Linux ALSA驱动之二：声卡的创建流程
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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：
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