Linux音频驱动-Card创建
概述在上节Linux音频驱动-ALSA概述中介绍了整个ALSA的构成，接口以及函数入口的分析。本节将从声卡的创建开始，也就是card的创建。card是整个声卡的最底层结构，用于管理该声卡下的所有设备包括control, pcm,timer等。在linux系统中使用struct snd_card结构代表一个card。数据结构声卡的主要数据结构struct snd_card
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概述
在上节
Linux音频驱动-ALSA概述中介绍了整个ALSA的构成，接口以及函数入口的分析。
本节将从声卡的创建开始，也就是card的创建。card是整个声卡的最底层结构，用于管理该声卡下的所有设备包括control, pcm,timer等。在linux系统中使用struct snd_card结构代表一个card。
数据结构
声卡的主要数据结构struct snd_card
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 */
struct completion *release_completion;
struct device *dev; /* device assigned to this card */
struct device card_dev; /* cardX object for sysfs */
bool registered; /* card_dev is registered? */
#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
};
.number: soundcard的序号，通常为0。
.id: card的标识符，通常是字符串形式。
.driver/shortname/longname: 会在具体驱动中设置，主要反映在/proc/asound/cards中。
.private_data: card的私有数据。
.devices: 用于管理该card下所有的设备。
.controls: 用于管理该card下的所有control设备。
.ctl_files: 用于管理该card下的active的control设备。
.dev: 和card相关的设备。
.card_dev: card用于在sys中显示，用于代表该card。
registered: 代表是否在系统中注册了card。
创建并且初始化card
几乎所有的音频驱动都会在刚驱动的开始创建card，通常会使用snd_card_new函数。
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;
card = kzalloc(sizeof(*card) + extra_size, GFP_KERNEL);
if (!card)
return -ENOMEM;
if (extra_size > 0)
card->private_data = (char *)card + sizeof(struct snd_card);
if (xid)
strlcpy(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);
kfree(card);
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);
card->dev = parent;
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);
#ifdef CONFIG_PM
mutex_init(&card->power_lock);
init_waitqueue_head(&card->power_sleep);
#endif
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_attr_groups;
err = kobject_set_name(&card->card_dev.kobj, "card%d", idx);
if (err < 0)
goto __error;
/* 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);
if (err < 0) {
dev_err(parent, "unable to register control minors\n");
goto __error;
}
err = snd_info_card_create(card);
if (err < 0) {
dev_err(parent, "unable to create card info\n");
goto __error_ctl;
}
*card_ret = card;
return 0;
__error_ctl:
snd_device_free_all(card);
__error:
put_device(&card->card_dev);
return err;
}
2. 根据card id赋值card->id成员，此成员通话是ASCII String。
3. 如果idx= -1，系统会自动分配一个索引编号。
4. card变量的一系列初始化，包括devices, controls等。
5. 初始化card_dev设备，设置parent, class, 设备属性，以及name。
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;
}
7. 调用snd_info_card_create函数在proc下创建card0目录。
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);
if ((entry = snd_info_create_module_entry(card->module, str, NULL)) == NULL)
return -ENOMEM;
entry->mode = S_IFDIR | S_IRUGO | S_IXUGO;
if (snd_info_register(entry) < 0) {
snd_info_free_entry(entry);
return -ENOMEM;
}
card->proc_root = entry;
return 0;
}声卡的注册
在声卡初始化，会在驱动程序中设置card，完后会调用snd_card_register注册此card到系统中去。
int snd_card_register(struct snd_card *card)
{
int err;
if (snd_BUG_ON(!card))
return -EINVAL;
if (!card->registered) {
err = device_add(&card->card_dev);
if (err < 0)
return err;
card->registered = true;
}
if ((err = snd_device_register_all(card)) < 0)
return err;
mutex_lock(&snd_card_mutex);
if (snd_cards[card->number]) {
/* already registered */
mutex_unlock(&snd_card_mutex);
return 0;
}
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);
init_info_for_card(card);
#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;
}
2. 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。
3. 调用snd_device_register_all注册所有card的设备，包括pcm, control等
int snd_device_register_all(struct snd_card *card)
{
struct snd_device *dev;
int err;
if (snd_BUG_ON(!card))
return -ENXIO;
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;
}
4. 会在snd_cards结构体数组中查找是否已经存在。
5. 通过给定的string, 设置一个独一无二的id name，先后会通过id,shortname, longname设置此 unique id。
6. 在snd_cards结构体数组中设置此card，根据number为下标。
7. 调用init_info_for_card函数在card0下创建id。
至此，整个声卡就完成了创建过程。
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