ANDROID音频系统散记之四：4.0音频系统HAL初探

昨天（2011-11-15）发布了Android4.0的源码，今天download下来，开始挺进4.0时代。简单看了一下，发现音频系统方面与2.3的有较多地方不同，下面逐一描述。

一、代码模块位置

1、AudioFlinger

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frameworks/base/services/audioflinger/
+-- Android.mk
+-- AudioBufferProvider.h
+-- AudioFlinger.cpp
+-- AudioFlinger.h
+-- AudioMixer.cpp
+-- AudioMixer.h
+-- AudioPolicyService.cpp
+-- AudioPolicyService.h
+-- AudioResampler.cpp
+-- AudioResamplerCubic.cpp
+-- AudioResamplerCubic.h
+-- AudioResampler.h
+-- AudioResamplerSinc.cpp
+-- AudioResamplerSinc.h

frameworks/base/services/audioflinger/

+--

Android.mk

+-- AudioBufferProvider.h

+-- AudioFlinger.cpp

+-- AudioFlinger.h

+-- AudioMixer.cpp

+-- AudioMixer.h

+-- AudioPolicyService.cpp

+-- AudioPolicyService.h

+-- AudioResampler.cpp

+-- AudioResamplerCubic.cpp

+-- AudioResamplerCubic.h

+-- AudioResampler.h

+-- AudioResamplerSinc.cpp

+-- AudioResamplerSinc.hAudioFlinger相关代码，好像这部分与2.3相差不大，至少接口是兼容的。值得注意的是：2.3位于这里的还有AudioHardwareGeneric、AudioHardwareInterface、 A2dpAudioInterface等一系列接口代码，现在都移除了。实际上，这些接口变更为legacy（有另外更好的实现方式，但也兼容之前的方法），取而代之的是要实现hardware/libhardware/include/hardware/audio.h提供的接口，这是一个较大的变化。

两种Audio Hardware HAL接口定义： 1/ legacy：hardware/libhardware_legacy/include/hardware_legacy /AudioHardwareInterface.h 2/ 非legacy：hardware/libhardware/include/hardware/audio.h

2、audio_hw

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hardware/libhardware_legacy/audio/
+-- A2dpAudioInterface.cpp
+-- A2dpAudioInterface.h
+-- Android.mk
+-- AudioDumpInterface.cpp
+-- AudioDumpInterface.h
+-- AudioHardwareGeneric.cpp
+-- AudioHardwareGeneric.h
+-- AudioHardwareInterface.cpp
+-- AudioHardwareStub.cpp
+-- AudioHardwareStub.h
+-- audio_hw_hal.cpp
+-- AudioPolicyCompatClient.cpp
+-- AudioPolicyCompatClient.h
+-- audio_policy_hal.cpp
+-- AudioPolicyManagerBase.cpp
+-- AudioPolicyManagerDefault.cpp
+-- AudioPolicyManagerDefault.h

hardware/libhardware_legacy/audio/

+--

A2dpAudioInterface.cpp

+-- A2dpAudioInterface.h

+-- Android.mk

+-- AudioDumpInterface.cpp

+-- AudioDumpInterface.h

+-- AudioHardwareGeneric.cpp

+-- AudioHardwareGeneric.h

+-- AudioHardwareInterface.cpp

+-- AudioHardwareStub.cpp

+-- AudioHardwareStub.h

+-- audio_hw_hal.cpp

+-- AudioPolicyCompatClient.cpp

+-- AudioPolicyCompatClient.h

+-- audio_policy_hal.cpp

+-- AudioPolicyManagerBase.cpp

+-- AudioPolicyManagerDefault.cpp

+-- AudioPolicyManagerDefault.h上面提及的AudioHardwareGeneric、 AudioHardwareInterface、A2dpAudioInterface等都放到libhardware_legacy里。事实上legacy也要封装成非legacy中的audio.h，确切的说需要一个联系legacy interface和not legacy interface的中间层，这里的audio_hw_hal.cpp就充当这样的一个角色了。因此，我们其实也可以把2.3之前的alsa_sound 这一套东西也搬过来。

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hardware/libhardware/modules/audio/
+-- Android.mk
+-- audio_hw.c
+-- audio_policy.c

hardware/libhardware/modules/audio/

+--

Android.mk

+-- audio_hw.c

+-- audio_policy.c这是一个stub（类似于2.3中的AudioHardwareStub），大多数函数只是简单的返回一个值，并没有实际操作，只是保证Android能得到一个audio hardware hal实例，从而启动运行，当然声音没有输出到外设的。在底层音频驱动或audio hardware hal还没有实现好的情况下，可以使用这个stub device，先让Android跑起来。

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device/samsung/tuna/audio/
+-- Android.mk
+-- audio_hw.c
+-- ril_interface.c
+-- ril_interface.h

device/samsung/tuna/audio/

+--

Android.mk

+-- audio_hw.c

+-- ril_interface.c

+-- ril_interface.h这是Samsung Tuna的音频设备抽象层，很有参考价值，计划以后就在它的基础上进行移植。它调用tinyalsa的接口，可见这个方案的底层音频驱动是alsa。

3、tinyalsa

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external/tinyalsa/
+-- Android.mk
+-- include
| +-- tinyalsa
| +-- asoundlib.h
+-- mixer.c ##类alsa-lib的control，作用音频部件开关、音量调节等
+-- pcm.c ##类alsa-lib的pcm，作用音频pcm数据回放录制
+-- README
+-- tinycap.c ## 类alsa_arecord
+-- tinymix.c ##类 alsa_amixer
+-- tinyplay.c ##类 alsa_aplay

external/tinyalsa/

+-- Android.mk

+-- include

| +-- tinyalsa

| +-- asoundlib.h

+-- mixer.c ##类alsa-lib的control，作用音频部件开关、音量调节等

+-- pcm.c ##类alsa-lib的pcm，作用音频pcm数据回放录制

+-- README

+-- tinycap.c ##类alsa_arecord

+-- tinymix.c ##类alsa_amixer

+-- tinyplay.c ##类alsa_aplay在2.3时代，Android还隐晦把它放在 android2.3.1-gingerbread/device/samsung/crespo/libaudio，现在终于把alsa-lib一脚踢开，小三变正室了，正名tinyalsa。这其实是历史的必然了，alsa-lib太过复杂繁琐了，我看得也很不爽；更重要的商业上面的考虑，必须移除被GNU GPL授权证所约束的部份，alsa-lib并不是个例。

注意：上面的hardware/libhardware_legacy/audio/、hardware/libhardware/modules /audio/、device/samsung/tuna/audio/是同层的。之一是legacy audio，用于兼容2.2时代的alsa_sound；之二是stub audio接口；之三是Samsung Tuna的音频抽象层实现。调用层次：AudioFlinger -> audio_hw -> tinyalsa。

二、Audio Hardware HAL加载

1、AudioFlinger

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// 加载audio hardware hal
static int load_audio_interface(const char *if_name, const hw_module_t **mod,
audio_hw_device_t **dev)
{
int rc;
//根据classid和if_name找到指定的动态库并加载，这里加载的是音频动态库，如libaudio.primary.tuna.so
rc = hw_get_module_by_class(AUDIO_HARDWARE_MODULE_ID, if_name, mod);
if (rc)
goto out;
//加载好的动态库模块必有个open方法，调用open方法打开音频设备模块
rc = audio_hw_device_open(*mod, dev);
LOGE_IF(rc, "couldn't open audio hw device in %s.%s (%s)",
AUDIO_HARDWARE_MODULE_ID, if_name, strerror(-rc));
if (rc)
goto out;
return 0;
out:
*mod = NULL;
*dev = NULL;
return rc;
}
//音频设备接口，hw_get_module_by_class需要根据这些字符串找到相关的音频模块库
static const char *audio_interfaces[] = {
"primary", //主音频设备，一般为本机codec
"a2dp", //a2dp设备，蓝牙高保真音频
"usb", //usb-audio设备，这个东东我2.3就考虑要实现了，现在终于支持了
};
#define ARRAY_SIZE(x) (sizeof((x))/sizeof(((x)[0])))
// ----------------------------------------------------------------------------
AudioFlinger::AudioFlinger()
: BnAudioFlinger(),
mPrimaryHardwareDev(0), mMasterVolume(1.0f), mMasterMute(false), mNextUniqueId(1),
mBtNrecIsOff(false)
{
}
void AudioFlinger::onFirstRef()
{
int rc = 0;
Mutex::Autolock _l(mLock);
mHardwareStatus = AUDIO_HW_IDLE;
//打开audio_interfaces数组定义的所有音频设备
for (size_t i = 0; i < ARRAY_SIZE(audio_interfaces); i++) {
const hw_module_t *mod;
audio_hw_device_t *dev;
rc = load_audio_interface(audio_interfaces[i], &mod, &dev);
if (rc)
continue;
LOGI("Loaded %s audio interface from %s (%s)", audio_interfaces[i],
mod->name, mod->id);
mAudioHwDevs.push(dev); //mAudioHwDevs 是一个Vector，存储已打开的audio hw devices
if (!mPrimaryHardwareDev) {
mPrimaryHardwareDev = dev;
LOGI("Using '%s' (%s.%s) as the primary audio interface",
mod->name, mod->id, audio_interfaces[i]);
}
}
mHardwareStatus = AUDIO_HW_INIT;
if (!mPrimaryHardwareDev || mAudioHwDevs.size() == 0) {
LOGE("Primary audio interface not found");
return;
}
//对audio hw devices进行一些初始化，如 mode、master volume的设置
for (size_t i = 0; i < mAudioHwDevs.size(); i++) {
audio_hw_device_t *dev = mAudioHwDevs[i];
mHardwareStatus = AUDIO_HW_INIT;
rc = dev->init_check(dev);
if (rc == 0) {
AutoMutex lock(mHardwareLock);
mMode = AUDIO_MODE_NORMAL;
mHardwareStatus = AUDIO_HW_SET_MODE;
dev->set_mode(dev, mMode);
mHardwareStatus = AUDIO_HW_SET_MASTER_VOLUME;
dev->set_master_volume(dev, 1.0f);
mHardwareStatus = AUDIO_HW_IDLE;
}
}
}

//加载audio hardware

hal

static int load_audio_interface(const char *if_name, const hw_module_t

**mod,

audio_hw_device_t **dev)

{

int rc;

//根据classid和if_name找到指定的动态库并加载，这里加载的是音频动态库，如libaudio.primary.tuna.so

rc = hw_get_module_by_class(AUDIO_HARDWARE_MODULE_ID, if_name, mod);

if (rc)

goto out;

//加载好的动态库模块必有个open方法，调用open方法打开音频设备模块

rc = audio_hw_device_open(*mod, dev);

LOGE_IF(rc, "couldn't open audio hw device in %s.%s (%s)",

AUDIO_HARDWARE_MODULE_ID, if_name, strerror(-rc));

if (rc)

goto out;

return 0;

out:

*mod = NULL;

*dev = NULL;

return rc;

}

//音频设备接口，hw_get_module_by_class需要根据这些字符串找到相关的音频模块库

static const char *audio_interfaces[] = {

"primary", //主音频设备，一般为本机codec

"a2dp", //a2dp设备，蓝牙高保真音频

"usb", //usb-audio设备，这个东东我2.3就考虑要实现了，现在终于支持了

};

#define ARRAY_SIZE(x) (sizeof((x))/sizeof(((x)[0])))

----------------------------------------------------------------------------

AudioFlinger::AudioFlinger()

: BnAudioFlinger(),

mPrimaryHardwareDev(0), mMasterVolume(1.0f), mMasterMute(false),

mNextUniqueId(1),

mBtNrecIsOff(false)

{

}

void AudioFlinger::onFirstRef()

{

int rc = 0;

Mutex::Autolock _l(mLock);

/* TODO: move all this work into an Init() function */

mHardwareStatus = AUDIO_HW_IDLE;

//打开audio_interfaces数组定义的所有音频设备

for (size_t i = 0; i < ARRAY_SIZE(audio_interfaces); i++) {

const hw_module_t *mod;

audio_hw_device_t *dev;

rc = load_audio_interface(audio_interfaces[i], &mod,

&dev);

if (rc)

continue;

LOGI("Loaded %s audio interface from %s (%s)",

audio_interfaces[i],

mod->name, mod->id);

mAudioHwDevs.push(dev); //mAudioHwDevs是一个Vector，存储已打开的audio hw

devices

if (!mPrimaryHardwareDev) {

mPrimaryHardwareDev = dev;

LOGI("Using '%s' (%s.%s) as the primary audio interface",

mod->name, mod->id, audio_interfaces[i]);

}

mHardwareStatus = AUDIO_HW_INIT;

if (!mPrimaryHardwareDev || mAudioHwDevs.size() == 0) {

LOGE("Primary audio interface not found");

return;

}

//对audio hw devices进行一些初始化，如mode、master volume的设置

for (size_t i = 0; i < mAudioHwDevs.size(); i++) {

audio_hw_device_t *dev = mAudioHwDevs[i];

mHardwareStatus = AUDIO_HW_INIT;

rc = dev->init_check(dev);

if (rc == 0) {

AutoMutex lock(mHardwareLock);

mMode = AUDIO_MODE_NORMAL;

mHardwareStatus = AUDIO_HW_SET_MODE;

dev->set_mode(dev, mMode);

mHardwareStatus = AUDIO_HW_SET_MASTER_VOLUME;

dev->set_master_volume(dev, 1.0f);

mHardwareStatus = AUDIO_HW_IDLE;

}

以上对AudioFlinger进行的分析，主要是通过hw_get_module_by_class()找到模块接口名字if_name相匹配的模块库，加载，然后audio_hw_device_open()调用模块的open方法，完成音频设备模块的初始化。

留意AudioFlinger的构造函数只有简单的私有变量的初始化操作了，把音频设备初始化放到onFirstRef()，Android终于改进了这一点，好的设计根本不应该把可能会失败的操作放到构造函数中。onFirstRef是RefBase类的一个虚函数，在构造sp的时候就会被调用。因此，在构造sp<AudioFlinger>的时候就会触发onFirstRef方法，从而完成音频设备模块初始化。

2、hw_get_module_by_class

我们接下来看看hw_get_module_by_class，实现在hardware/libhardware/ hardware.c中，它作用加载指定名字的模块库（.so文件），这个应该是用于加载所有硬件设备相关的库文件，并不只是音频设备。

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int hw_get_module_by_class(const char *class_id, const char *inst,
const struct hw_module_t **module)
{
int status;
int i;
const struct hw_module_t *hmi = NULL;
char prop[PATH_MAX];
char path[PATH_MAX];
char name[PATH_MAX];
if (inst)
snprintf(name, PATH_MAX, "%s.%s", class_id, inst);
else
strlcpy(name, class_id, PATH_MAX);
//这里我们以音频库为例，AudioFlinger调用到这个函数时，
//class_id=AUDIO_HARDWARE_MODULE_ID="audio"，inst="primary" (或"a2dp"或"usb")
//那么此时name="audio.primary"
for (i=0 ; i<HAL_VARIANT_KEYS_COUNT+1 ; i++) {
if (i < HAL_VARIANT_KEYS_COUNT) {
//通过property_get找到厂家标记如"ro.product.board=tuna"，这时prop="tuna"
if (property_get(variant_keys[i], prop, NULL) == 0) {
continue;
}
snprintf(path, sizeof(path), "%s/%s.%s.so",
HAL_LIBRARY_PATH2, name, prop); //#define HAL_LIBRARY_PATH2 "/vendor/lib/hw"
if (access(path, R_OK) == 0) break;
snprintf(path, sizeof(path), "%s/%s.%s.so",
HAL_LIBRARY_PATH1, name, prop); //#define HAL_LIBRARY_PATH1 "/system/lib/hw"
if (access(path, R_OK) == 0) break;
} else {
snprintf(path, sizeof(path), "%s/%s.default.so", //如没有指定的库文件，则加载default.so，即stub-device
HAL_LIBRARY_PATH1, name);
if (access(path, R_OK) == 0) break;
}
}
//到这里，完成一个模块库的完整路径名称，如path="/system/lib/hw /audio.primary.tuna.so"
//如何生成audio.primary.tuna.so？请看相关的Android.mk文件，其中有定义 LOCAL_MODULE := audio.primary.tuna
status = -ENOENT;
if (i < HAL_VARIANT_KEYS_COUNT+1) {
status = load(class_id, path, module); //加载模块库
}
return status;
}

int

hw_get_module_by_class(const char *class_id, const char *inst,

const struct hw_module_t **module)

{

int status;

int i;

const struct hw_module_t *hmi = NULL;

char prop[PATH_MAX];

char path[PATH_MAX];

char name[PATH_MAX];

if (inst)

snprintf(name, PATH_MAX, "%s.%s", class_id, inst);

else

strlcpy(name, class_id, PATH_MAX);

//这里我们以音频库为例，AudioFlinger调用到这个函数时，

//class_id=AUDIO_HARDWARE_MODULE_ID="audio"，inst="primary"(或"a2dp"

或"usb")

//那么此时name="audio.primary"

* Here we rely on the fact that calling dlopen multiple times on

* the same .so will simply increment a refcount (and not load

* a new copy of the library).

* We also assume that dlopen() is thread-safe.

/* Loop through the configuration variants looking for a module */

for (i=0 ; i<HAL_VARIANT_KEYS_COUNT+1 ; i++) {

if (i < HAL_VARIANT_KEYS_COUNT) {

//通过property_get找到厂家标记如"ro.product.board=tuna"，这时prop="tuna"

if (property_get(variant_keys[i], prop, NULL) == 0) {

continue;

}

snprintf(path, sizeof(path), "%s/%s.%s.so",

HAL_LIBRARY_PATH2, name, prop); //#define

HAL_LIBRARY_PATH2 "/vendor/lib/hw"

if (access(path, R_OK) == 0) break;

snprintf(path, sizeof(path), "%s/%s.%s.so",

HAL_LIBRARY_PATH1, name, prop); //#define

HAL_LIBRARY_PATH1 "/system/lib/hw"

if (access(path, R_OK) == 0) break;

} else {

snprintf(path, sizeof(path), "%s/%s.default.so",

//如没有指定的库文件，则加载default.so，即stub-device

HAL_LIBRARY_PATH1, name);

if (access(path, R_OK) == 0) break;

}

//到这里，完成一个模块库的完整路径名称，如path="/system/lib/hw/audio.primary.tuna.so"

//如何生成audio.primary.tuna.so？请看相关的Android.mk文件，其中有定义LOCAL_MODULE :=

audio.primary.tuna

status = -ENOENT;

if (i < HAL_VARIANT_KEYS_COUNT+1) {

/* load the module, if this fails, we're doomed, and we should

not try

* to load a different variant. */

status = load(class_id, path, module); //加载模块库

}

return status;

}

load()函数不详细分析了，它通过dlopen加载库文件，然后dlsym找到hal_module_info的首地址。我们先看看 hal_module_info的定义： view plain print ?

typedef struct hw_module_t {
uint32_t tag;
uint16_t version_major;
uint16_t version_minor;
const char *id;
const char *name;
const char *author;
struct hw_module_methods_t* methods;
void* dso;
uint32_t reserved[32-7];
} hw_module_t;
typedef struct hw_module_methods_t {
int (*open)(const struct hw_module_t* module, const char* id,
struct hw_device_t** device);
} hw_module_methods_t;

/**

* Every hardware module must have a data structure named

HAL_MODULE_INFO_SYM

* and the fields of this data structure must begin with hw_module_t

* followed by module specific information.

typedef struct hw_module_t {

/** tag must be initialized to HARDWARE_MODULE_TAG */

uint32_t tag;

/** major version number for the module */

uint16_t version_major;

/** minor version number of the module */

uint16_t version_minor;

/** Identifier of module */

const char *id;

/** Name of this module */

const char *name;

/** Author/owner/implementor of the module */

const char *author;

/** Modules methods */

struct hw_module_methods_t* methods;

/** module's dso */

void* dso;

/** padding to 128 bytes, reserved for future use */

uint32_t reserved[32-7];

} hw_module_t;

typedef struct hw_module_methods_t {

/** Open a specific device */

int (*open)(const struct hw_module_t* module, const char* id,

struct hw_device_t** device);

} hw_module_methods_t;这个结构体很重要，注释很详细。dlsym拿到这个结构体的首地址后，就可以调用 Modules methods进行设备模块的初始化了。设备模块中，都应该按照这个格式初始化好这个结构体，否则dlsym找不到它，也就无法调用Modules methods进行初始化了。

例如，在audio_hw.c中，它是这样定义的： view plain print ?

static struct hw_module_methods_t hal_module_methods = {
.open = adev_open,
};
struct audio_module HAL_MODULE_INFO_SYM = {
.common = {
.tag = HARDWARE_MODULE_TAG,
.version_major = 1,
.version_minor = 0,
.id = AUDIO_HARDWARE_MODULE_ID,
.name = "Tuna audio HW HAL",
.author = "The Android Open Source Project",
.methods = &hal_module_methods,
},
};

static

struct hw_module_methods_t hal_module_methods = {

.open = adev_open,

};

struct audio_module HAL_MODULE_INFO_SYM = {

.common = {

.tag = HARDWARE_MODULE_TAG,

.version_major = 1,

.version_minor = 0,

.id = AUDIO_HARDWARE_MODULE_ID,

.name = "Tuna audio HW HAL",

.author = "The Android Open Source Project",

.methods = &hal_module_methods,

};

3、audio_hw

好了，经过一番周折，又dlopen又dlsym的，终于进入我们的audio_hw。这部分没什么好说的，按照 hardware/libhardware/include/hardware/audio.h定义的接口实现就行了。这些接口全扔到一个结构体里面的，这样做的好处是：不必用大量的dlsym来获取各个接口函数的地址，只需找到这个结构体即可，从易用性和可扩充性来说，都是首选方式。

接口定义如下：

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struct audio_hw_device {
struct hw_device_t common;
uint32_t (*get_supported_devices)(const struct audio_hw_device *dev);
int (*init_check)(const struct audio_hw_device *dev);
int (*set_voice_volume)(struct audio_hw_device *dev, float volume);
int (*set_master_volume)(struct audio_hw_device *dev, float volume);
int (*set_mode)(struct audio_hw_device *dev, int mode);
int (*set_mic_mute)(struct audio_hw_device *dev, bool state);
int (*get_mic_mute)(const struct audio_hw_device *dev, bool *state);
int (*set_parameters)(struct audio_hw_device *dev, const char *kv_pairs);
char * (*get_parameters)(const struct audio_hw_device *dev,
const char *keys);
size_t (*get_input_buffer_size)(const struct audio_hw_device *dev,
uint32_t sample_rate, int format,
int channel_count);
int (*open_output_stream)(struct audio_hw_device *dev, uint32_t devices,
int *format, uint32_t *channels,
uint32_t *sample_rate,
struct audio_stream_out **out);
void (*close_output_stream)(struct audio_hw_device *dev,
struct audio_stream_out* out);
int (*open_input_stream)(struct audio_hw_device *dev, uint32_t devices,
int *format, uint32_t *channels,
uint32_t *sample_rate,
audio_in_acoustics_t acoustics,
struct audio_stream_in **stream_in);
void (*close_input_stream)(struct audio_hw_device *dev,
struct audio_stream_in *in);
int (*dump)(const struct audio_hw_device *dev, int fd);
};
typedef struct audio_hw_device audio_hw_device_t;

struct

audio_hw_device {

struct hw_device_t common;

/**

* used by audio flinger to enumerate what devices are supported by

* each audio_hw_device implementation.

* Return value is a bitmask of 1 or more values of audio_devices_t

uint32_t (*get_supported_devices)(const struct audio_hw_device

*dev);

/**

* check to see if the audio hardware interface has been

initialized.

* returns 0 on success, -ENODEV on failure.

int (*init_check)(const struct audio_hw_device *dev);

/** set the audio volume of a voice call. Range is between 0.0 and

1.0 */

int (*set_voice_volume)(struct audio_hw_device *dev, float volume);

/**

* set the audio volume for all audio activities other than voice

call.

* Range between 0.0 and 1.0. If any value other than 0 is returned,

* the software mixer will emulate this capability.

int (*set_master_volume)(struct audio_hw_device *dev, float volume);

/**

* setMode is called when the audio mode changes. AUDIO_MODE_NORMAL

mode

* is for standard audio playback, AUDIO_MODE_RINGTONE when a

ringtone is

* playing, and AUDIO_MODE_IN_CALL when a call is in progress.

int (*set_mode)(struct audio_hw_device *dev, int mode);

/* mic mute */

int (*set_mic_mute)(struct audio_hw_device *dev, bool state);

int (*get_mic_mute)(const struct audio_hw_device *dev, bool *state);

/* set/get global audio parameters */

int (*set_parameters)(struct audio_hw_device *dev, const char

*kv_pairs);

* Returns a pointer to a heap allocated string. The caller is

responsible

* for freeing the memory for it.

char * (*get_parameters)(const struct audio_hw_device *dev,

const char *keys);

/* Returns audio input buffer size according to parameters passed or

* 0 if one of the parameters is not supported

size_t (*get_input_buffer_size)(const struct audio_hw_device *dev,

uint32_t sample_rate, int format,

int channel_count);

/** This method creates and opens the audio hardware output stream

int (*open_output_stream)(struct audio_hw_device *dev, uint32_t

devices,

int *format, uint32_t *channels,

uint32_t *sample_rate,

struct audio_stream_out **out);

void (*close_output_stream)(struct audio_hw_device *dev,

struct audio_stream_out* out);

/** This method creates and opens the audio hardware input stream */

int (*open_input_stream)(struct audio_hw_device *dev, uint32_t

devices,

int *format, uint32_t *channels,

uint32_t *sample_rate,

audio_in_acoustics_t acoustics,

struct audio_stream_in **stream_in);

void (*close_input_stream)(struct audio_hw_device *dev,

struct audio_stream_in *in);

/** This method dumps the state of the audio hardware */

int (*dump)(const struct audio_hw_device *dev, int fd);

};

typedef struct audio_hw_device audio_hw_device_t;

注：这是比较标准的C接口设计方法了，但是个人感觉还是用C++比较好，直观易读。2.3之前都是用C++实现这些接口设计的，到了4.0，不知道为何采纳用C？不会理由是做底层的不懂C++吧？！

三、Audio Hardware HAL的legacy实现

之前提到两种Audio Hardware HAL接口定义：

1/ legacy：hardware/libhardware_legacy/include/hardware_legacy /AudioHardwareInterface.h

2/ 非legacy：hardware/libhardware/include/hardware/audio.h

前者是2.3及之前的音频设备接口定义，后者是4.0的接口定义。

为了兼容以前的设计，4.0实现一个中间层：hardware/libhardware_legacy/audio/audio_hw_hal.cpp，结构与其他的audio_hw.c大同小异，差别在于open方法：

view plain print ?

static int legacy_adev_open(const hw_module_t* module, const char* name,
hw_device_t** device)
{
......
ladev->hwif = createAudioHardware();
if (!ladev->hwif) {
ret = -EIO;
goto err_create_audio_hw;
}
......
}

static

int legacy_adev_open(const hw_module_t* module, const char* name,

hw_device_t** device)

{

......

ladev->hwif = createAudioHardware();

if (!ladev->hwif) {

ret = -EIO;

goto err_create_audio_hw;

}

......

}看到那个熟悉的createAudioHardware()没有？这是以前我提到的Vendor Specific Audio接口，然后新的接口再调用ladev->hwif的函数就是了。因此老一套的alsa-lib、alsa-utils和alsa_sound也可以照搬过来，这里的文件被编译成静态库的，因此你需要修改 alsa_sound里面的Android.mk文件，链接这个静态库。还有alsa_sound的命名空间原来是“android”，现在需要改成 “android_audio_legacy”。

四、a2dp Audio HAL的实现

4.0的a2dp audio hal放到bluez里实现了，我找了好一会才找到：

external/Bluetooth/bluez/audio/android_audio_hw.c

大致与上面提到的audio_hw.c类似，因为都是基于audio.h定义的接口来实现的。

如果需要编译这个库，须在BoardConfig.mk里定义：

BOARD_HAVE_BLUETOOTH := true

开始还提到现在支持3种audio设备了，分别是primary、a2dp和usb。目前剩下usb audio hal我没有找到，不知是否需要自己去实现？其实alsa-driver都支持大部分的usb-audio设备了，因此上层也可调用tinyalsa的接口，就像samsung tuna的audio_hw.c那样。

五、音质改进？？？

可使用audio echo cancel和更好的resampler（SRC）？？？

ANDROID音频系统散记之四：4.0音频系统HAL初探

一、代码模块位置

1、AudioFlinger

2、audio_hw

3、tinyalsa

二、Audio Hardware HAL加载

1、AudioFlinger

2、hw_get_module_by_class

3、audio_hw

三、Audio Hardware HAL的legacy实现

四、a2dp Audio HAL的实现

五、音质改进？？？

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