Android 7.0 Audio: AudioPolicyService和他的码农APM

AudioPolicyService负责音频策略处理，包括根据手机的各种状态和器件的存在、插拔状态、用户使能状态，给不同的audio stream选择不同的输入输出设备，控制不同设备的增益等。

代码层面，该类则主要是维护3个线程looper，通过不同的辅助类，向client提供服务。

AudioPolicyService的相关类关系如下，

同AudioFlinger，APS在audioServer里启动。AudioPolicyService有两种实现方案，由USE_LEGACY_AUDIO_POLICY控制，

如果是LEGACY方法，使用一个ops函数集操作接口，类似驱动模块的写法，在AudioPolicyClientImplLegacy.cpp实现所有函数功能，这些函数实际上就是对AF的功能封装，在其构造函数中初始，这种方法以及废弃，我们就不具体分析。

另外一种方法，在AudioPolicyService的onFirstRef中，创建了如下几个重要的线程和实例。

void AudioPolicyService::onFirstRef()

{

// start tone playback thread

mTonePlaybackThread = new AudioCommandThread(String8("ApmTone"), this);

// start audio commands thread

mAudioCommandThread = new AudioCommandThread(String8("ApmAudio"), this);

// start output activity command thread

mOutputCommandThread = new AudioCommandThread(String8("ApmOutput"), this);

mAudioPolicyClient = new AudioPolicyClient(this);

mAudioPolicyManager = createAudioPolicyManager(mAudioPolicyClient);

spaudioPolicyEffects = new AudioPolicyEffects();

}

AudioCommandThread是AudioPolicyService的内部类，继承自Thread，在其onFirstRef启动线程，

void AudioPolicyService::AudioCommandThread::onFirstRef()

{

run(mName.string(), ANDROID_PRIORITY_AUDIO);

}

它对外提供各种接口，如startOutputCommand，向其队列发消息，如START_OUTPUT，然后由其threadLoop处理这些消息，这样的好处是避免消息拥塞，避免调用栈过长，阻塞系统等，代码结构清晰，系统耦合性也低。

bool AudioPolicyService::AudioCommandThread::threadLoop()

{

nsecs_t waitTime = -1;

mLock.lock();

while (!exitPending())

{

sp svc;

while (!mAudioCommands.isEmpty() && !exitPending()) {

nsecs_t curTime = systemTime();

// commands are sorted by increasing time stamp: execute them from index 0 and up

if (mAudioCommands[0]->mTime <= curTime) {

sp command = mAudioCommands[0];

mAudioCommands.removeAt(0);

mLastCommand = command;

switch (command->mCommand) {

…

case START_OUTPUT: {

StartOutputData *data = (StartOutputData *)command->mParam.get();

ALOGV("AudioCommandThread() processing start output %d",

data->mIO);

svc = mService.promote();

if (svc == 0) {

command->mStatus = UNKNOWN_ERROR;

break;

}

mLock.unlock();

command->mStatus = svc->doStartOutput(data->mIO, data->mStream, data->mSession);

mLock.lock();

…

threadLoop主要调用ToneGenerator，AudioSystem，AF,还有自身的功能实现函数AudioPolicyIntefaceImpl来完成相应的消息处理。实际上，APII也只是封装了对APM（AudioPolicyManager）的调用而已。

AudioPolicyService还有一个NotificationClient子类，提供接口registerClient让客户端注册，貌似只有AS一个客户，和AF类似。

AudioSystem.cpp(frameworks\av\media\libmedia): af->registerClient(afc);

AudioSystem.cpp(frameworks\av\media\libmedia): ap->registerClient(apc);

AudioSystem.cpp(frameworks\av\media\libmedia): af->registerClient(afc);

void AudioPolicyService::registerClient(const sp& client)

{

sp notificationClient = new NotificationClient(this,

client,

uid);

ALOGV("registerClient() client %p, uid %d", client.get(), uid);

mNotificationClients.add(uid, notificationClient);

}

AudioPolicyClient : publicAudioPolicyClientInterface，代码实现在AudioPolicyClientImpl.cpp，这个类也只是一个功能封装类，调用了AF，AudioSystem，APS的接口。在APS的构造函数中作为APM的构造参数传入，所以它是APM调用AF，AudioSystem，APS的桥梁。

mAudioPolicyClient = new AudioPolicyClient(this);

mAudioPolicyManager = createAudioPolicyManager(mAudioPolicyClient);

对于APM类，

APS创建了它的实例，自己的文件不怎么使用它，但通过looper，使用APII来调用APM的功能。我们先分析这些表面的调用关系，之后要思考其背后的设计意图。

通过后面分析APM，会发现前面围绕APS讲了一堆的策略、架构、汇报关系的东西，其实真正干活的只有APM，它就是APS架构里面的码农，干了个体力活还不显眼，毕竟大家喜欢看架构和策略方面的东西。

APM还是有些技能的，首先他有几个工具，他把他们交给AudioPolicyConfig去管理（在AudioPolicyConfig.h中），

HwModuleCollection &mHwModules;

DeviceVector &mAvailableOutputDevices;

DeviceVector &mAvailableInputDevices;

sp &mDefaultOutputDevices;

VolumeCurvesCollection *mVolumeCurves;

在APM的构造函数里，

首先从/system/etc/audio_policy_configuration.xml加载相关配置信息，并序列化、以及初始，不再使用/system/etc/audio_policy.conf文件，这里会用到PolicySerializer和EngineInstance。

后面就是围绕AudioPolicyConfig管理的几个类，根据配置文件进行初始化，如，代码太多，不一一细化。

AudioPolicyManager::AudioPolicyManager(AudioPolicyClientInterface *clientInterface) {

1.加载配置信息

#ifdef USE_XML_AUDIO_POLICY_CONF

mVolumeCurves = new VolumeCurvesCollection();

AudioPolicyConfig config(mHwModules, mAvailableOutputDevices, mAvailableInputDevices,

mDefaultOutputDevice, speakerDrcEnabled,

static_cast(mVolumeCurves));

PolicySerializer serializer;

if (serializer.deserialize(AUDIO_POLICY_XML_CONFIG_FILE, config) != NO_ERROR) {

config.setDefault();

}

#endif

audio_policy::EngineInstance *engineInstance = audio_policy::EngineInstance::getInstance();

mEngine = engineInstance->queryInterface();

mEngine->setObserver(this);

status_t status = mEngine->initCheck();

…

2. 加载配置文件里的模块，并一一初始化

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

mHwModules[i]->mHandle = mpClientInterface->loadHwModule(mHwModules[i]->getName());

2.1 打开输出流设备

for (size_t j = 0; j < mHwModules[i]->mOutputProfiles.size(); j++)

{

const sp outProfile = mHwModules[i]->mOutputProfiles[j];

sp outputDesc = new SwAudioOutputDescriptor(outProfile,

mpClientInterface);

audio_io_handle_t output = AUDIO_IO_HANDLE_NONE;

status_t status = mpClientInterface->openOutput(outProfile->getModuleHandle(),

&output,

&config,

&outputDesc->mDevice,

address,

&outputDesc->mLatency,

outputDesc->mFlags);

addOutput(output, outputDesc);

setOutputDevice(outputDesc,

outputDesc->mDevice,

true,

NULL,

address.string());

}

2.2 打开输入流设备

for (size_t j = 0; j < mHwModules[i]->mInputProfiles.size(); j++)

{

const sp inProfile = mHwModules[i]->mInputProfiles[j];

sp inputDesc =

new AudioInputDescriptor(inProfile);

audio_io_handle_t input = AUDIO_IO_HANDLE_NONE;

status_t status = mpClientInterface->openInput(inProfile->getModuleHandle(),

&input,

&config,

&inputDesc->mDevice,

address,

AUDIO_SOURCE_MIC,

AUDIO_INPUT_FLAG_NONE);

}

3.设置输入输出状态和设备使能专题

// make sure all attached devices have been allocated a unique ID

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

if (!mAvailableOutputDevices[i]->isAttached()) {

ALOGW("Output device %08x unreachable", mAvailableOutputDevices[i]->type());

mAvailableOutputDevices.remove(mAvailableOutputDevices[i]);

continue;

}

// The device is now validated and can be appended to the available devices of the engine

mEngine->setDeviceConnectionState(mAvailableOutputDevices[i],

AUDIO_POLICY_DEVICE_STATE_AVAILABLE);

i++;

}

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

if (!mAvailableInputDevices[i]->isAttached()) {

ALOGW("Input device %08x unreachable", mAvailableInputDevices[i]->type());

mAvailableInputDevices.remove(mAvailableInputDevices[i]);

continue;

}

// The device is now validated and can be appended to the available devices of the engine

mEngine->setDeviceConnectionState(mAvailableInputDevices[i],

AUDIO_POLICY_DEVICE_STATE_AVAILABLE);

i++;

}

updateDevicesAndOutputs();

}

一份完整的audio_policy_configuration.xml如下，可对照这个配置分解来分析APM的构造过程，

Earpiece

Speaker

Telephony Tx

Built-In Mic

Built-In Back Mic

FM Tuner

Telephony Rx

Speaker

samplingRates="48000" channelMasks="AUDIO_CHANNEL_OUT_STEREO"/>

flags="AUDIO_OUTPUT_FLAG_FAST|AUDIO_OUTPUT_FLAG_RAW">

samplingRates="48000" channelMasks="AUDIO_CHANNEL_OUT_STEREO"/>

flags="AUDIO_OUTPUT_FLAG_DEEP_BUFFER">

samplingRates="48000" channelMasks="AUDIO_CHANNEL_OUT_STEREO"/>

flags="AUDIO_OUTPUT_FLAG_DIRECT">

samplingRates="8000,11025,16000,22050,32000,44100,48000,64000,88200,96000,128000,176400,192000" channelMasks="dynamic"/>

flags="AUDIO_OUTPUT_FLAG_DIRECT|AUDIO_OUTPUT_FLAG_DIRECT_PCM">

samplingRates="8000,11025,12000,16000,22050,24000,32000,44100,48000,64000,88200,96000,128000,176400,192000"

channelMasks="AUDIO_CHANNEL_OUT_MONO,AUDIO_CHANNEL_OUT_STEREO,AUDIO_CHANNEL_OUT_2POINT1,AUDIO_CHANNEL_OUT_QUAD,AUDIO_CHANNEL_OUT_PENTA,AUDIO_CHANNEL_OUT_5POINT1,AUDIO_CHANNEL_OUT_6POINT1,AUDIO_CHANNEL_OUT_7POINT1"/>

samplingRates="8000,11025,12000,16000,22050,24000,32000,44100,48000,64000,88200,96000,128000,176400,192000"

flags="AUDIO_OUTPUT_FLAG_DIRECT|AUDIO_OUTPUT_FLAG_COMPRESS_OFFLOAD|AUDIO_OUTPUT_FLAG_NON_BLOCKING">

samplingRates="8000,11025,12000,16000,22050,24000,32000,44100,48000"