Android 音频数据流(2): 从 AudioTrack 到 ALSA 驱动

Android 音频数据流(2): 从 AudioTrack 到 ALSA 驱动

注意:本文基于 Android 8.1 进行分析
Qidi 2020.11.17 (Markdown & Haroopad & EnterpriseArchitect)


0. 前言

在上一篇文章《Android 音频数据流(1): 从 MediaPlayer 到 AudioTrack》中,我们分析了音频数据被从数据源读取并写入 AudioTrack 的过程。这一篇文章再来看看数据从 AudioTrack 又是按照什么流程最终写入到 ALSA 驱动节点的。

相对于上一篇文章来说,这部分的逻辑要简洁得多。


1. 类图

先看类关系。简单来说,每有一个 AudioTrack 实例,就会相应地有一个 Track 实例;Track 实例既会从 匿名共享内存(AshMem) 中读取数据,又会被加入到 AudioFlinger::PlaybackThread 中进行统一管理;PlaybackThread 会把数据写入 HAL 层:

Android 音频数据流(2): 从 AudioTrack 到 ALSA 驱动_第1张图片


2. 时序图

再看写数据过程:

Android 音频数据流(2): 从 AudioTrack 到 ALSA 驱动_第2张图片

其中 AOSP 部分的逻辑(浅绿色部分)是固定的;HAL 层的实现(粉色和红色部分)则因公司而异,为了流程的完整性,图中列出的实现仅供参考


正如上文所说,上层部件每创建一个 AudioTrack 实例,都会有一个 Track 实例相应被构造出来。上层部件调用 AudioTrack::write() 写入 匿名共享内存(AshMem) 的音频数据,会在这个 Track 实例中被读取出来,随后,该 Track 实例会被加入到 PlaybackThread 的成员变量 mTracks 中,对应的音频数据也会在 AudioFlinger 中作混音处理。

AudioTrack::start() 被调用后,上述 Track 实例就被加入到了 PlaybackThreadmActiveTracks 成员中。这是一个 SortedVector> 容器类型的变量,它所包含的 Track 实例是动态变化的,代表所有当前处于活跃(播放)状态的 Track 实例。 AudioFlinger 中的混音对象,就是该容器中的这些 Track


AudioFlinger 中有独立的线程负责完成音频数据的混音、读(录音)和 写(播放)工作。其中,MixerThread 负责混音的过程,主要分为 2 步:

  • 汇总数据,即把所有数据写入同一个内存区。在 prepareTracks_l() 函数中完成;
  • 处理数据,即真正进行混音操作。在 threadLoop_mix() 函数中完成。

因为 MixerThread 继承自 PlaybackThread,所以其 threadLoop() 函数的关键代码逻辑如下:


bool AudioFlinger::PlaybackThread::threadLoop()
{
     
    //......

    while (!exitPending())
    {
     
        //......

        Vector< sp<EffectChain> > effectChains;

        {
      // scope for mLock

            Mutex::Autolock _l(mLock);

            processConfigEvents_l();

            //......
            // mMixerStatusIgnoringFastTracks is also updated internally
            mMixerStatus = prepareTracks_l(&tracksToRemove);

            mActiveTracks.updatePowerState(this);

            // prevent any changes in effect chain list and in each effect chain
            // during mixing and effect process as the audio buffers could be deleted
            // or modified if an effect is created or deleted
            lockEffectChains_l(effectChains);
        } // mLock scope ends

        if (mBytesRemaining == 0) {
     
            mCurrentWriteLength = 0;
            if (mMixerStatus == MIXER_TRACKS_READY) {
     
                // threadLoop_mix() sets mCurrentWriteLength
                threadLoop_mix();
            } else if ((mMixerStatus != MIXER_DRAIN_TRACK)
                        && (mMixerStatus != MIXER_DRAIN_ALL)) {
     
                // threadLoop_sleepTime sets mSleepTimeUs to 0 if data
                // must be written to HAL
                threadLoop_sleepTime();
                if (mSleepTimeUs == 0) {
     
                    mCurrentWriteLength = mSinkBufferSize;
                }
            }
            //......
            if (mMixerBufferValid) {
     
                void *buffer = mEffectBufferValid ? mEffectBuffer : mSinkBuffer;
                audio_format_t format = mEffectBufferValid ? mEffectBufferFormat : mFormat;

                // mono blend occurs for mixer threads only (not direct or offloaded)
                // and is handled here if we're going directly to the sink.
                if (requireMonoBlend() && !mEffectBufferValid) {
     
                    mono_blend(mMixerBuffer, mMixerBufferFormat, mChannelCount, mNormalFrameCount,
                               true /*limit*/);
                }

                memcpy_by_audio_format(buffer, format, mMixerBuffer, mMixerBufferFormat,
                        mNormalFrameCount * mChannelCount);
            }

            //......
        }
        //......
}

再来细看 prepareTracks_l() 函数。在这里,所有处于活跃状态的 Track 实例从 AshMem 中读出音频数据,并写入 mMixerBuffer 指向的内存区。各 Track 实例的音频参数也在这个过程中被写入 AudioMixer 对象:

AudioFlinger::PlaybackThread::mixer_state AudioFlinger::MixerThread::prepareTracks_l(
        Vector< sp<Track> > *tracksToRemove)
{
     

    //......
    // find out which tracks need to be processed
    size_t count = mActiveTracks.size();

	//......

    for (size_t i=0 ; i<count ; i++) {
     
        const sp<Track> t = mActiveTracks[i];

        // this const just means the local variable doesn't change
        Track* const track = t.get();

        // process fast tracks

        {
        // local variable scope to avoid goto warning

        audio_track_cblk_t* cblk = track->cblk();

        //......

        size_t framesReady = track->framesReady();
        //......
        if ((framesReady >= minFrames) && track->isReady() &&
                !track->isPaused() && !track->isTerminated())
        {
     
            ALOGVV("track %d s=%08x [OK] on thread %p", name, cblk->mServer, this);

            mixedTracks++;

            // track->mainBuffer() != mSinkBuffer or mMixerBuffer means
            // there is an effect chain connected to the track
            chain.clear();
            if (track->mainBuffer() != mSinkBuffer &&
                    track->mainBuffer() != mMixerBuffer) {
     
                if (mEffectBufferEnabled) {
     
                    mEffectBufferValid = true; // Later can set directly.
                }
                chain = getEffectChain_l(track->sessionId());
                // Delegate volume control to effect in track effect chain if needed
                if (chain != 0) {
     
                    tracksWithEffect++;
                } else {
     
                    ALOGW("prepareTracks_l(): track %d attached to effect but no chain found on "
                            "session %d",
                            name, track->sessionId());
                }
            }

            //......
            // XXX: these things DON'T need to be done each time
            mAudioMixer->setBufferProvider(name, track);
            mAudioMixer->enable(name);

            mAudioMixer->setParameter(name, param, AudioMixer::VOLUME0, &vlf);
            mAudioMixer->setParameter(name, param, AudioMixer::VOLUME1, &vrf);
            mAudioMixer->setParameter(name, param, AudioMixer::AUXLEVEL, &vaf);
            mAudioMixer->setParameter(
                name,
                AudioMixer::TRACK,
                AudioMixer::FORMAT, (void *)track->format());
            mAudioMixer->setParameter(
                name,
                AudioMixer::TRACK,
                AudioMixer::CHANNEL_MASK, (void *)(uintptr_t)track->channelMask());
            mAudioMixer->setParameter(
                name,
                AudioMixer::TRACK,
                AudioMixer::MIXER_CHANNEL_MASK, (void *)(uintptr_t)mChannelMask);
            // limit track sample rate to 2 x output sample rate, which changes at re-configuration
            uint32_t maxSampleRate = mSampleRate * AUDIO_RESAMPLER_DOWN_RATIO_MAX;
            uint32_t reqSampleRate = track->mAudioTrackServerProxy->getSampleRate();
            if (reqSampleRate == 0) {
     
                reqSampleRate = mSampleRate;
            } else if (reqSampleRate > maxSampleRate) {
     
                reqSampleRate = maxSampleRate;
            }
            mAudioMixer->setParameter(
                name,
                AudioMixer::RESAMPLE,
                AudioMixer::SAMPLE_RATE,
                (void *)(uintptr_t)reqSampleRate);

            AudioPlaybackRate playbackRate = track->mAudioTrackServerProxy->getPlaybackRate();
            mAudioMixer->setParameter(
                name,
                AudioMixer::TIMESTRETCH,
                AudioMixer::PLAYBACK_RATE,
                &playbackRate);

            /*
             * Select the appropriate output buffer for the track.
             *
             * Tracks with effects go into their own effects chain buffer
             * and from there into either mEffectBuffer or mSinkBuffer.
             *
             * Other tracks can use mMixerBuffer for higher precision
             * channel accumulation.  If this buffer is enabled
             * (mMixerBufferEnabled true), then selected tracks will accumulate
             * into it.
             *
             */
            if (mMixerBufferEnabled
                    && (track->mainBuffer() == mSinkBuffer
                            || track->mainBuffer() == mMixerBuffer)) {
     
                mAudioMixer->setParameter(
                        name,
                        AudioMixer::TRACK,
                        AudioMixer::MIXER_FORMAT, (void *)mMixerBufferFormat);
                mAudioMixer->setParameter(
                        name,
                        AudioMixer::TRACK,
                        AudioMixer::MAIN_BUFFER, (void *)mMixerBuffer);
                // TODO: override track->mainBuffer()?
                mMixerBufferValid = true;
            } else {
     
                //......
            }
            mAudioMixer->setParameter(
                name,
                AudioMixer::TRACK,
                AudioMixer::AUX_BUFFER, (void *)track->auxBuffer());

            //......
        } else {
     
            //......
        }

        }   // local variable scope to avoid goto warning

    }

    //......

    // remove all the tracks that need to be...
    removeTracks_l(*tracksToRemove);

    if (getEffectChain_l(AUDIO_SESSION_OUTPUT_MIX) != 0) {
     
        mEffectBufferValid = true;
    }

    if (mEffectBufferValid) {
     
        // as long as there are effects we should clear the effects buffer, to avoid
        // passing a non-clean buffer to the effect chain
        memset(mEffectBuffer, 0, mEffectBufferSize);
    }
    //......
    return mixerStatus;
}

现在 AudioMixer 对象已经持有了各个 Track 的数据和参数。随后在 MixerThread::threadLoop_mix() 中, AudioMixer::process() 被调用,开始真正进行混音处理:

void AudioFlinger::MixerThread::threadLoop_mix()
{
     
    // mix buffers...
    mAudioMixer->process();
    // ......

}

混音完成后,还是在 PlaybackThread::threadLoop() 中,代码执行到 PlaybackThread::threadLoop_write(),混音后的数据在这里被写入 HAL 层:


bool AudioFlinger::PlaybackThread::threadLoop()
{
     
    //......

    while (!exitPending())
    {
     
        //......
        if (mEffectBufferValid) {
     
            //ALOGV("writing effect buffer to sink buffer format %#x", mFormat);

            if (requireMonoBlend()) {
     
                mono_blend(mEffectBuffer, mEffectBufferFormat, mChannelCount, mNormalFrameCount,
                           true /*limit*/);
            }

            memcpy_by_audio_format(mSinkBuffer, mFormat, mEffectBuffer, mEffectBufferFormat,
                    mNormalFrameCount * mChannelCount);
        }

        // enable changes in effect chain
        unlockEffectChains(effectChains);

        if (!waitingAsyncCallback()) {
     
            // mSleepTimeUs == 0 means we must write to audio hardware
            if (mSleepTimeUs == 0) {
     
                ssize_t ret = 0;
                // We save lastWriteFinished here, as previousLastWriteFinished,
                // for throttling. On thread start, previousLastWriteFinished will be
                // set to -1, which properly results in no throttling after the first write.
                nsecs_t previousLastWriteFinished = lastWriteFinished;
                nsecs_t delta = 0;
                if (mBytesRemaining) {
     
                    // FIXME rewrite to reduce number of system calls
                    mLastWriteTime = systemTime();  // also used for dumpsys
                    ret = threadLoop_write();
                    lastWriteFinished = systemTime();
                    delta = lastWriteFinished - mLastWriteTime;
                    if (ret < 0) {
     
                        mBytesRemaining = 0;
                    } else {
     
                        mBytesWritten += ret;
                        mBytesRemaining -= ret;
                        mFramesWritten += ret / mFrameSize;
                    }
                } else if ((mMixerStatus == MIXER_DRAIN_TRACK) ||
                        (mMixerStatus == MIXER_DRAIN_ALL)) {
     
                    threadLoop_drain();
                }
                if (mType == MIXER && !mStandby) {
     
                    // write blocked detection
                    if (delta > maxPeriod) {
     
                        mNumDelayedWrites++;
                        if ((lastWriteFinished - lastWarning) > kWarningThrottleNs) {
     
                            ATRACE_NAME("underrun");
                            ALOGW("write blocked for %llu msecs, %d delayed writes, thread %p",
                                    (unsigned long long) ns2ms(delta), mNumDelayedWrites, this);
                            lastWarning = lastWriteFinished;
                        }
                    }

                    //......
                }

            } else {
     
                //......
            }
        }

        // Finally let go of removed track(s), without the lock held
        // since we can't guarantee the destructors won't acquire that
        // same lock.  This will also mutate and push a new fast mixer state.
        threadLoop_removeTracks(tracksToRemove);
        tracksToRemove.clear();

        // FIXME I don't understand the need for this here;
        //       it was in the original code but maybe the
        //       assignment in saveOutputTracks() makes this unnecessary?
        clearOutputTracks();

        // Effect chains will be actually deleted here if they were removed from
        // mEffectChains list during mixing or effects processing
        effectChains.clear();

        // FIXME Note that the above .clear() is no longer necessary since effectChains
        // is now local to this block, but will keep it for now (at least until merge done).
    }

    threadLoop_exit();

    if (!mStandby) {
     
        threadLoop_standby();
        mStandby = true;
    }

    releaseWakeLock();

    ALOGV("Thread %p type %d exiting", this, mType);
    return false;
}

HAL 层的实现因公司而异,一般各公司也不允许将这部分代码对外公开,所以此处略去细节。

但无论各家 HAL 的具体内容如何,肯定都是对 /hardware/libhardware/include/hardware/audio.h 中定义的各接口的实现( audio_stream_out 结构里声明了 write() 接口),且数据去向也是一致的 —— 最终都是调用 ALSA-Lib,通过 snd_pcm_writei() 将数据写入驱动


至此,我们对 Android 音频数据流的 从数据源读取数据 到 写入驱动设备节点 的过程就大致清楚了。

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