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 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 view plaincopy to clipboardprint? struct snd_card *card; int err; ....
err = snd_card_create(index, id, THIS_MODULE, 0, &card); 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 view plaincopy to clipboardprint?
// 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; // 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：
view plaincopy to clipboardprint? 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); 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：
view plaincopy to clipboardprint?
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); 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 view plaincopy to clipboardprint? strcpy(card->driver, \
strcpy(card->shortname, \ sprintf(card->longname, \
card->shortname, chip->ioport, chip->irq); strcpy(card->driver, \
strcpy(card->shortname, \sprintf(card->longname, \
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
view plaincopy to clipboardprint? err = snd_card_register(card); if (err < 0) {
snd_card_free(card); return err; }
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：
view plaincopy to clipboardprint?
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, \ 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);