new(card, &pxa2xx_ac97_pcm_client, &pxa2xx_ac97_pcm);

if (ret)

goto err;

////(2)////

ret = pxa2xx_ac97_hw_probe(dev);

if (ret)

goto err;

////(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);

if (ret)

goto err_remove;

////(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);

////(5)////

ret = snd_card_register(card);

if (ret == 0) {

platform_set_drvdata(dev, card);

return 0;

}

err_remove:

pxa2xx_ac97_hw_remove(dev);

err:

if (card)

snd_card_free(card);

err_dev:

return ret;

}

static int __devexit pxa2xx_ac97_remove(struct platform_device *dev)

{

struct snd_card *card = platform_get_drvdata(dev);

if (card) {

snd_card_free(card);

platform_set_drvdata(dev, NULL);

pxa2xx_ac97_hw_remove(dev);

}

return 0;

}

static struct platform_driver pxa2xx_ac97_driver = {

.probe = pxa2xx_ac97_probe,

.remove = __devexit_p(pxa2xx_ac97_remove),

.driver = {

.name = "pxa2xx-ac97",

.owner = THIS_MODULE,

#ifdef CONFIG_PM

.pm = &pxa2xx_ac97_pm_ops,

#endif

},

};

static int __init pxa2xx_ac97_init(void)

{

return platform_driver_register(&pxa2xx_ac97_driver);

}

static void __exit pxa2xx_ac97_exit(void)

{

platform_driver_unregister(&pxa2xx_ac97_driver);

}

module_init(pxa2xx_ac97_init);

module_exit(pxa2xx_ac97_exit);

MODULE_AUTHOR("Nicolas Pitre");

MODULE_DESCRIPTION("AC97 driver for the Intel PXA2xx chip");

驱动程序通常由probe回调函数开始，对一下2.1中的步骤，是否有相似之处？

经过以上的创建步骤之后，声卡的逻辑结构如下图所示：



图 2.2.1 声卡的软件逻辑结构

下面的章节里我们分别讨论一下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。

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/**

* snd_card_create - create and initialize a soundcard structure

* @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.

*

* Returns zero if successful or a negative error code.

*/

int snd_card_create(int idx, const char *xid,

struct module *module, int extra_size,

struct snd_card **card_ret)

首先，根据extra_size参数的大小分配内存，该内存区可以作为芯片的专有数据使用（见前面的介绍）：

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card = kzalloc(sizeof(*card) + extra_size, GFP_KERNEL);

if (!card)

return -ENOMEM;

拷贝声卡的ID字符串：

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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：

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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结构中必要的字段：

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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);

init_waitqueue_head(&card->shutdown_sleep);

#ifdef CONFIG_PM

mutex_init(&card->power_lock);

init_waitqueue_head(&card->power_sleep);

#endif

建立逻辑设备：Control

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/* 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);

建立proc文件中的info节点：通常就是/proc/asound/card0

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err = snd_info_card_create(card);

把第一步分配的内存指针放入private_data字段中：

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if (extra_size > 0)

card->private_data = (char *)card + sizeof(struct snd_card);

4. snd_card_register()

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

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/**

* 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.

*

* Returns zero otherwise a negative error code if the registrain failed.

*/

int snd_card_register(struct snd_card *card)

首先，创建sysfs下的设备：

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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：

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static char *sound_devnode(struct device *dev, mode_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。

接下来的步骤，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devices链表，遍历所有的snd_device，并且调用snd_device的ops->dev_register()来实现各自设备的注册的。

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if ((err = snd_device_register_all(card)) < 0)

return err;

最后就是建立一些相应的proc和sysfs下的文件或属性节点，代码就不贴了。

至此，整个声卡完成了建立过程。