alsa相關的配置文件 .

在根文件系統下，alsa相關的配置文件有：

在/system/usr/share/alsa目錄下：

├── alsa.conf
├── cards
│   └── aliases.conf
└── pcm
    ├── center_lfe.conf
    ├── default.conf
    ├── dmix.conf
    ├── dpl.conf
    ├── dsnoop.conf
    ├── front.conf
    ├── iec958.conf
    ├── modem.conf
    ├── rear.conf
    ├── side.conf
    ├── surround40.conf
    ├── surround41.conf
    ├── surround50.conf
    ├── surround51.conf
    └── surround71.conf

├── alsa.conf
├── cards
│   └── aliases.conf
└── pcm
    ├── center_lfe.conf
    ├── default.conf
    ├── dmix.conf
    ├── dpl.conf
    ├── dsnoop.conf
    ├── front.conf
    ├── iec958.conf
    ├── modem.conf
    ├── rear.conf
    ├── side.conf
    ├── surround40.conf
    ├── surround41.conf
    ├── surround50.conf
    ├── surround51.conf
    └── surround71.conf

├── alsa.conf
├── cards
│   └── aliases.conf
└── pcm
    ├── center_lfe.conf
    ├── default.conf
    ├── dmix.conf
    ├── dpl.conf
    ├── dsnoop.conf
    ├── front.conf
    ├── iec958.conf
    ├── modem.conf
    ├── rear.conf
    ├── side.conf
    ├── surround40.conf
    ├── surround41.conf
    ├── surround50.conf
    ├── surround51.conf
    └── surround71.conf

├── alsa.conf ├── cards │ └── aliases.conf └── pcm ├── center_lfe.conf ├── default.conf ├── dmix.conf ├── dpl.conf ├── dsnoop.conf ├── front.conf ├── iec958.conf ├── modem.conf ├── rear.conf ├── side.conf ├── surround40.conf ├── surround41.conf ├── surround50.conf ├── surround51.conf └── surround71.conf 

和

/system/etc/asound.state

 

這些配置文件是如何工作的？

alsa-lib是如何解析這些腳本的？

上層app是如何實現不同需要中的通道選擇，音量調整等？？？比如手機使用過程中打電話/錄音/藍牙耳機輸入/外放喇叭輸入.......

 

下面分別分析這些配置腳本。

+++++++++++++++++++++++++++++++++++++++++++++++

 

【/system/etc/asound.state】

asound.state用來控制音頻通道的初始設置。使用方法：

1 使用alsa_ctl store來將當前音頻的初始設置導出爲asound.state文件。

2 修改文件內容：設置播放/錄製的通道，增益等。

3 使用alsa_ctl restore 重新加載修改過的配置。

4 在init.rc中添加一行，用於系統啓動是自動加載此配置：

service asound_conf /system/bin/alsa_ctl restore
    oneshot

service asound_conf /system/bin/alsa_ctl restore
    oneshot

service asound_conf /system/bin/alsa_ctl restore
    oneshot

service asound_conf /system/bin/alsa_ctl restore oneshot  

asound.state中的信息是抽取在驅動裏面註冊的snd_controls的信息。

 

 

 

 

【/system/usr/share/alsa/alsa.conf】

 

這裏主要參考整理 http://hi.baidu.com/zmjdx/blog/item/35d570347a36e947241f1475.html

 

ALSA的配置文件alsa.conf定義了一些簡單的語法，通過這些語法記錄了alsa環境變量。

 

該文件開頭處包含了用戶可以配置的hook.

用戶自定義的配置信息可以保存在/etc/asound.conf或~/.asoundrc裏，當然也可以自己定義的位置。

我們感興趣的是,alsa lib是如何解析這些配置的。

 

首先我們可以從使用alsa lib時，最先的入口函數snd_pcm_open處開始說起：

int snd_pcm_open(snd_pcm_t **pcmp, const char *name,
snd_pcm_stream_t stream, int mode)
{
int err;
assert(pcmp && name);
err = snd_config_update();
if (err < 0)
return err;
return snd_pcm_open_noupdate(pcmp, snd_config, name, stream, mode, 0);
}

int snd_pcm_open(snd_pcm_t **pcmp, const char *name,
snd_pcm_stream_t stream, int mode)
{
int err;
assert(pcmp && name);
err = snd_config_update();
if (err < 0)
return err;
return snd_pcm_open_noupdate(pcmp, snd_config, name, stream, mode, 0);
}

int snd_pcm_open(snd_pcm_t **pcmp, const char *name,
snd_pcm_stream_t stream, int mode)
{
int err;
assert(pcmp && name);
err = snd_config_update();
if (err < 0)
return err;
return snd_pcm_open_noupdate(pcmp, snd_config, name, stream, mode, 0);
}

int snd_pcm_open(snd_pcm_t **pcmp, const char *name, snd_pcm_stream_t stream, int mode) { int err; assert(pcmp && name); err = snd_config_update(); if (err < 0) return err; return snd_pcm_open_noupdate(pcmp, snd_config, name, stream, mode, 0); }  

其中調用了函數snd_config_update(),這個函數就是加載alsa的配置文件中配置信息的。

這個函數直接調用了 snd_config_update_r,見下面函數註釋：

/**
 * /brief Updates a configuration tree by rereading the configuration files (if needed).
 * /param _top Address of the handle to the top level node.
 * /param _update Address of a pointer to private update information.
 * /param cfgs A list of configuration file names, delimited with ':'.
 *             If /p cfgs is set to /c NULL, the default global configuration
 *             file is used ("/usr/share/alsa/alsa.conf").
 * /return A non-negative value if successful, otherwise a negative error code.
 * /retval 0 No action is needed.
 * /retval 1 The configuration tree has been rebuilt.
 *
 * The global configuration files are specified in the environment variable
 * /c ALSA_CONFIG_PATH.
 *
 * /warning If the configuration tree is reread, all string pointers and
 * configuration node handles previously obtained from this tree become invalid.
 */
int snd_config_update_r(snd_config_t **_top, snd_config_update_t **_update, const char *cfgs)
{
int err;
const char *configs, *c;
unsigned int k;
size_t l;
snd_config_update_t *local;
snd_config_update_t *update;
snd_config_t *top;
assert(_top && _update);
top = *_top;
update = *_update;
configs = cfgs;
if (!configs) {
configs = getenv(ALSA_CONFIG_PATH_VAR);
if (!configs || !*configs)
configs = ALSA_CONFIG_PATH_DEFAULT;
}
//...............
}

/**
 * /brief Updates a configuration tree by rereading the configuration files (if needed).
 * /param _top Address of the handle to the top level node.
 * /param _update Address of a pointer to private update information.
 * /param cfgs A list of configuration file names, delimited with ':'.
 *             If /p cfgs is set to /c NULL, the default global configuration
 *             file is used ("/usr/share/alsa/alsa.conf").
 * /return A non-negative value if successful, otherwise a negative error code.
 * /retval 0 No action is needed.
 * /retval 1 The configuration tree has been rebuilt.
 *
 * The global configuration files are specified in the environment variable
 * /c ALSA_CONFIG_PATH.
 *
 * /warning If the configuration tree is reread, all string pointers and
 * configuration node handles previously obtained from this tree become invalid.
 */
int snd_config_update_r(snd_config_t **_top, snd_config_update_t **_update, const char *cfgs)
{
int err;
const char *configs, *c;
unsigned int k;
size_t l;
snd_config_update_t *local;
snd_config_update_t *update;
snd_config_t *top;
assert(_top && _update);
top = *_top;
update = *_update;
configs = cfgs;
if (!configs) {
configs = getenv(ALSA_CONFIG_PATH_VAR);
if (!configs || !*configs)
configs = ALSA_CONFIG_PATH_DEFAULT;
}
//...............
}

/**
 * /brief Updates a configuration tree by rereading the configuration files (if needed).
 * /param _top Address of the handle to the top level node.
 * /param _update Address of a pointer to private update information.
 * /param cfgs A list of configuration file names, delimited with ':'.
 *             If /p cfgs is set to /c NULL, the default global configuration
 *             file is used ("/usr/share/alsa/alsa.conf").
 * /return A non-negative value if successful, otherwise a negative error code.
 * /retval 0 No action is needed.
 * /retval 1 The configuration tree has been rebuilt.
 *
 * The global configuration files are specified in the environment variable
 * /c ALSA_CONFIG_PATH.
 *
 * /warning If the configuration tree is reread, all string pointers and
 * configuration node handles previously obtained from this tree become invalid.
 */
int snd_config_update_r(snd_config_t **_top, snd_config_update_t **_update, const char *cfgs)
{
int err;
const char *configs, *c;
unsigned int k;
size_t l;
snd_config_update_t *local;
snd_config_update_t *update;
snd_config_t *top;
assert(_top && _update);
top = *_top;
update = *_update;
configs = cfgs;
if (!configs) {
configs = getenv(ALSA_CONFIG_PATH_VAR);
if (!configs || !*configs)
configs = ALSA_CONFIG_PATH_DEFAULT;
}
//...............
}

/** * /brief Updates a configuration tree by rereading the configuration files (if needed). * /param _top Address of the handle to the top level node. * /param _update Address of a pointer to private update information. * /param cfgs A list of configuration file names, delimited with ':'. * If /p cfgs is set to /c NULL, the default global configuration * file is used ("/usr/share/alsa/alsa.conf"). * /return A non-negative value if successful, otherwise a negative error code. * /retval 0 No action is needed. * /retval 1 The configuration tree has been rebuilt. * * The global configuration files are specified in the environment variable * /c ALSA_CONFIG_PATH. * * /warning If the configuration tree is reread, all string pointers and * configuration node handles previously obtained from this tree become invalid. */ int snd_config_update_r(snd_config_t **_top, snd_config_update_t **_update, const char *cfgs) { int err; const char *configs, *c; unsigned int k; size_t l; snd_config_update_t *local; snd_config_update_t *update; snd_config_t *top; assert(_top && _update); top = *_top; update = *_update; configs = cfgs; if (!configs) { configs = getenv(ALSA_CONFIG_PATH_VAR); if (!configs || !*configs) configs = ALSA_CONFIG_PATH_DEFAULT; } //............... } 

參數一和二：snd_config, snd_config_global_update 都是全局變量，定義在conf.c中。

第三個參數cfgs，是包含配置文件名的字符串，snd_config_update調用它時沒有傳遞該參數，所以爲空。

 

snd_config_udpate_r首先分析第三個參數cfgs,如果爲空，就獲取系統環境變量ALSA_CONFIG_PATH_VAR值，如果還是爲空，就取ALSA_CONFIG_PATH_DEFAULT.

/** The name of the default files used by #snd_config_update. */
#define ALSA_CONFIG_PATH_DEFAULT ALSA_CONFIG_DIR "/alsa.conf"
#define ALSA_CONFIG_DIR "/usr/share/alsa"

/** The name of the default files used by #snd_config_update. */
#define ALSA_CONFIG_PATH_DEFAULT ALSA_CONFIG_DIR "/alsa.conf"
#define ALSA_CONFIG_DIR "/usr/share/alsa"

/** The name of the default files used by #snd_config_update. */
#define ALSA_CONFIG_PATH_DEFAULT ALSA_CONFIG_DIR "/alsa.conf"
#define ALSA_CONFIG_DIR "/usr/share/alsa"

/** The name of the default files used by #snd_config_update. */ #define ALSA_CONFIG_PATH_DEFAULT ALSA_CONFIG_DIR "/alsa.conf" #define ALSA_CONFIG_DIR "/usr/share/alsa"  

然後提取cfgs裏的文件名。

注意cfgs可以包含多個文件名，以：或空格分開。並把每個文件的文件信息保存在snd_config_update_t變量local中，其成員count記錄了有多少個文件，finfo則是對應的文件信息鏈表。

 

接下來分析第二個參數update，如果該參數值爲空（前面提到過，最開始是爲空），就重新去讀配置文件，否則與local變量比較，如果發現不一樣（配置文件被修改過）,也會跳轉到重讀配置文件的代碼。

重新讀取配置文件的代碼主要做三件事：

第一，以第一個參數snd_config_t * top爲參數調用snd_config_top(top);

第二，打開local中保存的每一個配置文件，並調用 snd_config_load(top,in),其中in是snd_input_t類型，是alsa內部定義的結構，代表輸入流; 

第三，snd_config_hooks(top,NULL);