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

AudioFlinger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。

2、audio_hw

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。

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。

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

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。

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

//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.sorc = 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 devicesif (!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;}}
}

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);elsestrlcpy(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-deviceHAL_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.tunastatus = -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;
}

/*** 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;

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,},
};

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？！

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。

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）？？？

--to be continued…