alsa数据结构流程

以声卡驱动的数据结构为切入点分析:

/* SoC card */
struct snd_soc_card {
    const char *name;
    struct device *dev;
    struct snd_card *snd_card; //在snd_soc_instantiate_card中利用snd_card_create创建声卡
    struct module *owner;

    struct list_head list;
    struct mutex mutex;

    bool instantiated;

    int (*probe)(struct snd_soc_card *card);
    int (*late_probe)(struct snd_soc_card *card);
    int (*remove)(struct snd_soc_card *card);

    /* the pre and post PM functions are used to do any PM work before and
     * after the codec and DAI's do any PM work. */
    int (*suspend_pre)(struct snd_soc_card *card);
    int (*suspend_post)(struct snd_soc_card *card);
    int (*resume_pre)(struct snd_soc_card *card);
    int (*resume_post)(struct snd_soc_card *card);

    /* callbacks */
    int (*set_bias_level)(struct snd_soc_card *,
                  enum snd_soc_bias_level level);
    int (*set_bias_level_post)(struct snd_soc_card *,
                   enum snd_soc_bias_level level);

    long pmdown_time;

    /* CPU <--> Codec DAI links  */
    struct snd_soc_dai_link *dai_link;
    int num_links;
    struct snd_soc_pcm_runtime *rtd;
    int num_rtd;

    /* optional codec specific configuration */
    struct snd_soc_codec_conf *codec_conf;
    int num_configs;

    /*optional auxiliary devices such as amplifiers or codecs with DAI link unused*/
    struct snd_soc_aux_dev *aux_dev;
    int num_aux_devs;
    struct snd_soc_pcm_runtime *rtd_aux;
    int num_aux_rtd;

    /* Card-specific routes and widgets.*/
    struct snd_soc_dapm_widget *dapm_widgets;
    int num_dapm_widgets;
    struct snd_soc_dapm_route *dapm_routes;
    int num_dapm_routes;

    struct work_struct deferred_resume_work; 

//属于这个card的已经探测到的设备列表
 /* lists of probed devices belonging to this card */
    struct list_head codec_dev_list;
    struct list_head platform_dev_list;
    struct list_head dai_dev_list;

    struct list_head widgets;
    struct list_head paths;
    struct list_head dapm_list;

    /* Generic DAPM context for the card */
    struct snd_soc_dapm_context dapm;

#ifdef CONFIG_DEBUG_FS
    struct dentry *debugfs_card_root;
    struct dentry *debugfs_pop_time;
#endif
    u32 pop_time;

    void *drvdata;
};

/* SoC machine DAI configuration, glues（胶水） a codec and cpu DAI together */
struct snd_soc_pcm_runtime  {
    struct device dev;
    struct snd_soc_card *card;
    struct snd_soc_dai_link *dai_link;

    unsigned int complete:1;
    unsigned int dev_registered:1;

    /* Symmetry data - only valid if symmetry is being enforced */
    unsigned int rate;
    long pmdown_time;

    /* runtime devices */
    struct snd_pcm *pcm;
    struct snd_soc_codec *codec;
    struct snd_soc_platform *platform;
    struct snd_soc_dai *codec_dai;
    struct snd_soc_dai *cpu_dai;

    struct delayed_work delayed_work;
};

1) struct snd_soc_codec - 由与平台无关的codec驱动实现。

2) struct snd_soc_platform - 由与imx平台相关的DAI驱动实现，主要实现了音频数据的DMA传输功能。

3) struct snd_soc_dai_link - 将平台相关的DAI与平台无关的codec联系起来。 

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。

   1. struct snd_card {  
   2.     int number;              /* number of soundcard (index to snd_cards) */  
   3.     char id[16];             /* id string of this card */  
   4.     char driver[16];         /* driver name */  
   5.     char shortname[32];      /* short name of this soundcard */  
   6.     char longname[80];       /* name of this soundcard */  
   7.     char mixername[80];      /* mixer name */  
   8.     char components[128];    /* card components delimited with space */  
   9.     struct module *module;   /* top-level module */  
  10.   
  11.     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 */  
  12.     void (*private_free) (struct snd_card *card); /* callback for freeing of private data */  
  13.     struct list_head devices;   /* devices 记录该声卡下所有逻辑设备的链表 */  
  14.   
  15.     unsigned int last_numid;    /* last used numeric ID */  
  16.     struct rw_semaphore controls_rwsem; /* controls list lock */  
  17.     rwlock_t ctl_files_rwlock;  /* ctl_files list lock */  
  18.     int controls_count;     /* count of all controls */  
  19.     int user_ctl_count;     /* count of all user controls */  
  20.     struct list_head controls;  /* all controls for this card 记录该声卡下所有的控制单元的链表 */  
  21.     struct list_head ctl_files; /* active control files */  
  22.   
  23.     struct snd_info_entry *proc_root;   /* root for soundcard specific files */  
  24.     struct snd_info_entry *proc_id; /* the card id */  
  25.     struct proc_dir_entry *proc_root_link;  /* number link to real id */  
  26.   
  27.     struct list_head files_list;    /* all files associated to this card */  
  28.     struct snd_shutdown_f_ops *s_f_ops; /* file operations in the shutdown state */  
  29.     spinlock_t files_lock;      /* lock the files for this card */  
  30.     int shutdown;           /* this card is going down */  
  31.     int free_on_last_close;     /* free in context of file_release */  
  32.     wait_queue_head_t shutdown_sleep;  
  33.     struct device *dev;     /* device assigned to this card */  
  34. #ifndef CONFIG_SYSFS_DEPRECATED  
  35.     struct device *card_dev;    /* cardX object for sysfs */  
  36. #endif  
  37.  
  38. #ifdef CONFIG_PM  
  39.     unsigned int power_state;   /* power state */  
  40.     struct mutex power_lock;    /* power lock */  
  41.     wait_queue_head_t power_sleep;  
  42. #endif  
  43.  
  44. #if defined(CONFIG_SND_MIXER_OSS) || defined(CONFIG_SND_MIXER_OSS_MODULE)  
  45.     struct snd_mixer_oss *mixer_oss;  
  46.     int mixer_oss_change_count;  
  47. #endif  
  48. };

创建声卡的功能部件（逻辑设备），例如PCM，Mixer，MIDI等。
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()

controlC0 --> 用于声卡的控制，例如通道选择，混音，麦克风的控制等
midiC0D0 --> 用于播放midi音频
pcmC0D0c --〉 用于录音的pcm设备
pcmC0D0p --〉 用于播放的pcm设备
seq --〉 音序器
timer --〉 定时器

core 该目录包含了ALSA驱动的中间层，它是整个ALSA驱动的核心部分i2c ALSA自己的I2C控制代码soc 针对system-on-chip体系的中间层代码soc/codecs 针对soc体系的各种codec的代码，与平台无关

每个声卡最多可以包含4个pcm的实例，每个pcm实例对应一个pcm设备文件。

ALSA已经为我们实现了功能强劲的PCM中间层，自己的驱动中只要实现一些底层的需要访问硬件的函数即可。

大多数情况下，在嵌入式设备中，一个pcm实例已经足够了。一个pcm实例由一个playback stream和一个capture stream组成，这两个stream又分别有一个或多个substreams组成。



pcm中间层的几个重要的结构体的关系图

struct snd_pcm {

struct snd_card *card;

struct list_head list;

int device; /* device number */

unsigned int info_flags;

unsigned short dev_class;

unsigned short dev_subclass;

char id[64];

char name[80];

struct snd_pcm_str streams[2];

struct mutex open_mutex;

wait_queue_head_t open_wait;

void *private_data;

void (*private_free) (struct snd_pcm *pcm);

struct device *dev; /* actual hw device this belongs to */

#if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE)

struct snd_pcm_oss oss;

#endif

};

1 snd_pcm是挂在snd_card下面的一个snd_device

2 snd_pcm中的字段：streams[2]，该数组中的两个元素指向两个snd_pcm_str结构，分别代表playback stream和capture stream。

3 snd_pcm_str中的substream字段，指向snd_pcm_substream结构

4 snd_pcm_substream是pcm中间层的核心，绝大部分任务都是在substream中处理，尤其是他的ops（snd_pcm_ops）字段，

许多user空间的应用程序通过alsa-lib对驱动程序的请求都是由该结构中的函数处理。它的runtime字段则指向snd_pcm_runtime结构，

snd_pcm_runtime记录这substream的一些重要的软件和硬件运行环境和参数。

struct snd_pcm_str {

int stream; /* stream (direction) */

struct snd_pcm *pcm;

/* -- substreams -- */

unsigned int substream_count;

unsigned int substream_opened;

struct snd_pcm_substream *substream;

#if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE)

/* -- OSS things -- */

struct snd_pcm_oss_stream oss;

#endif

#ifdef CONFIG_SND_VERBOSE_PROCFS

struct snd_info_entry *proc_root;

struct snd_info_entry *proc_info_entry;

#ifdef CONFIG_SND_PCM_XRUN_DEBUG

unsigned int xrun_debug; /* 0 = disabled, 1 = verbose, 2 = stacktrace */

struct snd_info_entry *proc_xrun_debug_entry;

#endif

#endif

};

struct snd_pcm_substream {

struct snd_pcm *pcm;

struct snd_pcm_str *pstr;

void *private_data; /* copied from pcm->private_data */

int number;

char name[32]; /* substream name */

int stream; /* stream (direction) */

struct pm_qos_request_list latency_pm_qos_req; /* pm_qos request */

size_t buffer_bytes_max; /* limit ring buffer size */

struct snd_dma_buffer dma_buffer;

unsigned int dma_buf_id;

size_t dma_max;

/* -- hardware operations -- */

struct snd_pcm_ops *ops;

/* -- runtime information -- */

struct snd_pcm_runtime *runtime;

/* -- timer section -- */

struct snd_timer *timer; /* timer */

unsigned timer_running: 1; /* time is running */

/* -- next substream -- */

struct snd_pcm_substream *next;

/* -- linked substreams -- */

struct list_head link_list; /* linked list member */

struct snd_pcm_group self_group; /* fake group for non linked substream (with substream lock inside) */

struct snd_pcm_group *group; /* pointer to current group */

/* -- assigned files -- */

void *file;

int ref_count;

atomic_t mmap_count;

unsigned int f_flags;

void (*pcm_release)(struct snd_pcm_substream *);

struct pid *pid;

#if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE)

/* -- OSS things -- */

struct snd_pcm_oss_substream oss;

#endif

#ifdef CONFIG_SND_VERBOSE_PROCFS

struct snd_info_entry *proc_root;

struct snd_info_entry *proc_info_entry;

struct snd_info_entry *proc_hw_params_entry;

struct snd_info_entry *proc_sw_params_entry;

struct snd_info_entry *proc_status_entry;

struct snd_info_entry *proc_prealloc_entry;

struct snd_info_entry *proc_prealloc_max_entry;

#endif

/* misc flags */

unsigned int hw_opened: 1;

};

新建一个pcm可以用下面一张新建pcm的调用的序列图进行描述：




新建pcm的序列图

snd_pcm_set_ops 设置操作该pcm的控制/操作接口函数，参数中的snd_pcm_ops结构中的函数通常就是我们驱动要实现的函数

snd_card_register 注册声卡，在这个阶段会遍历声卡下的所有逻辑设备，并且调用各设备的注册回调函数，

对于pcm，就是第二步提到的snd_pcm_dev_register函数，该回调函数建立了和用户空间应用程序（alsa-lib）通信所用的设备文件节点:/dev/snd/pcmCxxDxxp

和/dev/snd/pcmCxxDxxc （这些设备节点由alsa-lib使用，用户不直接使用）。

snd_pcm_dev_register调用snd_register_device_for_dev，snd_register_device_for_dev调用device_create创建设备节点。snd_register_device_for_dev有个参数

snd_pcm_f_ops，它是一个标准的文件系统file_operations结构数组，它的定义在sound/core/pcm_native.c中：并被记录在snd_minors[minor]中的字段f_ops中。

/*

* Register section

*/

//used in pcm.c

const struct file_operations snd_pcm_f_ops[2] = {

{

.owner = THIS_MODULE,

.write = snd_pcm_write,

.aio_write = snd_pcm_aio_write,

.open = snd_pcm_playback_open,

.release = snd_pcm_release,

.llseek = no_llseek,

.poll = snd_pcm_playback_poll,

.unlocked_ioctl = snd_pcm_playback_ioctl,

.compat_ioctl = snd_pcm_ioctl_compat,

.mmap = snd_pcm_mmap,

.fasync = snd_pcm_fasync,

.get_unmapped_area = snd_pcm_get_unmapped_area,

},

{

.owner = THIS_MODULE,

.read = snd_pcm_read,

.aio_read = snd_pcm_aio_read,

.open = snd_pcm_capture_open,

.release = snd_pcm_release,

.llseek = no_llseek,

.poll = snd_pcm_capture_poll,

.unlocked_ioctl = snd_pcm_capture_ioctl,

.compat_ioctl = snd_pcm_ioctl_compat,

.mmap = snd_pcm_mmap,

.fasync = snd_pcm_fasync,

.get_unmapped_area = snd_pcm_get_unmapped_area,

}

};

在sound/core/sound.c中有alsa_sound_init()函数，其中有：

register_chrdev(major, "alsa", &snd_fops)，参数major与之前创建pcm设备是device_create时的major是同一个，这样的结果是，

当应用程序open设备文件/dev/snd/pcmCxDxp时，会进入snd_fops的open回调函数。

static const struct file_operations snd_fops =

{

.owner = THIS_MODULE,

.open = snd_open,

.llseek = noop_llseek,

};

snd_open函数，它首先从inode中取出此设备号，然后以次设备号为索引，从snd_minors全局数组中取出当初注册pcm设备时填充的snd_minor结构

，然后从snd_minor结构中取出pcm设备的f_ops，并且把file->f_op替换为pcm设备的f_ops，紧接着直接调用pcm设备的f_ops->open()，然后返回。

因为file->f_op已经被替换，以后，应用程序的所有read/write/ioctl调用都会进入pcm设备自己的回调函数中，也就是snd_pcm_f_ops结构中定义的回调。

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