1. audioflinger建立過程

在Android8.0的音訊系統中，AudioFlinger是一個C++的Binder服務，執行在HAL程序中，它是在audioserver.c

//frameworks/av/media/audioserver/audioserver.rc
service audioserver /system/bin/audioserver
    class main
    user audioserver
    # media gid needed for /dev/fm (radio) and for /data/misc/media (tee)
    group audio camera drmrpc inet media mediadrm net_bt net_bt_admin net_bw_acct oem_2901
    ioprio rt 4
 

    writepid /dev/cpuset/foreground/tasks /dev/stune/foreground/tasks
    onrestart restart audio-hal-2-0

on property:vts.native_server.on=1
    stop audioserver
on property:vts.native_server.on=0
    start audioserver

audioserver的入口函式是main（），程式碼如下：

int main(int argc __unused, char **argv)
{
    ......
    sp<
 
ProcessState> proc(ProcessState::self());
        sp<IServiceManager> sm = defaultServiceManager();
        ALOGI("ServiceManager: %p", sm.get());
        AudioFlinger::instantiate();   //建立AudioFlinger服務
        AudioPolicyService::instantiate(); //建立AudioPolicyService服務
        AAudioService::instantiate
 
();  //建立AAudioService服務
        RadioService::instantiate();   //建立RadioService服務
        SoundTriggerHwService::instantiate();
#ifdef VRAUDIOSERVICE_ENABLE
        VRAudioServiceNative::instantiate();
#endif
        ProcessState::self()->startThreadPool();
// FIXME: remove when BUG 31748996 is fixed
        android::hardware::ProcessState::self()->startThreadPool();
        IPCThreadState::self()->joinThreadPool();
}

AudioFlinger::instantiate()並不屬於AudioFlinger的內部類，而是BinderService類的一個實現包括AudioFlinger,AudioPolicy等在內的幾個服務都繼承自這個統一的Binder的服務類，具體實現在BinderService.h中

// frameworks/native/include/binder/BinderService.h
    static status_t publish(bool allowIsolated = false) {
        sp<IServiceManager> sm(defaultServiceManager());
        //SERVICE是檔案中定義的一個模板，AudioFlinger呼叫了instantiate()函式，
        //所以當前的SERVICE為AudioFlinger
        return sm->addService(
                String16(SERVICE::getServiceName()),
                new SERVICE(), allowIsolated);
    }

    static void instantiate() { publish(); }

可以看出publish()函式所做的事獲取到ServiceManager的代理，然後new一個呼叫instantiate的那個service的物件並把它新增到ServiceManager中。
所以下一步就是去分析AudioFlinger的構造函數了

AudioFlinger::AudioFlinger()
    : BnAudioFlinger(),
      mMediaLogNotifier(new AudioFlinger::MediaLogNotifier()),
      mPrimaryHardwareDev(NULL),
      mAudioHwDevs(NULL),
      mHardwareStatus(AUDIO_HW_IDLE),
      mMasterVolume(1.0f),
      mMasterMute(false),
      // mNextUniqueId(AUDIO_UNIQUE_ID_USE_MAX),
      mMode(AUDIO_MODE_INVALID),
      mBtNrecIsOff(false),
      mIsLowRamDevice(true),
      mIsDeviceTypeKnown(false),
      mGlobalEffectEnableTime(0),
      mSystemReady(false)
{
    // unsigned instead of audio_unique_id_use_t, because ++ operator is unavailable for enum
    for (unsigned use = AUDIO_UNIQUE_ID_USE_UNSPECIFIED; use < AUDIO_UNIQUE_ID_USE_MAX; use++) {
        // zero ID has a special meaning, so unavailable
        mNextUniqueIds[use] = AUDIO_UNIQUE_ID_USE_MAX;
    }

    getpid_cached = getpid();
    const bool doLog = property_get_bool("ro.test_harness", false);
    if (doLog) {
        mLogMemoryDealer = new MemoryDealer(kLogMemorySize, "LogWriters",
                MemoryHeapBase::READ_ONLY);
        (void) pthread_once(&sMediaLogOnce, sMediaLogInit);
    }

    // reset battery stats.
    // if the audio service has crashed, battery stats could be left
    // in bad state, reset the state upon service start.
    //重置電池統計資訊。 如果音訊服務崩潰，電池狀態可能會處於不良狀態，在服務啟動時重置狀態
    BatteryNotifier::getInstance().noteResetAudio();

    //建立裝置HAL層介面，用於hidl繫結
    mDevicesFactoryHal = DevicesFactoryHalInterface::create();
    //建立效果HAL層介面，用於hidl繫結
    mEffectsFactoryHal = EffectsFactoryHalInterface::create();

    mMediaLogNotifier->run("MediaLogNotifier");
}

1.1 DevicesFactoryHalInterface

sp<DevicesFactoryHalInterface> DevicesFactoryHalInterface::create() {
    return new DevicesFactoryHalHybrid();
}

DevicesFactoryHalHybrid::DevicesFactoryHalHybrid()
        : mLocalFactory(new DevicesFactoryHalLocal()),
          mHidlFactory(
#ifdef USE_LEGACY_LOCAL_AUDIO_HAL
                  nullptr
#else
                  new DevicesFactoryHalHidl()
#endif
                       ) {
}

DevicesFactoryHalHybrid::~DevicesFactoryHalHybrid() {
}

1.2 EffectsFactoryHalInterface

sp<EffectsFactoryHalInterface> EffectsFactoryHalInterface::create() {
    return new EffectsFactoryHalHidl();
}

EffectsFactoryHalHidl::EffectsFactoryHalHidl() : ConversionHelperHidl("EffectsFactory") {
    mEffectsFactory = IEffectsFactory::getService();
    if (mEffectsFactory == 0) {
        ALOGE("Failed to obtain IEffectsFactory service, terminating process.");
        exit(1);
    }
}

接下來再看看它的onFirstRef()函式。

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

    /* TODO: move all this work into an Init() function */
    char val_str[PROPERTY_VALUE_MAX] = { 0 };
    if (property_get("ro.audio.flinger_standbytime_ms", val_str, NULL) >= 0) {
        uint32_t int_val;
        if (1 == sscanf(val_str, "%u", &int_val)) {
            mStandbyTimeInNsecs = milliseconds(int_val);
            ALOGI("Using %u mSec as standby time.", int_val);
        } else {
            mStandbyTimeInNsecs = kDefaultStandbyTimeInNsecs;
            ALOGI("Using default %u mSec as standby time.",
                    (uint32_t)(mStandbyTimeInNsecs / 1000000));
        }
    }

    mPatchPanel = new PatchPanel(this);

    mMode = AUDIO_MODE_NORMAL;

    gAudioFlinger = this;
}

onFirstRef()函式也很簡單，僅僅讀取屬性”ro.audio.flinger_standbytime_ms”的值並賦給變數mStandbyTimeInNsecs ，若這個屬性沒有設定，則使用預設值kDefaultStandbyTimeInNsecs。

2. audioflinger中的執行緒關係圖

    PlaybackThread和RecordThread分別用於播放與錄音。PlaybackThread中派生出DirectOutputThread與MixerThread。AudioFlinger中建立的播放執行緒大都是MixerThread，用於混合多路聲音輸出，而DirectOutputThread用於將聲音直接傳送給底層播放，不需要混音處理。
  DuplicatingThread用於將聲音同時輸出到藍芽裝置和其他裝置，因為藍芽裝置和其他裝置的連線是分開的，所以輸出藍芽音訊資料需要一條單獨的通道，而輸出到DSP的資料也需要一條單獨的通道。

3.小結

    本文旨在介紹一下audioflinger的啟動過程及它包含執行緒之間的關係。具體audioflinger是如何工作的？後面會深入的一步步分析。下一篇將會先介紹audiopolicyservice以及它如何關聯audioflinger等等，未完待續。。。