Android-MediaCodec详解

前言:

安卓中播放视频一般有两个接口，一个是mediaplayer，一个是mediacodec，其中mediaplayer是对mediacodec / genericsource / render 的封装，对APP而言使用比较方便。相比而言mediacodec留给用户发挥的空间更多，APP可以自己实现很多功能。目前只有MXplayer使用的是mediaplayer接口，其他APP一般都是使用的mediacodec接口。本文接下来就会讲解mediacodec的具体实现过程。

 

上层实现:

上层通过MediaCodec.java接口调用MediaCodec。具体过程如下:

// 参考cts/tests/tests/media/src/android/media/cts/MediaCodecTest.java
MediaExtractor mediaExtractor = null;
MediaCodec mediaCodec = null;
try{
    mediaExtractor = getMediaExtractorForMimeType(inputResourceId, "video/");
    MediaFormat mediaFormat =
          mediaExtractor.getTrackFormat(mediaExtractor.getSampleTrackIndex()); // 获取video的类型
    String mimeType = mediaFormat.getString(MediaFormat.KEY_MIME); // 获取mimeType
    mediaCodec = MediaCodec.createDecoderByType(mime);　// 创建decoder
    mediaCodec.configure(format, null, null, 0);　//　configure
    mediaCodec.start(); // start
    while(!eos && !Thread.interrupted()){
        int bufferIndex = mediaCodec.dequeueInputBuffer(0); // dequeueInputBuffer
        if (bufferIndex != NO_BUFFER_INDEX) {
            ByteBuffer buffer = mediaCodec.getInputBuffers()[bufferIndex]; //getInputBuffers 
            int size = mediaExtractor.readSampleData(buffer, 0);
            long timestampUs = mediaExtractor.getSampleTime();
            mediaExtractor.advance();
            signaledEos = mediaExtractor.getSampleTrackIndex() == -1
                || timestampUs == lastBufferTimestampUs;
            mediaCodec.queueInputBuffer(bufferIndex, // queueInputBuffer
                0,
                size,
                timestampUs,
                signaledEos ? MediaCodec.BUFFER_FLAG_END_OF_STREAM : 0);
          }
        // If we don't have an output buffer, try to get one now.
        if (outputBufferIndex == NO_BUFFER_INDEX) {
             outputBufferIndex = mediaCodec.dequeueOutputBuffer(outputBufferInfo, 0); // dequeueOutputBuffer
        }
        if (outputBufferIndex == MediaCodec.INFO_OUTPUT_BUFFERS_CHANGED
                || outputBufferIndex == MediaCodec.INFO_OUTPUT_FORMAT_CHANGED
                || outputBufferIndex == MediaCodec.INFO_TRY_AGAIN_LATER) {
              outputBufferIndex = NO_BUFFER_INDEX;
         } else if (outputBufferIndex != NO_BUFFER_INDEX) {
                  eos = (outputBufferInfo.flags & 
               MediaCodec.BUFFER_FLAG_END_OF_STREAM) != 0;

              boolean render = outputBufferInfo.size > 0;
              mediaCodec.releaseOutputBuffer(outputBufferIndex, render); //releaseOutputBuffer
              if (render) {
                  outputSurface.awaitNewImage();
              }
              outputBufferIndex = NO_BUFFER_INDEX;
         }
      }

      return eos;
} catch (IOException e) {
    throw new RuntimeException("error reading input resource", e);
} finally {
    if (mediaCodec != null) {
        mediaCodec.stop(); // stop
        mediaCodec.release(); // release
    }
    if (mediaExtractor != null) {
        mediaExtractor.release();
    }
    if (outputSurface != null) {
        outputSurface.release();
    }
}

MediaCodec的具体实现:

createDecoderByType
setCallback
configure
start
dequeueInputBuffer
getInputBuffers
queueInputBuffer
dequeueOutputBuffer
releaseOutputBuffer
stop
release

1. createDecoderByType

public static MediaCodec createDecoderByType(@NonNull String type)
            throws IOException {
        return new MediaCodec(type, true /* nameIsType */, false /* encoder */);
}

根据type创建decoder。此处MediaCode的构造函数如下:

    private MediaCodec(
            @NonNull String name, boolean nameIsType, boolean encoder) {
        Looper looper;
        if ((looper = Looper.myLooper()) != null) {
            mEventHandler = new EventHandler(this, looper);
        } else if ((looper = Looper.getMainLooper()) != null) {
            mEventHandler = new EventHandler(this, looper);
        } else {
            mEventHandler = null;
        }
        mCallbackHandler = mEventHandler;
        mOnFrameRenderedHandler = mEventHandler;

        mBufferLock = new Object();

        native_setup(name, nameIsType, encoder);
    }

此处首先设置了callback和rennder的handler，然后调用native_setup函数。native_setup的具体实现在frameworks/base/media/jni/android_media_MediaCodec.cpp中。

static void android_media_MediaCodec_native_setup(
        JNIEnv *env, jobject thiz,
        jstring name, jboolean nameIsType, jboolean encoder) {
    if (name == NULL) {
        jniThrowException(env, "java/lang/NullPointerException", NULL);
        return;
    }

    const char *tmp = env->GetStringUTFChars(name, NULL);

    if (tmp == NULL) {
        return;
    }

    sp codec = new JMediaCodec(env, thiz, tmp, nameIsType, encoder);

    const status_t err = codec->initCheck();
    if (err == NAME_NOT_FOUND) {
        // fail and do not try again.
        jniThrowException(env, "java/lang/IllegalArgumentException",
                String8::format("Failed to initialize %s, error %#x", tmp, err));
        env->ReleaseStringUTFChars(name, tmp);
        return;
    } if (err == NO_MEMORY) {
        throwCodecException(env, err, ACTION_CODE_TRANSIENT,
                String8::format("Failed to initialize %s, error %#x", tmp, err));
        env->ReleaseStringUTFChars(name, tmp);
        return;
    } else if (err != OK) {
        // believed possible to try again
        jniThrowException(env, "java/io/IOException",
                String8::format("Failed to find matching codec %s, error %#x", tmp, err));
        env->ReleaseStringUTFChars(name, tmp);
        return;
    }

    env->ReleaseStringUTFChars(name, tmp);

    codec->registerSelf();

    setMediaCodec(env,thiz, codec);
}

在JMediaCodec的构造函数中会调用native mediacodec的creatByType函数创建decoder。

2. configure

MediaCodec.java中configure的调用过程如下:

    private void configure(
            @Nullable MediaFormat format, @Nullable Surface surface,
            @Nullable MediaCrypto crypto, @Nullable IHwBinder descramblerBinder,
            @ConfigureFlag int flags) {
        if (crypto != null && descramblerBinder != null) {
            throw new IllegalArgumentException("Can't use crypto and descrambler together!");
        }

        String[] keys = null;
        Object[] values = null;

        if (format != null) {
            Map formatMap = format.getMap();
            keys = new String[formatMap.size()];
            values = new Object[formatMap.size()];

            int i = 0;
            for (Map.Entry entry: formatMap.entrySet()) {
                if (entry.getKey().equals(MediaFormat.KEY_AUDIO_SESSION_ID)) {
                    int sessionId = 0;
                    try {
                        sessionId = (Integer)entry.getValue();
                    }
                    catch (Exception e) {
                        throw new IllegalArgumentException("Wrong Session ID Parameter!");
                    }
                    keys[i] = "audio-hw-sync";
                    values[i] = AudioSystem.getAudioHwSyncForSession(sessionId);
                } else {
                    keys[i] = entry.getKey();
                    values[i] = entry.getValue();
                }
                ++i;
            }
        }

        mHasSurface = surface != null;

        native_configure(keys, values, surface, crypto, descramblerBinder, flags);
    }

此处就是把MediaFormat格式的format参数转换为底层可以识别的key-value格式。然后调用native_configure，实际调用android_media_MediaCodec_native_configure。

static void android_media_MediaCodec_native_configure(
        JNIEnv *env,
        jobject thiz,
        jobjectArray keys, jobjectArray values,
        jobject jsurface,
        jobject jcrypto,
        jobject descramblerBinderObj,
        jint flags) {
    sp codec = getMediaCodec(env, thiz);

    if (codec == NULL) {
        throwExceptionAsNecessary(env, INVALID_OPERATION);
        return;
    }

    sp format;
    status_t err = ConvertKeyValueArraysToMessage(env, keys, values, &format);

    if (err != OK) {
        jniThrowException(env, "java/lang/IllegalArgumentException", NULL);
        return;
    }

    sp bufferProducer;
    if (jsurface != NULL) {
        sp surface(android_view_Surface_getSurface(env, jsurface));
        if (surface != NULL) {
            bufferProducer = surface->getIGraphicBufferProducer();
        } else {
            jniThrowException(
                    env,
                    "java/lang/IllegalArgumentException",
                    "The surface has been released");
            return;
        }
    }

    sp crypto;
    if (jcrypto != NULL) {
        crypto = JCrypto::GetCrypto(env, jcrypto);
    }

    sp descrambler;
    if (descramblerBinderObj != NULL) {
        descrambler = GetDescrambler(env, descramblerBinderObj);
    }

    err = codec->configure(format, bufferProducer, crypto, descrambler, flags);

    throwExceptionAsNecessary(env, err);
}

status_t JMediaCodec::configure(
        const sp &format,
        const sp &bufferProducer,
        const sp &crypto,
        const sp &descrambler,
        int flags) {
    sp client;
    if (bufferProducer != NULL) {
        mSurfaceTextureClient =
            new Surface(bufferProducer, true /* controlledByApp */);
    } else {
        mSurfaceTextureClient.clear();
    }

    return mCodec->configure(
            format, mSurfaceTextureClient, crypto, descrambler, flags);
}

然后就是调用native的MediaCodec::configure函数，此处就是解析format的内容并设置给底层。此处也会把surface设置给底层。

MediaCodec::configure中会发送消息kWhatConfigure，收到消息后调用ACodec的configurecodec。ACodec的configurecodec中调用setupVideoEncoder or setupVideoDecoder。在setupVideoDecoder函数中有两个参数，从APP传递的msg，和需要在此赋值的outputFormat。setupVideoDecoder从msg中获取colorFormat，然后调用setVideoPortFormatType将colorFormat设置下去，如果返回错误，表示不支持这个格式，然后会调用setSupportedOutputFormat(在APP不设置color-format时也会调用此函数)。setSupportedOutputFormat函数中会for循环去找当前支持的格式。并把找到的第一个格式设置给底层。

3. start

public final void start() {
    native_start();　// 调用native start,实际会调用android_media_MediaCodec_start
    synchronized(mBufferLock) {
        cacheBuffers(true /* input */);
        cacheBuffers(false /* input */);
    }
}

static void android_media_MediaCodec_start(JNIEnv *env, jobject thiz) {
    ALOGV("android_media_MediaCodec_start");

    sp codec = getMediaCodec(env, thiz);

    if (codec == NULL) {
        throwExceptionAsNecessary(env, INVALID_OPERATION);
        return;
    }

    status_t err = codec->start();　// JMediaCodec::start

    throwExceptionAsNecessary(env, err, ACTION_CODE_FATAL, "start failed");
}

status_t JMediaCodec::start() {
    return mCodec->start(); // MediaCodec::start
}

通过java->jni->native，最终调用的是MediaCodec::start()。

status_t MediaCodec::start() {
    sp msg = new AMessage(kWhatStart, this);

    status_t err;
    Vector resources;
    MediaResource::Type type = (mFlags & kFlagIsSecure) ?
            MediaResource::kSecureCodec : MediaResource::kNonSecureCodec;
    MediaResource::SubType subtype =
            mIsVideo ? MediaResource::kVideoCodec : MediaResource::kAudioCodec;
    resources.push_back(MediaResource(type, subtype, 1));
    // Don't know the buffer size at this point, but it's fine to use 1 because
    // the reclaimResource call doesn't consider the requester's buffer size for now.
    resources.push_back(MediaResource(MediaResource::kGraphicMemory, 1));
    for (int i = 0; i <= kMaxRetry; ++i) {
        if (i > 0) {
            // Don't try to reclaim resource for the first time.
            if (!mResourceManagerService->reclaimResource(resources)) {
                break;
            }
            // Recover codec from previous error before retry start.
            err = reset();
            if (err != OK) {
                ALOGE("retrying start: failed to reset codec");
                break;
            }
            sp response;
            err = PostAndAwaitResponse(mConfigureMsg, &response);
            if (err != OK) {
                ALOGE("retrying start: failed to configure codec");
                break;
            }
        }

        sp response;
        err = PostAndAwaitResponse(msg, &response);
        if (!isResourceError(err)) {
            break;
        }
    }
    return err;
}

此处如果有资源不够的问题就会去回收资源，然后reset，否则就发送kWhatStart消息。然后开始调用ACodec::initiateStart()。ACodec::initiateStart()会发送消息kWhatStart，然后调用ACodec::LoadedState::onStart()。ACodec::LoadedState::onStart()此函数中给omx发送消息，设置state为mLoadedToIdleState。

　　mLoadedToIdleState状态之后如果收到OMX_EventCmdComplete就会转换到Executing状态。

4. dequeueInputBuffer / getInputBuffers / queueInputBuffer / dequeueOutputBuffer / releaseOutputBuffer

// step1 请求一个输入缓存的bufferIndex
int bufferIndex = mediaCodec.dequeueInputBuffer(TIME_OUT_US /* timeoutUs */);
// step2 根据bufferIndex得到buffer
ByteBuffer buffer = mediaCodec.getInputBuffer(bufferIndex);
// step3 给buffer中写入数据
int size = mediaExtractor.readSampleData(buffer, 0 /* offset */);
// step4 buffer入队
mediaCodec.queueInputBuffer(bufferIndex,0 /* offset */,size,
    timestampUs,
    signaledEos ? MediaCodec.BUFFER_FLAG_END_OF_STREAM : 0);

　　当MediaCodec处于Executing状态之后就可以对数据进行处理了。首先通过dequeueInputBuffer(long timeoutUs)请求一个输入缓存，timeoutUs代表等待时间，设置为-1代表无限等待。通过getInputBuffers()得到的是输入缓存数组，通过index和输入缓存数组可以得到当前请求的输入缓存，在使用前要clear一下，避免之前的缓存数据影响当前数据。接着就是把数据添加到输入缓存中，并调用queueInputBuffer(...)把缓存数据入队。

MediaCodec.BufferInfo outputBufferInfo = new MediaCodec.BufferInfo();
int outputBufferIndex = mediaCodec.dequeueOutputBuffer(outputBufferInfo, TIMEOUT_USEC);
outputBuffers = mediaCodec.getOutputBuffers();
configData.put(outputBuffers[outputBufferIndex]);
mediaCodec.releaseOutputBuffer(outputBufferIndex, false /* render */);

　　获取输出缓存和获取输入缓存类似，首先通过dequeueOutputBuffer(BufferInfo info, long timeoutUs)来请求一个输出缓存，这里需要传入一个BufferInfo对象，用于存储ByteBuffer的信息。然后通过返回的index得到输出缓存，并通过BufferInfo获取ByteBuffer的信息。然后把获取的buffer取出，最后relese这个buffer。

4.1 dequeueInputBuffer(只有在没有inputsurface的时候可以用)

status_t MediaCodec::dequeueInputBuffer(size_t *index, int64_t timeoutUs) {
    sp msg = new AMessage(kWhatDequeueInputBuffer, this);
    msg->setInt64("timeoutUs", timeoutUs);
    ......
    if ((err = PostAndAwaitResponse(msg, &response)) != OK) {
        return err;
    }
    ......
}

bool MediaCodec::handleDequeueInputBuffer(const sp &replyID, bool newRequest) {
    ......
    ssize_t index = dequeuePortBuffer(kPortIndexInput);
    ......
    sp response = new AMessage;
    response->setSize("index", index);
    response->postReply(replyID);
    return true;
}

ssize_t MediaCodec::dequeuePortBuffer(int32_t portIndex) {
    CHECK(portIndex == kPortIndexInput || portIndex == kPortIndexOutput);

    List *availBuffers = &mAvailPortBuffers[portIndex];

    if (availBuffers->empty()) {
        return -EAGAIN;
    }

    size_t index = *availBuffers->begin();
    availBuffers->erase(availBuffers->begin());

    BufferInfo *info = &mPortBuffers[portIndex][index];
    CHECK(!info->mOwnedByClient);
    {
        Mutex::Autolock al(mBufferLock);
        info->mOwnedByClient = true;

        // set image-data
        if (info->mData->format() != NULL) {
            sp imageData;
            if (info->mData->format()->findBuffer("image-data", &imageData)) {
                info->mData->meta()->setBuffer("image-data", imageData);
            }
            int32_t left, top, right, bottom;
            if (info->mData->format()->findRect("crop", &left, &top, &right, &bottom)) {
                info->mData->meta()->setRect("crop-rect", left, top, right, bottom);
            }
        }
    }

    return index;
}

　　所以,dequeueInputBuffer最终返回的index就是mAvailPortBuffers[kPortIndexInput]->begin()。

问题: kPortIndexInput和mAvailPortBuffers的具体来历。

　　kPortIndexInput是0。kPortIndexIOutput是1。

　　mAvailPortBuffers在updateBuffers时push_back。updateBuffers在MediaCodec收到kWhatDrainThisBuffer或kWhatFillThisBuffer时被调用。

　　BufferCallback::onInputBufferAvailable中发送kWhatFillThisBuffer。

　　ACodecBufferChannel::fillThisBuffer中调用onInputBufferAvailable

　　ACodec::BaseState::postFillThisBuffer中调用fillThisBuffer

　　ACodec::ExecutingState::resume()调用submitOutputBuffers()然后遍历所有可用buffer调用postFillThisBuffer。

　　ACodec在IdleToExecutingState收到OMX_EventCmdComplete就会调用resume，然后进入Executing State。

此处具体过程如下:

bool ACodec::IdleToExecutingState::onOMXEvent(
        OMX_EVENTTYPE event, OMX_U32 data1, OMX_U32 data2) {
    switch (event) {
        case OMX_EventCmdComplete:
        {   .....
            mCodec->mExecutingState->resume();
            mCodec->changeState(mCodec->mExecutingState);
            return true;
            .....
        }
    }
}

void ACodec::ExecutingState::resume() {
	ALOGV("ZYang: resume");
    .....
    submitOutputBuffers();
    .....
    // Post all available input buffers
    for (size_t i = 0; i < mCodec->mBuffers[kPortIndexInput].size(); i++) {
        BufferInfo *info = &mCodec->mBuffers[kPortIndexInput].editItemAt(i);
        if (info->mStatus == BufferInfo::OWNED_BY_US) {
            postFillThisBuffer(info);
        }
    }
    mActive = true;
}

void ACodec::BaseState::postFillThisBuffer(BufferInfo *info) {
    .....
    info->mData->setFormat(mCodec->mInputFormat);
    mCodec->mBufferChannel->fillThisBuffer(info->mBufferID);
    info->mData.clear();
    info->mStatus = BufferInfo::OWNED_BY_UPSTREAM;
}

void ACodecBufferChannel::fillThisBuffer(IOMX::buffer_id bufferId) {
    .....
    mCallback->onInputBufferAvailable(
            std::distance(array->begin(), it),
            it->mClientBuffer);
}

void BufferCallback::onInputBufferAvailable(
        size_t index, const sp &buffer) {
    // 发送消息kWhatFillThisBuffer 
}

void MediaCodec::onMessageReceived(const sp &msg) {
    switch (msg->what()) {
        case kWhatCodecNotify:
        {
            int32_t what;
            CHECK(msg->findInt32("what", &what));
            switch (what) {
                case kWhatFillThisBuffer:
                {
                    /* size_t index = */updateBuffers(kPortIndexInput, msg);
                    .....
                 }
         }
}

size_t MediaCodec::updateBuffers(
        int32_t portIndex, const sp &msg) {
    .....
    sp buffer = static_cast(obj.get());

    {
        .....
        mPortBuffers[portIndex][index].mData = buffer;
    }
    mAvailPortBuffers[portIndex].push_back(index);
    return index;
}

问题: mCodec->mBuffers的来历?

void ACodec::LoadedToIdleState::stateEntered() {
    ....
    if ((err = allocateBuffers()) != OK) {
        .....
        mCodec->changeState(mCodec->mLoadedState);
    }
}

status_t ACodec::LoadedToIdleState::allocateBuffers() {
    status_t err = mCodec->allocateBuffersOnPort(kPortIndexInput);
    if (err != OK) {
        return err;
    }

    err = mCodec->allocateBuffersOnPort(kPortIndexOutput);
    if (err != OK) {
        return err;
    }
    err = mCodec->allocateBuffersOnPort(kPortIndexInputExtradata);
    err = mCodec->allocateBuffersOnPort(kPortIndexOutputExtradata);
    if (err != OK) {
        err = OK; // Ignore Extradata buffer allocation failure
    }

    mCodec->mCallback->onStartCompleted();

    return OK;
}

status_t ACodec::allocateBuffersOnPort(OMX_U32 portIndex) {
    // 会给mBuffers[portIndex].push(info);
    .....
}

5. stop

public final void stop() {
    native_stop();
    freeAllTrackedBuffers();

    synchronized (mListenerLock) {
        if (mCallbackHandler != null) {
            mCallbackHandler.removeMessages(EVENT_SET_CALLBACK);
            mCallbackHandler.removeMessages(EVENT_CALLBACK);
        }
        if (mOnFrameRenderedHandler != null) {
            mOnFrameRenderedHandler.removeMessages(EVENT_FRAME_RENDERED);
        }
    }
}

static void android_media_MediaCodec_stop(JNIEnv *env, jobject thiz) {
    ALOGV("android_media_MediaCodec_stop");

    sp codec = getMediaCodec(env, thiz);

    if (codec == NULL) {
        throwExceptionAsNecessary(env, INVALID_OPERATION);
        return;
    }

    status_t err = codec->stop();

    throwExceptionAsNecessary(env, err);
}

 

MediaCodec和APP的数据交互:

1. dequeueOutputBuffer

APP需要从底层拿出yuv去渲染就会调用dequeueOutputBuffer，在native层的处理如下:

status_t MediaCodec::dequeueOutputBuffer( // APP调用到native的接口
        size_t *index,
        size_t *offset,
        size_t *size,
        int64_t *presentationTimeUs,
        uint32_t *flags,
        int64_t timeoutUs) {
    sp msg = new AMessage(kWhatDequeueOutputBuffer, this);　// 发送消息，此处会设置timeoutUs
    msg->setInt64("timeoutUs", timeoutUs);

    sp response;
    status_t err;
    if ((err = PostAndAwaitResponse(msg, &response)) != OK) {
        return err;
    }

    CHECK(response->findSize("index", index));
    CHECK(response->findSize("offset", offset));
    CHECK(response->findSize("size", size));
    CHECK(response->findInt64("timeUs", presentationTimeUs));
    CHECK(response->findInt32("flags", (int32_t *)flags));

    return OK;
}

void MediaCodec::onMessageReceived(const sp &msg) {
    case kWhatDequeueOutputBuffer:　// 收到消息的处理
        {
            sp replyID;
            CHECK(msg->senderAwaitsResponse(&replyID));

            if (handleDequeueOutputBuffer(replyID, true /* new request */)) {
                break;
            }

            int64_t timeoutUs;
            CHECK(msg->findInt64("timeoutUs", &timeoutUs));

            if (timeoutUs == 0ll) {
                PostReplyWithError(replyID, -EAGAIN);
                break;
            }

            mFlags |= kFlagDequeueOutputPending;
            mDequeueOutputReplyID = replyID;

            if (timeoutUs > 0ll) {
                sp timeoutMsg =
                    new AMessage(kWhatDequeueOutputTimedOut, this);
                timeoutMsg->setInt32(
                        "generation", ++mDequeueOutputTimeoutGeneration);
                timeoutMsg->post(timeoutUs);
            }
            break;
        }
｝

bool MediaCodec::handleDequeueOutputBuffer(const sp &replyID, bool newRequest) {
    if {
        // 此处有很多异常处理
    } else {
        sp response = new AMessage;
        ssize_t index = dequeuePortBuffer(kPortIndexOutput); // 从outputbuffer中取出一个buffer给APP

        const sp &buffer =
            mPortBuffers[kPortIndexOutput][index].mData; // 取出的buffer就是此处的buffer
　　　　　// 把buffer的信息传给response，返回给app
        response->setSize("index", index);
        response->setSize("offset", buffer->offset());
        response->setSize("size", buffer->size());

        int64_t timeUs;
        CHECK(buffer->meta()->findInt64("timeUs", &timeUs));

        statsBufferReceived(timeUs);

        response->setInt64("timeUs", timeUs);

        int32_t flags;
        CHECK(buffer->meta()->findInt32("flags", &flags));

        response->setInt32("flags", flags);
        response->postReply(replyID);
    }

    return true;
}

ssize_t MediaCodec::dequeuePortBuffer(int32_t portIndex) {
    CHECK(portIndex == kPortIndexInput || portIndex == kPortIndexOutput);

    List *availBuffers = &mAvailPortBuffers[portIndex]; //取出output的可用buffer

    size_t index = *availBuffers->begin(); // 从buffer中取出第一个的index
    availBuffers->erase(availBuffers->begin()); // erase第一个buffer

    BufferInfo *info = &mPortBuffers[portIndex][index]; // 再此取出buffer info
    CHECK(!info->mOwnedByClient);
    {
        Mutex::Autolock al(mBufferLock);
        info->mOwnedByClient = true;

        // set image-data
        if (info->mData->format() != NULL) {
            sp imageData;
            if (info->mData->format()->findBuffer("image-data", &imageData)) {
                info->mData->meta()->setBuffer("image-data", imageData);
            }
            int32_t left, top, right, bottom;
            if (info->mData->format()->findRect("crop", &left, &top, &right, &bottom)) {
                info->mData->meta()->setRect("crop-rect", left, top, right, bottom);
            }
        }
    }

    return index;
}

// 问题: mPortBuffers和availBuffers的来由
MediaCodec收到kWhatDrainThisBuffer和kWhatFillThisBuffer时分别会update output和input buffer，把此时收到消息带的bufer放入mPortBuffers和availBuffers。

size_t MediaCodec::updateBuffers(
        int32_t portIndex, const sp &msg) {
    CHECK(portIndex == kPortIndexInput || portIndex == kPortIndexOutput);
    size_t index;
    CHECK(msg->findSize("index", &index));
    sp obj;
    CHECK(msg->findObject("buffer", &obj));// 从msg中获取buffer的信息
    sp buffer = static_cast(obj.get());　// 创建MediaCodecBuffer

    {
        Mutex::Autolock al(mBufferLock);
        if (mPortBuffers[portIndex].size() <= index) {　// 如果buffer数量不够，更新size
            mPortBuffers[portIndex].resize(align(index + 1, kNumBuffersAlign));
        }
        mPortBuffers[portIndex][index].mData = buffer; //把buffer加入mPortBuffers
    }
    mAvailPortBuffers[portIndex].push_back(index);　// 把buffer的index加入mAvailPortBuffers

    return index;
}

kWhatDrainThisBuffer和kWhatFillThisBuffer的来由

//kWhatDrainThisBuffer消息的来由
ACodec::BaseState::onOMXFillBufferDone(……）｛ // acodec收到omx的FBD消息
    mCodec->mBufferChannel->drainThisBuffer(info->mBufferID, flags);
    // 此处会调用drainThisBuffer
｝

void ACodecBufferChannel::drainThisBuffer(
        IOMX::buffer_id bufferId,
        OMX_U32 omxFlags) {
    mCallback->onOutputBufferAvailable( // 调用onOutputBufferAvailable
            std::distance(array->begin(), it),
            it->mClientBuffer);
}

void BufferCallback::onOutputBufferAvailable(
        size_t index, const sp &buffer) {
    sp notify(mNotify->dup());
    notify->setInt32("what", kWhatDrainThisBuffer);// 此处发送消息kWhatDrainThisBuffer
    notify->setSize("index", index);
    notify->setObject("buffer", buffer);
    notify->post();
}
// 总结一下就是acodec收到omx的fbd之后就会告诉mediacodec有可用的outoutbuffer，然后mediacodec更新outputbuffer的list。

//kWhatFillThisBuffer消息的来由
// ACodec收到EDB或者kWhatInputBufferFilled
void ACodec::BaseState::postFillThisBuffer(BufferInfo *info) {
    if (mCodec->mPortEOS[kPortIndexInput]) {
        return;
    }

    CHECK_EQ((int)info->mStatus, (int)BufferInfo::OWNED_BY_US);

    info->mData->setFormat(mCodec->mInputFormat);
    mCodec->mBufferChannel->fillThisBuffer(info->mBufferID);
    info->mData.clear();
    info->mStatus = BufferInfo::OWNED_BY_UPSTREAM;
}

void ACodecBufferChannel::fillThisBuffer(IOMX::buffer_id bufferId) {
    mCallback->onInputBufferAvailable(　// 调用onInputBufferAvailable
            std::distance(array->begin(), it),
            it->mClientBuffer);
｝
void BufferCallback::onInputBufferAvailable(
        size_t index, const sp &buffer) {
    sp notify(mNotify->dup());
    notify->setInt32("what", kWhatFillThisBuffer);// 此处发送消息kWhatFillThisBuffer
    notify->setSize("index", index);
    notify->setObject("buffer", buffer);
    notify->post();
}

2. releaseOutputBuffer

当APP渲染完一帧图片之后就会调用此函数把buffer返还给mediacodec。

具体流程如下:

status_t MediaCodec::releaseOutputBuffer(size_t index) {
    sp msg = new AMessage(kWhatReleaseOutputBuffer, this); // 发送消息
    msg->setSize("index", index);

    sp response;
    return PostAndAwaitResponse(msg, &response);
}

void MediaCodec::onMessageReceived(const sp &msg) {
    case kWhatReleaseOutputBuffer:
        {
            sp replyID;
            CHECK(msg->senderAwaitsResponse(&replyID));

            status_t err = onReleaseOutputBuffer(msg); // onReleaseOutputBuffer

            PostReplyWithError(replyID, err);
            break;
        }
}

status_t MediaCodec::onReleaseOutputBuffer(const sp &msg) {
    size_t index;
    CHECK(msg->findSize("index", &index));

    int32_t render;
    if (!msg->findInt32("render", &render)) {
        render = 0;
    }

    BufferInfo *info = &mPortBuffers[kPortIndexOutput][index];


    // synchronization boundary for getBufferAndFormat
    sp buffer;
    {
        Mutex::Autolock al(mBufferLock);
        info->mOwnedByClient = false;
        buffer = info->mData;
        info->mData.clear();
    }

    if (render && buffer->size() != 0) { // 如果需要render
        int64_t mediaTimeUs = -1;
        buffer->meta()->findInt64("timeUs", &mediaTimeUs);

        int64_t renderTimeNs = 0;
        if (!msg->findInt64("timestampNs", &renderTimeNs)) {
            // use media timestamp if client did not request a specific render timestamp
            ALOGV("using buffer PTS of %lld", (long long)mediaTimeUs);
            renderTimeNs = mediaTimeUs * 1000;
        }

        if (mSoftRenderer != NULL) {
            std::list doneFrames = mSoftRenderer->render(
                    buffer->data(), buffer->size(), mediaTimeUs, renderTimeNs,
                    mPortBuffers[kPortIndexOutput].size(), buffer->format());

            // if we are running, notify rendered frames
            if (!doneFrames.empty() && mState == STARTED && mOnFrameRenderedNotification != NULL) {
                sp notify = mOnFrameRenderedNotification->dup();
                sp data = new AMessage;
                if (CreateFramesRenderedMessage(doneFrames, data)) {
                    notify->setMessage("data", data);
                    notify->post();
                }
            }
        }
        mBufferChannel->renderOutputBuffer(buffer, renderTimeNs);
    } else { // 不需要render
        mBufferChannel->discardBuffer(buffer);
    }

    return OK;
}

// 需要render时
status_t ACodecBufferChannel::renderOutputBuffer(
        const sp &buffer, int64_t timestampNs) {
    std::shared_ptr> array(
            std::atomic_load(&mOutputBuffers));
    BufferInfoIterator it = findClientBuffer(array, buffer);

    ALOGV("renderOutputBuffer #%d", it->mBufferId);
    sp msg = mOutputBufferDrained->dup();
    msg->setObject("buffer", buffer);
    msg->setInt32("buffer-id", it->mBufferId);
    msg->setInt32("render", true);
    msg->setInt64("timestampNs", timestampNs);
    msg->post(); // 发送消息kWhatOutputBufferDrained
    return OK;
}

// 不需要render时
status_t ACodecBufferChannel::discardBuffer(const sp &buffer) {
    std::shared_ptr> array(
            std::atomic_load(&mInputBuffers));
    BufferInfoIterator it = findClientBuffer(array, buffer);

    ALOGV("discardBuffer #%d", it->mBufferId);
    sp msg = mOutputBufferDrained->dup();
    msg->setObject("buffer", it->mCodecBuffer);
    msg->setInt32("buffer-id", it->mBufferId);
    msg->setInt32("discarded", true);
    msg->post(); // 发送消息kWhatOutputBufferDrained
    return OK;
}

// 处理消息kWhatOutputBufferDrained
void ACodec::BaseState::onOutputBufferDrained(const sp &msg) {
    IOMX::buffer_id bufferID;
    CHECK(msg->findInt32("buffer-id", (int32_t*)&bufferID));
    sp obj;
    CHECK(msg->findObject("buffer", &obj));
    sp buffer = static_cast(obj.get());　// 拿到buffer
    int32_t discarded = 0;
    msg->findInt32("discarded", &discarded);

    ssize_t index;
    BufferInfo *info = mCodec->findBufferByID(kPortIndexOutput, bufferID, &index); // 拿到buffer info
    BufferInfo::Status status = BufferInfo::getSafeStatus(info);
    if (status != BufferInfo::OWNED_BY_DOWNSTREAM) {
        ALOGE("Wrong ownership in OBD: %s(%d) buffer #%u", _asString(status), status, bufferID);
        mCodec->dumpBuffers(kPortIndexOutput);
        mCodec->signalError(OMX_ErrorUndefined, FAILED_TRANSACTION);
        return;
    }

    int64_t timeUs = -1;
    buffer->meta()->findInt64("timeUs", &timeUs);
    bool skip = mCodec->getDSModeHint(msg, timeUs);

    info->mData = buffer; // 把buffer给info->mData
    int32_t render;
    if (!skip && mCodec->mNativeWindow != NULL
            && msg->findInt32("render", &render) && render != 0
            && !discarded && buffer->size() != 0) {
        // 如果需要render
        android_native_rect_t crop;
        if (buffer->format()->findRect("crop", &crop.left, &crop.top, &crop.right, &crop.bottom)) {
            // NOTE: native window uses extended right-bottom coordinate
            ++crop.right;
            ++crop.bottom;
            if (memcmp(&crop, &mCodec->mLastNativeWindowCrop, sizeof(crop)) != 0) {
                mCodec->mLastNativeWindowCrop = crop;
                status_t err = native_window_set_crop(mCodec->mNativeWindow.get(), &crop);
                ALOGW_IF(err != NO_ERROR, "failed to set crop: %d", err);
            }
        }

        int32_t dataSpace;
        if (buffer->format()->findInt32("android._dataspace", &dataSpace)
                && dataSpace != mCodec->mLastNativeWindowDataSpace) {
            status_t err = native_window_set_buffers_data_space(
                    mCodec->mNativeWindow.get(), (android_dataspace)dataSpace);
            mCodec->mLastNativeWindowDataSpace = dataSpace;
            ALOGW_IF(err != NO_ERROR, "failed to set dataspace: %d", err);
        }
        if (buffer->format()->contains("hdr-static-info")) {
            HDRStaticInfo info;
            if (ColorUtils::getHDRStaticInfoFromFormat(buffer->format(), &info)
                && memcmp(&mCodec->mLastHDRStaticInfo, &info, sizeof(info))) {
                setNativeWindowHdrMetadata(mCodec->mNativeWindow.get(), &info);
                mCodec->mLastHDRStaticInfo = info;
            }
        }

        // save buffers sent to the surface so we can get render time when they return
        int64_t mediaTimeUs = -1;
        buffer->meta()->findInt64("timeUs", &mediaTimeUs);
        if (mediaTimeUs >= 0) {
            mCodec->mRenderTracker.onFrameQueued(
                    mediaTimeUs, info->mGraphicBuffer, new Fence(::dup(info->mFenceFd)));
        }

        int64_t timestampNs = 0;
        if (!msg->findInt64("timestampNs", ×tampNs)) {
            // use media timestamp if client did not request a specific render timestamp
            if (buffer->meta()->findInt64("timeUs", ×tampNs)) {
                ALOGV("using buffer PTS of %lld", (long long)timestampNs);
                timestampNs *= 1000;
            }
        }

        status_t err;
        err = native_window_set_buffers_timestamp(mCodec->mNativeWindow.get(), timestampNs);
        ALOGW_IF(err != NO_ERROR, "failed to set buffer timestamp: %d", err);

        info->checkReadFence("onOutputBufferDrained before queueBuffer");
        err = mCodec->mNativeWindow->queueBuffer(
                    mCodec->mNativeWindow.get(), info->mGraphicBuffer.get(), info->mFenceFd);
        info->mFenceFd = -1;
        if (err == OK) {
            info->mStatus = BufferInfo::OWNED_BY_NATIVE_WINDOW;
        } else {
            ALOGE("queueBuffer failed in onOutputBufferDrained: %d", err);
            mCodec->signalError(OMX_ErrorUndefined, makeNoSideEffectStatus(err));
            info->mStatus = BufferInfo::OWNED_BY_US;
            // keeping read fence as write fence to avoid clobbering
            info->mIsReadFence = false;
        }
    } else {
        if (mCodec->mNativeWindow != NULL && (discarded || buffer->size() != 0)) {
            // move read fence into write fence to avoid clobbering
            info->mIsReadFence = false;
            ATRACE_NAME("frame-drop");
        }
        info->mStatus = BufferInfo::OWNED_BY_US;　// 重点:buffer OWNED_BY_US
    }

    PortMode mode = getPortMode(kPortIndexOutput);

    switch (mode) {
        case KEEP_BUFFERS:
        {
            if (info->mStatus == BufferInfo::OWNED_BY_NATIVE_WINDOW) {
                info = mCodec->dequeueBufferFromNativeWindow();
            }
            break;
        }

        case RESUBMIT_BUFFERS:
        {
            if (!mCodec->mPortEOS[kPortIndexOutput]) {
                if (info->mStatus == BufferInfo::OWNED_BY_NATIVE_WINDOW) {

                    info = mCodec->dequeueBufferFromNativeWindow(); // 从nativewindow dequeue buffer
                }

                if (info != NULL) {
                    ALOGV("[%s] calling fillBuffer %u",
                         mCodec->mComponentName.c_str(), info->mBufferID);
                    info->checkWriteFence("onOutputBufferDrained::RESUBMIT_BUFFERS");
                    status_t err = mCodec->fillBuffer(info);
                    }
                }
            }
            break;
        }

        case FREE_BUFFERS:
        {
            status_t err = mCodec->freeBuffer(kPortIndexOutput, index);
            if (err != OK) {
                mCodec->signalError(OMX_ErrorUndefined, makeNoSideEffectStatus(err));
            }
            break;
        }

        default:
            ALOGE("Invalid port mode: %d", mode);
            return;
    }
}