Android音频子系统源码分析之AudioFlinger的实现

音频系统的播放接口由AudioTrack提供,每一个音频都会对应一个AudioTrack实例,它会通过iBinder来远程调用AudioFlinger的createTrack函数.AudioFlinger 会根据传入的frameCOunt参数申请一块内存,AudioTrack可以通过IaudioTrack接口的getCblk()函数获得指向该内存的Imemory接口,然后AudioTrack通过该Imemeory接口的pointer()函数获得指向该内存快的指针,这块内存的开始部分是audio_track_cblk_t结构,紧接着是大小为feameSize的buffer内存.

代码路径位于frameworks/base/media/libmedia/AudioTrack.cpp

status_t AudioTrack::set(
        int streamType,
        uint32_t sampleRate,
        int format,
        int channelMask,
        int frameCount,
        uint32_t flags,
        callback_t cbf,
        void* user,
        int notificationFrames,
        const sp<IMemory>& sharedBuffer,
        bool threadCanCallJava,
        int sessionId)
{

    LOGV_IF(sharedBuffer != 0, "sharedBuffer: %p, size: %d", sharedBuffer->pointer(), sharedBuffer->size());

    AutoMutex lock(mLock);
    if (mAudioTrack != 0) {
        LOGE("Track already in use");
        return INVALID_OPERATION;
    }
   。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。
    /* create the IAudioTrack */
    status_t status = createTrack_l(streamType,
                                  sampleRate,
                                  (uint32_t)format,
                                  (uint32_t)channelMask,
                                  frameCount,
                                  flags,
                                  sharedBuffer,
                                  output,
                                  true);

    if (status != NO_ERROR) {
        return status;
    }

    if (cbf != 0) {
        mAudioTrackThread = new AudioTrackThread(*this, threadCanCallJava);
        if (mAudioTrackThread == 0) {
          LOGE("Could not create callback thread");
          return NO_INIT;
        }
    }

   。。。。。。。。。。。。。。。。。。。
    AudioSystem::acquireAudioSessionId(mSessionId);
    mRestoreStatus = NO_ERROR;
    return NO_ERROR;
}

status_t AudioTrack::createTrack_l(
        int streamType,
        uint32_t sampleRate,
        uint32_t format,
        uint32_t channelMask,
        int frameCount,
        uint32_t flags,
        const sp<IMemory>& sharedBuffer,
        audio_io_handle_t output,
        bool enforceFrameCount)
{
    status_t status;
    const sp<IAudioFlinger>& audioFlinger = AudioSystem::get_audio_flinger();
    if (audioFlinger == 0) {
       LOGE("Could not get audioflinger");
       return NO_INIT;
    }

    int afSampleRate;
    if (AudioSystem::getOutputSamplingRate(&afSampleRate, streamType) != NO_ERROR) {
        return NO_INIT;
    }
    int afFrameCount;
    if (AudioSystem::getOutputFrameCount(&afFrameCount, streamType) != NO_ERROR) {
        return NO_INIT;
    }
    uint32_t afLatency;
    if (AudioSystem::getOutputLatency(&afLatency, streamType) != NO_ERROR) {
        return NO_INIT;
    }

    mNotificationFramesAct = mNotificationFramesReq;
    if (!audio_is_linear_pcm(format)) {
        if (sharedBuffer != 0) {
            frameCount = sharedBuffer->size();
        }
    } else {
        // Ensure that buffer depth covers at least audio hardware latency
        uint32_t minBufCount = afLatency / ((1000 * afFrameCount)/afSampleRate);
        if (minBufCount < 2) minBufCount = 2;

        int minFrameCount = (afFrameCount*sampleRate*minBufCount)/afSampleRate;

        if (sharedBuffer == 0) {
            if (frameCount == 0) {
                frameCount = minFrameCount;
            }
            if (mNotificationFramesAct == 0) {
                mNotificationFramesAct = frameCount/2;
            }
            // Make sure that application is notified with sufficient margin
            // before underrun
            if (mNotificationFramesAct > (uint32_t)frameCount/2) {
                mNotificationFramesAct = frameCount/2;
            }
            if (frameCount < minFrameCount) {
                if (enforceFrameCount) {
                    LOGE("Invalid buffer size: minFrameCount %d, frameCount %d", minFrameCount, frameCount);
                    return BAD_VALUE;
                } else {
                    frameCount = minFrameCount;
                }
            }
        } else {
            // Ensure that buffer alignment matches channelcount
            int channelCount = popcount(channelMask);
            if (((uint32_t)sharedBuffer->pointer() & (channelCount | 1)) != 0) {
                LOGE("Invalid buffer alignement: address %p, channelCount %d", sharedBuffer->pointer(), channelCount);
                return BAD_VALUE;
            }
            frameCount = sharedBuffer->size()/channelCount/sizeof(int16_t);
        }
    }

    sp<IAudioTrack> track = audioFlinger->createTrack(getpid(),
                                                      streamType,
                                                      sampleRate,
                                                      format,
                                                      channelMask,
                                                      frameCount,
                                                      ((uint16_t)flags) << 16,
                                                      sharedBuffer,
                                                      output,
                                                      &mSessionId,
                                                      &status);

    if (track == 0) {
        LOGE("AudioFlinger could not create track, status: %d", status);
        return status;
    }
    sp<IMemory> cblk = track->getCblk();
    if (cblk == 0) {
        LOGE("Could not get control block");
        return NO_INIT;
    }
    mAudioTrack.clear();
    mAudioTrack = track;
    mCblkMemory.clear();
    mCblkMemory = cblk;
    mCblk = static_cast<audio_track_cblk_t*>(cblk->pointer());
    android_atomic_or(CBLK_DIRECTION_OUT, &mCblk->flags);
    if (sharedBuffer == 0) {
        mCblk->buffers = (char*)mCblk + sizeof(audio_track_cblk_t);
    } else {
        mCblk->buffers = sharedBuffer->pointer();
         // Force buffer full condition as data is already present in shared memory
        mCblk->stepUser(mCblk->frameCount);
    }

    mCblk->volumeLR = (uint32_t(uint16_t(mVolume[RIGHT] * 0x1000)) << 16) | uint16_t(mVolume[LEFT] * 0x1000);
    mCblk->sendLevel = uint16_t(mSendLevel * 0x1000);
    mAudioTrack->attachAuxEffect(mAuxEffectId);
    mCblk->bufferTimeoutMs = MAX_STARTUP_TIMEOUT_MS;
    mCblk->waitTimeMs = 0;
    mRemainingFrames = mNotificationFramesAct;
    mLatency = afLatency + (1000*mCblk->frameCount) / sampleRate;
    return NO_ERROR;
}
/**
 *createTrack_l
 */
status_t AudioTrack::createTrack_l(
        int streamType,
        uint32_t sampleRate,
        uint32_t format,
        uint32_t channelMask,
        int frameCount,
        uint32_t flags,
        const sp<IMemory>& sharedBuffer,
        audio_io_handle_t output,
        bool enforceFrameCount)
{
    status_t status;
    const sp<IAudioFlinger>& audioFlinger = AudioSystem::get_audio_flinger();
    if (audioFlinger == 0) {
       LOGE("Could not get audioflinger");
       return NO_INIT;
    }

    。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。

    sp<IAudioTrack> track = audioFlinger->createTrack(getpid(),
                                                      streamType,
                                                      sampleRate,
                                                      format,
                                                      channelMask,
                                                      frameCount,//通过这个参数来申请一块内存
                                                      ((uint16_t)flags) << 16,
                                                      sharedBuffer,
                                                      output,
                                                      &mSessionId,
                                                      &status);

    if (track == 0) {
        LOGE("AudioFlinger could not create track, status: %d", status);
        return status;
    }
    /*获取IMemeory 接口*/
    sp<IMemory> cblk = track->getCblk();
    if (cblk == 0) {
        LOGE("Could not get control block");
        return NO_INIT;
    }
    mAudioTrack.clear();
    mAudioTrack = track;
    mCblkMemory.clear();
    mCblkMemory = cblk;
    /*通过pointer指针来指向该内存块*/
    mCblk = static_cast<audio_track_cblk_t*>(cblk->pointer());
    android_atomic_or(CBLK_DIRECTION_OUT, &mCblk->flags);
    if (sharedBuffer == 0) {//给buffer起始地址赋值
        mCblk->buffers = (char*)mCblk + sizeof(audio_track_cblk_t);/*内存的其实部分让frameSize 的buffer内存也指向这里*/
    } else {
        mCblk->buffers = sharedBuffer->pointer();
         // Force buffer full condition as data is already present in shared memory
        mCblk->stepUser(mCblk->frameCount);
    }

    mCblk->volumeLR = (uint32_t(uint16_t(mVolume[RIGHT] * 0x1000)) << 16) | uint16_t(mVolume[LEFT] * 0x1000);
    mCblk->sendLevel = uint16_t(mSendLevel * 0x1000);
    mAudioTrack->attachAuxEffect(mAuxEffectId);
    mCblk->bufferTimeoutMs = MAX_STARTUP_TIMEOUT_MS;
    mCblk->waitTimeMs = 0;
    mRemainingFrames = mNotificationFramesAct;
    mLatency = afLatency + (1000*mCblk->frameCount) / sampleRate;
    return NO_ERROR;
}

在上面的函数中通过AudioSystem:: AudioSystem::get_audio_flinger()函数可以获得IAudioFlinger的实例,再通过该实例调用createTrack申请内存,初始化buffer等，
如此看来当我们要播放音乐的时候应用程序要通过IAudioFlinger接口，调用IAudioFlinger的createTrack()AP,好吧我再回到sp<IAudioTrack> AudioFlinger::createTrack()
这个函数的代码中代码路径frameworks/base/services/audioflinger/AudioFlinger.cpp