AndroidQ | AudioPolicy

AudioPolicyService是负责音频策略的制定，AudioFlinger负责音频策略的具体执行，两个模块各司其职。

1.AudioPolicyService

对于应用开发者来说，需要播放音频的时候只会去创建个AudioTrack去实现，其中的参数有主要的streamType、sampleRate、format等，还有的默认就不会设置参数，他不会去关心你当前设备的具体音频播放模块是什么样的，是speaker还是lineout、btout。系统跑起来后有许多应用都要播放音频，有的是播放music，有的是播放ring，具体每个音频对应的音量、音效等都是需要一个策略来管理的，这就是AudioPolicyService的任务，当然不止我上面说的这些。

AudioPolicyService和AudioFlinger一样，也是一个server，其提供的功能我们从其接口来入手(接口决定了功能)，接口为IAudioPolicyService：

class IAudioPolicyService : public IInterface
{
public:
    DECLARE_META_INTERFACE(AudioPolicyService);
	...
    virtual audio_io_handle_t getOutput(audio_stream_type_t stream) = 0;
	...
}

AudioSystem中又封装了AudioPolicyService的功能，AudioSystem首先要获取AudioPolicyService的Proxy：

const sp<IAudioPolicyService> AudioSystem::get_audio_policy_service()
{
    sp<IAudioPolicyService> ap;
    sp<AudioPolicyServiceClient> apc;
    {
        Mutex::Autolock _l(gLockAPS);
        if (gAudioPolicyService == 0) {
            sp<IServiceManager> sm = defaultServiceManager();
            sp<IBinder> binder;
            do {
                binder = sm->getService(String16("media.audio_policy"));
                if (binder != 0)
                    break;
                ALOGW("AudioPolicyService not published, waiting...");
                usleep(500000); // 0.5 s
            } while (true);
            if (gAudioPolicyServiceClient == NULL) {
                gAudioPolicyServiceClient = new AudioPolicyServiceClient();
            }
            binder->linkToDeath(gAudioPolicyServiceClient);
            gAudioPolicyService = interface_cast<IAudioPolicyService>(binder);
			...
        }
        ap = gAudioPolicyService;
    }
	...
    return ap;
}

以getOutput为例，看下AudioSystem的功能封装

audio_io_handle_t AudioSystem::getOutput(audio_stream_type_t stream)
{
    const sp<IAudioPolicyService>& aps = AudioSystem::get_audio_policy_service();
    if (aps == 0) return 0;
    return aps->getOutput(stream);
}

这样就最终会调用到AudioPolicyService的Server端，Server端通过继承BinderService和BnAudioPolicyService来实现Binder功能：

class AudioPolicyService :
    public BinderService<AudioPolicyService>,
    public BnAudioPolicyService,
    public IBinder::DeathRecipient
{
	...
}

AudioPolicyService在第一次被引用的时候会创建关键的对象AudioPolicyClient和AudioPolicyManager

void AudioPolicyService::onFirstRef()
{
    {
        Mutex::Autolock _l(mLock);

        // 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);
    }
    // load audio processing modules
    sp<AudioPolicyEffects>audioPolicyEffects = new AudioPolicyEffects();
    {
        Mutex::Autolock _l(mLock);
        mAudioPolicyEffects = audioPolicyEffects;
    }

    mUidPolicy = new UidPolicy(this);
    mUidPolicy->registerSelf();

    mSensorPrivacyPolicy = new SensorPrivacyPolicy(this);
    mSensorPrivacyPolicy->registerSelf();
}

相关类的关系图

看这个图可以看到两个关键的接口AudioPolicyInterface和AudioPolicyClientInterface，这两个接口主要就是定义AudioPolicy该有的功能，关于这两个接口的命名我们来看下Android的解释

// The AudioPolicyInterface and AudioPolicyClientInterface classes define the communication interfaces
// between the platform specific audio policy manager and Android generic audio policy manager.
// The platform specific audio policy manager must implement methods of the AudioPolicyInterface class.
// This implementation makes use of the AudioPolicyClientInterface to control the activity and
// configuration of audio input and output streams.
//
// The platform specific audio policy manager is in charge of the audio routing and volume control
// policies for a given platform.
// The main roles of this module are:
// - keep track of current system state (removable device connections, phone state, user requests…).
// System state changes and user actions are notified to audio policy manager with methods of the AudioPolicyInterface.
// - process getOutput() queries received when AudioTrack objects are created: Those queries
// return a handler on an output that has been selected, configured and opened by the audio policy manager and that
// must be used by the AudioTrack when registering to the AudioFlinger with the createTrack() method.
// When the AudioTrack object is released, a putOutput() query is received and the audio policy manager can decide
// to close or reconfigure the output depending on other streams using this output and current system state.
// - similarly process getInput() and putInput() queries received from AudioRecord objects and configure audio inputs.
// - process volume control requests: the stream volume is converted from an index value (received from UI) to a float value
// applicable to each output as a function of platform specific settings and current output route (destination device). It
// also make sure that streams are not muted if not allowed (e.g. camera shutter sound in some countries).
//
// The platform specific audio policy manager is provided as a shared library by platform vendors (as for libaudio.so)
// and is linked with libaudioflinger.so

一般接口函数的调用流程遵循如下关系：AudioSystem-》AudioPolicyService-》AudioPolicyManager-》AudioPolicyClient-》AudioFlinger，接下来我们主要分析接口的实现。

2.AudioPolicyManager

AudioPolicyManager是AudioPolicy的管理者，主要就是实现audiopolicy的功能，所以他直接继承于AudioPolicyInterface。

我们来看一张图，发现没，两个类的方法大部分是一样的，这就说明AudioPolicyService一般会调用AudioPolicyManager中的方法来实现具体功能。在AudioPolicyService接口的具体实现中，区分了调用AudioPolicyManager和AudioPolicyClient的实现，调用AudioPolicyManager的实现是在文件AudioPolicyInterfaceImpl.cpp中。

在目录frameworks\av\services\audiopolicy中，有两个目录manager和managerdefault，分别是为了创建AudioPolicyManager和实现AudioPolicyManager的功能。

2.1 createAudioPolicyManager

extern "C" AudioPolicyInterface* createAudioPolicyManager(
        AudioPolicyClientInterface *clientInterface)
{
    AudioPolicyManager *apm = new AudioPolicyManager(clientInterface);
    status_t status = apm->initialize();
    if (status != NO_ERROR) {
        delete apm;
        apm = nullptr;
    }
    return apm;
}

2.2 解析AudioPolicy配置文件

AudioPolicyManager::AudioPolicyManager(AudioPolicyClientInterface *clientInterface)
        : AudioPolicyManager(clientInterface, false /*forTesting*/)
{
    loadConfig();
}

void AudioPolicyManager::loadConfig() {
    if (deserializeAudioPolicyXmlConfig(getConfig()) != NO_ERROR) {
        ALOGE("could not load audio policy configuration file, setting defaults");
        getConfig().setDefault();
    }
}

AudioPolicy的配置文件有如下这些

audio_policy_configuration.xml
audio_policy_configuration_a2dp_offload_disabled.xml
audio_policy_configuration_bluetooth_legacy_hal.xml
…

配置文件太长，就不在这里粘贴了，我主要说下最后解析出来的配置文件对应到代码中哪些类型

• module节点对应到HwModule
• mixPort节点对应到AudioPort
• profile节点对应到AudioProfile
• gains对应到AudioGain，最后会一起加入到AudioPort中，这是其组成的一部分
• devicePort对应到DeviceDescriptor
• route对应到AudioRoute，最终会加入到HwModule中

具体每个类的相互关系如下图：

XML配置文件中，还有一类是关于音量的，这个是体现在VolumeCurve中

2.3 Manager

AudioPolicyService作为一个service，它一般负责命令的交互，具体到Policy的实际管理就是在类AudioPolicyManager中处理，AudioPolicyManager中负责太多的功能，要管理每个流的音量，设备的打开关闭，寻找合适的输出设备等等，我们来看其中的一个功能startOutput，它首先是通过AudioSystem来调用下来的

status_t AudioPolicyManager::startOutput(audio_port_handle_t portId)
{
...
    status = startSource(outputDesc, client, &delayMs);
...
}

status_t AudioPolicyManager::startSource(const sp<SwAudioOutputDescriptor>& outputDesc,
                                         const sp<TrackClientDescriptor>& client,
                                         uint32_t *delayMs)
{
...
        checkAndSetVolume(curves, client->volumeSource(),
...
}

status_t AudioPolicyManager::checkAndSetVolume(IVolumeCurves &curves,
                                               VolumeSource volumeSource,
                                               int index,
                                               const sp<AudioOutputDescriptor>& outputDesc,
                                               DeviceTypeSet deviceTypes,
                                               int delayMs,
                                               bool force)
{
...
    if (isVoiceVolSrc || isBtScoVolSrc) {
...
            mpClientInterface->setVoiceVolume(voiceVolume, delayMs);
...
    }
    return NO_ERROR;
}

这里又最终调用到AudioPolicyClient中的函数

3.AudioPolicyClient

AudioPolicyClient中的实现都在文件AudioPolicyClientImpl.cpp中

status_t AudioPolicyService::AudioPolicyClient::setVoiceVolume(float volume, int delay_ms)
{
    return mAudioPolicyService->setVoiceVolume(volume, delay_ms);
}