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Android4.2.2的Stagefright中编解码器数据流的维护

本文均属自己阅读源码的点滴总结,转账请注明出处谢谢。

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Android源码版本Version:4.2.2; 硬件平台 全志A31

 

前沿:

在前面的博文中,基本提到的是stagefright相关的控制流,具体分析了android架构中的MediaExtractor、AwesomePlayer、StagefrightPlayer、OMXCodec等的创建,底层OMXNodinstance实例的创建。分析了OMX最底层插件库、编解码器组件的架构以及如何创建属于我们自己的OMX Plugin。

分析源码架构的另一个关键是数据流的分析,从这里开始,我们将对stagefright中的编解码缓存区进行分析:

1.

回到OMXCodec的创建过程的源码:

status_t AwesomePlayer::initVideoDecoder(uint32_t flags) {.......    mVideoSource = OMXCodec::Create(            mClient.interface(), mVideoTrack->getFormat(),//提取视频流的格式, mClient:BpOMX;mVideoTrack->getFormat()            false, // createEncoder,不创建编码器false            mVideoTrack,            NULL, flags, USE_SURFACE_ALLOC ? mNativeWindow : NULL);//创建一个解码器mVideoSource    if (mVideoSource != NULL) {        int64_t durationUs;        if (mVideoTrack->getFormat()->findInt64(kKeyDuration, &durationUs)) {            Mutex::Autolock autoLock(mMiscStateLock);            if (mDurationUs < 0 || durationUs > mDurationUs) {                mDurationUs = durationUs;            }        }        status_t err = mVideoSource->start();//启动解码器OMXCodec,完成解码器的init初始化操作.............}

在Android4.2.2下Stagefright多媒体架构中的A31的OMX插件和Codec组件 博文我们对于OMXCodec::create已经做了详细的分析,这里来关注mVideoSource->start的相关功能,即OMXCodec::start的处理:

status_t OMXCodec::start(MetaData *meta) {    Mutex::Autolock autoLock(mLock);........    return init();//进行初始化操作}

这里调用init()的过程,将会进行buffer的申请操作,为后续的流操作打下基础:

status_t OMXCodec::init() {    // mLock is held..........    err = allocateBuffers();//缓存区的分配    if (err != (status_t)OK) {        return err;    }    if (mQuirks & kRequiresLoadedToIdleAfterAllocation) {        err = mOMX->sendCommand(mNode, OMX_CommandStateSet, OMX_StateIdle);        CHECK_EQ(err, (status_t)OK);        setState(LOADED_TO_IDLE);    }............}

我们来看allocateBuffers的实现

 

2.关注allocateBuffersOnPort的实现

status_t OMXCodec::allocateBuffers() {    status_t err = allocateBuffersOnPort(kPortIndexInput);//输入缓存input口分配    if (err != OK) {        return err;    }    return allocateBuffersOnPort(kPortIndexOutput);//输出缓存input口分配}

这里分别将对输入和输出口进行Buffer的申请与分配,对于解码器,需要输入口来存储待解码的数据源,需要将解码后的数据源存储到输出口,而这也符合硬件的实现逻辑。以输入缓存区分配为例展开分析:

status_t OMXCodec::allocateBuffersOnPort(OMX_U32 portIndex) {.......    OMX_PARAM_PORTDEFINITIONTYPE def;    InitOMXParams(&def);    def.nPortIndex = portIndex;//输入口    err = mOMX->getParameter(            mNode, OMX_IndexParamPortDefinition, &def, sizeof(def));//获取输入口参数到def..........                err = mOMX->allocateBuffer(                        mNode, portIndex, def.nBufferSize, &buffer,                        &info.mData);........      info.mBuffer = buffer;//获取对应的buffer_id,有保存有底层的buffer的相关信息        info.mStatus = OWNED_BY_US;        info.mMem = mem;        info.mMediaBuffer = NULL; ...........        mPortBuffers[portIndex].push(info);//把当前的buffer恢复到mPortBuffers[2]中去

上述过程主要分为:

step1:先是获取底层解码器组件的当前的参数熟悉,一般这些参数都在建立OMX_Codec时完成的初始配置,前一博文中已经提到过。

step2:进行allocateBuffer的处理,这个函数的调用最终交给底层的OMX组件来完成,相关的实现将集成到A31的底层OMX编解码组件的处理流中进行分析。

step3:完成对分配好的buffer信息info,维护在mPortBuffers[0]这个端口中。

上述过程完成了输入与输出的Buffer分配,为后续解码操作buffer打下了基础。

 

3.mediaplay启动播放器

通过start的API调用,进入MediaplayerService::Client,再依次经过stagefrightplayer,AwesomePlayer。触发play的videoevent的发生.

void AwesomePlayer::postVideoEvent_l(int64_t delayUs) {    ATRACE_CALL();    if (mVideoEventPending) {        return;    }    mVideoEventPending = true;    mQueue.postEventWithDelay(mVideoEvent, delayUs < 0 ? 10000 : delayUs);}

根据前一博文的分析可知,该事件对应的处理函数为AwesomePlayer::onVideoEvent(),该部分代码量较大,提取核心内容read的处理进行分析:

   status_t err = mVideoSource->read(&mVideoBuffer, &options);//循环读数据实际的OMX_CODEC::read,读取到mVideoBuffer

read的核心是获取可以用于render的视频数据,这表明了read函数主要完成了从视频源读取元数据,并调用解码器完成解码生成可送显的数据。

 

4. read函数的实现

可以想象read函数的应该是一个比较复杂的过程,我们从OMX_Codec的read函数入手来分析:

status_t OMXCodec::read(        MediaBuffer **buffer, const ReadOptions *options) {    status_t err = OK;    *buffer = NULL;    Mutex::Autolock autoLock(mLock);        drainInputBuffers();//buffer,填充数据源        if (mState == EXECUTING) {            // Otherwise mState == RECONFIGURING and this code will trigger            // after the output port is reenabled.            fillOutputBuffers();        }    }...........}

read的核心逻辑总结为drainInputBuffers()和fillOutputBuffers(),我们对其依次进行深入的分析

 

5. drainInputBuffers()读取待解码的视频数据源到解码器的Inport

这里贴出其较为复杂的处理过程代码,主要分为以下3个部分进行分析:

(1)

bool OMXCodec::drainInputBuffer(BufferInfo *info) {
   if (mCodecSpecificDataIndex < mCodecSpecificData.size()) {        CHECK(!(mFlags & kUseSecureInputBuffers));        const CodecSpecificData *specific =            mCodecSpecificData[mCodecSpecificDataIndex];        size_t size = specific->mSize;        if (!strcasecmp(MEDIA_MIMETYPE_VIDEO_AVC, mMIME)                && !(mQuirks & kWantsNALFragments)) {            static const uint8_t kNALStartCode[4] =                    { 0x00, 0x00, 0x00, 0x01 };            CHECK(info->mSize >= specific->mSize + 4);            size += 4;            memcpy(info->mData, kNALStartCode, 4);            memcpy((uint8_t *)info->mData + 4,                   specific->mData, specific->mSize);        } else {            CHECK(info->mSize >= specific->mSize);            memcpy(info->mData, specific->mData, specific->mSize);//copy前面的数据字段        }        mNoMoreOutputData = http://www.mamicode.com/false;>...............(1)

这部分的内容主要是提取一部分解码器字段,填充到info->mData的存储空间中去。这部分主要基于视频源的格式,如mp4等在创建OXMCodec病configureCodec时就完成了这个mCodecSpecificData字段的添加,应该些解码需要的特殊字段吧。是否需要要看其视频源的格式。获取完这个字段信息后就是正式读取视频源的数据了。

 

(2)

  for (;;) {        MediaBuffer *srcBuffer;        if (mSeekTimeUs >= 0) {            if (mLeftOverBuffer) {                mLeftOverBuffer->release();                mLeftOverBuffer = NULL;            }            MediaSource::ReadOptions options;            options.setSeekTo(mSeekTimeUs, mSeekMode);            mSeekTimeUs = -1;            mSeekMode = ReadOptions::SEEK_CLOSEST_SYNC;            mBufferFilled.signal();            err = mSource->read(&srcBuffer, &options);//读取视频源中的真实数据这里是MPEG4Source的read            if (err == OK) {                int64_t targetTimeUs;                if (srcBuffer->meta_data()->findInt64(                            kKeyTargetTime, &targetTimeUs)                        && targetTimeUs >= 0) {                    CODEC_LOGV("targetTimeUs = %lld us", targetTimeUs);                    mTargetTimeUs = targetTimeUs;                } else {                    mTargetTimeUs = -1;                }            }        } else if (mLeftOverBuffer) {            srcBuffer = mLeftOverBuffer;            mLeftOverBuffer = NULL;            err = OK;        } else {            err = mSource->read(&srcBuffer);        }        if (err != OK) {            signalEOS = true;            mFinalStatus = err;            mSignalledEOS = true;            mBufferFilled.signal();            break;        }        if (mFlags & kUseSecureInputBuffers) {            info = findInputBufferByDataPointer(srcBuffer->data());            CHECK(info != NULL);        }        size_t remainingBytes = info->mSize - offset;//buffer中剩余的可以存储视频数据的空间        if (srcBuffer->range_length() > remainingBytes) {//当前读取的数据已经达到解码的数据量            if (offset == 0) {                CODEC_LOGE(                     "Codec‘s input buffers are too small to accomodate "                     "buffer read from source (info->mSize = %d, srcLength = %d)",                     info->mSize, srcBuffer->range_length());                srcBuffer->release();                srcBuffer = NULL;                setState(ERROR);                return false;            }            mLeftOverBuffer = srcBuffer;//把没读取的buffer记录下来            break;        }        bool releaseBuffer = true;        if (mFlags & kStoreMetaDataInVideoBuffers) {                releaseBuffer = false;                info->mMediaBuffer = srcBuffer;        }        if (mFlags & kUseSecureInputBuffers) {                // Data in "info" is already provided at this time.                releaseBuffer = false;                CHECK(info->mMediaBuffer == NULL);                info->mMediaBuffer = srcBuffer;        } else {            CHECK(srcBuffer->data() != NULL) ;            memcpy((uint8_t *)info->mData + offset,                    (const uint8_t *)srcBuffer->data()                        + srcBuffer->range_offset(),                    srcBuffer->range_length());//copy数据源数据到输入缓存,数据容量srcBuffer->range_length()        }        int64_t lastBufferTimeUs;        CHECK(srcBuffer->meta_data()->findInt64(kKeyTime, &lastBufferTimeUs));        CHECK(lastBufferTimeUs >= 0);        if (mIsEncoder && mIsVideo) {            mDecodingTimeList.push_back(lastBufferTimeUs);        }        if (offset == 0) {            timestampUs = lastBufferTimeUs;        }        offset += srcBuffer->range_length();//增加偏移量        if (!strcasecmp(MEDIA_MIMETYPE_AUDIO_VORBIS, mMIME)) {            CHECK(!(mQuirks & kSupportsMultipleFramesPerInputBuffer));            CHECK_GE(info->mSize, offset + sizeof(int32_t));            int32_t numPageSamples;            if (!srcBuffer->meta_data()->findInt32(                        kKeyValidSamples, &numPageSamples)) {                numPageSamples = -1;            }            memcpy((uint8_t *)info->mData + offset,                   &numPageSamples,                   sizeof(numPageSamples));            offset += sizeof(numPageSamples);        }        if (releaseBuffer) {            srcBuffer->release();            srcBuffer = NULL;        }        ++n;        if (!(mQuirks & kSupportsMultipleFramesPerInputBuffer)) {            break;        }        int64_t coalescedDurationUs = lastBufferTimeUs - timestampUs;        if (coalescedDurationUs > 250000ll) {            // Don‘t coalesce more than 250ms worth of encoded data at once.            break;        }    }...........

该部分是提取视频源数据的关键,主要通过 err = mSource->read(&srcBuffer, &options)来完成,mSource是在创建编解码器传入的,实际是一个对应于视频源格式的一个解析器MediaExtractor。比如在建立MP4的解析器MPEG4Extractor,通过新建一个new MPEG4Source。故最终这里调用的是MPEG4Source的read成员函数,其实际也维护着整个待解码的原始视频流。

我们可以看大在read函数后,会将待解码的数据流以for循环依次读入到底层的buffer空间中,只有当满足当前读取的原始数据片段比底层的input口的buffer剩余空间小srcBuffer->range_length() > remainingBytes,那就可以继续读取,否则直接break后,去进行下一步操作。或者如果一次待解码的数据时张是大于250ms也直接跳出。

这处理体现了处理的高效性。最终视频原始数据存储在info->mData的底层输入空间中。

 

(3)

    err = mOMX->emptyBuffer(            mNode, info->mBuffer, 0, offset,            flags, timestampUs);

触发底层的解码器组件进行处理。这部分留在后续对A31的底层编解码API操作时进行分析。

6.fillOutputBuffers对输出buffer口的填充,即实现解码过程:

void OMXCodec::fillOutputBuffers() {    CHECK_EQ((int)mState, (int)EXECUTING);...........     Vector<BufferInfo> *buffers = &mPortBuffers[kPortIndexOutput];输出端口    for (size_t i = 0; i < buffers->size(); ++i) {        BufferInfo *info = &buffers->editItemAt(i);        if (info->mStatus == OWNED_BY_US) {            fillOutputBuffer(&buffers->editItemAt(i));        }    }}
void OMXCodec::fillOutputBuffer(BufferInfo *info) {    CHECK_EQ((int)info->mStatus, (int)OWNED_BY_US);    if (mNoMoreOutputData) {        CODEC_LOGV("There is no more output data available, not "             "calling fillOutputBuffer");        return;    }    CODEC_LOGV("Calling fillBuffer on buffer %p", info->mBuffer);    status_t err = mOMX->fillBuffer(mNode, info->mBuffer);    if (err != OK) {        CODEC_LOGE("fillBuffer failed w/ error 0x%08x", err);        setState(ERROR);        return;    }    info->mStatus = OWNED_BY_COMPONENT;}

从上面的代码看来,fillOutputBuffer的实现比drainInputBuffers简单了很多。但相同的是,两者最终都讲控制权交给底层的解码器来完成。

 

7.等待解码数据被fill到outbuffer中,OMXCodecObserver完成回调处理

等待解码完成的这部分内容在read函数中通过以下函数来实现:

    while (mState != ERROR && !mNoMoreOutputData && mFilledBuffers.empty()) {        if ((err = waitForBufferFilled_l()) != OK) {//进入等待buffer被填充            return err;        }    }

上述表明,只要mFilledBuffers为空则进入等待填充pthread_cond_timedwait。而这个线程被唤醒是通过底层的组件回调来完成的,回调函数的注册哎底层编解码器Node完成的,实际最终的回调是交给OMXCodecObserver来完成的:

struct OMXCodecObserver : public BnOMXObserver {    OMXCodecObserver() {    }    void setCodec(const sp<OMXCodec> &target) {        mTarget = target;    }    // from IOMXObserver    virtual void onMessage(const omx_message &msg) {        sp<OMXCodec> codec = mTarget.promote();        if (codec.get() != NULL) {            Mutex::Autolock autoLock(codec->mLock);            codec->on_message(msg);//OMX_Codec的on_message处理            codec.clear();        }    }

最终可以看到是由OMX_Codec->on_message来进行消息的处理,这部分的内容主要包括EMPTY_BUFFER_DONE和FILL_BUFFER_DONE两个message处理,对FILL_BUFFER_DONE完成后的消息回调进行分析:

void OMXCodec::on_message(const omx_message &msg) {    if (mState == ERROR) {        /*         * only drop EVENT messages, EBD and FBD are still         * processed for bookkeeping purposes         */        if (msg.type == omx_message::EVENT) {            ALOGW("Dropping OMX EVENT message - we‘re in ERROR state.");            return;        }    }    switch (msg.type) {                                                                                                                                         case omx_message::FILL_BUFFER_DONE://底层回调callback告知当前                        ..............                mFilledBuffers.push_back(i);//当前的输出buffer信息维护在mFilledBuffers                mBufferFilled.signal();//发出信息用于渲染

可以看到这里对read线程进行了唤醒。

8.提取一个可用的解码后的数据帧

    size_t index = *mFilledBuffers.begin();    mFilledBuffers.erase(mFilledBuffers.begin());    BufferInfo *info = &mPortBuffers[kPortIndexOutput].editItemAt(index);//从获取解码后的视频源    CHECK_EQ((int)info->mStatus, (int)OWNED_BY_US);    info->mStatus = OWNED_BY_CLIENT;    info->mMediaBuffer->add_ref();//    if (mSkipCutBuffer != NULL) {        mSkipCutBuffer->submit(info->mMediaBuffer);    }    *buffer = info->mMediaBuffer;

获得了线程唤醒后的buffer,从这里获取到输出端口对应的Bufferinfo,作为最终的BufferInfo信息返回给read函数

9

经过5、6、7、8的处理过程,read最终返回可用于显示的mVideoBuffer,接下去就是如何送显的过程了。可以看到下面的代码,将会创建一个渲染器mVideoRenderer来完成这个解码后视频源的显示:

          

    if ((mNativeWindow != NULL)            && (mVideoRendererIsPreview || mVideoRenderer == NULL)) {//首次创建渲染器        mVideoRendererIsPreview = false;

        initRenderer_l();//初始化渲染器,新建一个AwesomeLocalRenderer    }

    if (mVideoRenderer != NULL) {        mSinceLastDropped++;        mVideoRenderer->render(mVideoBuffer);//启动渲染,即显示当前buffer        if (!mVideoRenderingStarted) {            mVideoRenderingStarted = true;            notifyListener_l(MEDIA_INFO, MEDIA_INFO_RENDERING_START);        }

    }

void AwesomePlayer::initRenderer_l() {    ATRACE_CALL();    if (mNativeWindow == NULL) {        return;    }    sp<MetaData> meta = mVideoSource->getFormat();    int32_t format;    const char *component;    int32_t decodedWidth, decodedHeight;    CHECK(meta->findInt32(kKeyColorFormat, &format));    CHECK(meta->findCString(kKeyDecoderComponent, &component));    CHECK(meta->findInt32(kKeyWidth, &decodedWidth));    CHECK(meta->findInt32(kKeyHeight, &decodedHeight));    int32_t rotationDegrees;    if (!mVideoTrack->getFormat()->findInt32(                kKeyRotation, &rotationDegrees)) {        rotationDegrees = 0;    }    mVideoRenderer.clear();    // Must ensure that mVideoRenderer‘s destructor is actually executed    // before creating a new one.    IPCThreadState::self()->flushCommands();    // Even if set scaling mode fails, we will continue anyway    setVideoScalingMode_l(mVideoScalingMode);    if (USE_SURFACE_ALLOC            && !strncmp(component, "OMX.", 4)            && strncmp(component, "OMX.google.", 11)            && strcmp(component, "OMX.Nvidia.mpeg2v.decode")) {//使用硬件渲染器,除去上述的解码器        // Hardware decoders avoid the CPU color conversion by decoding        // directly to ANativeBuffers, so we must use a renderer that        // just pushes those buffers to the ANativeWindow.        mVideoRenderer =            new AwesomeNativeWindowRenderer(mNativeWindow, rotationDegrees);//一般是使用硬件渲染机制    } else {        // Other decoders are instantiated locally and as a consequence        // allocate their buffers in local address space.  This renderer        // then performs a color conversion and copy to get the data        // into the ANativeBuffer.        mVideoRenderer = new AwesomeLocalRenderer(mNativeWindow, meta);    }}

可以看到这里有2个渲染器的创建分支,OMX和OMX.google说明底层的解码器用的是软解码,那么他渲染器也使用所谓的本地渲染器实际是软渲染器。故这里我们使用的是AwesomeNativeWindowRenderer渲染器,其结构如下所述:

struct AwesomeNativeWindowRenderer : public AwesomeRenderer {    AwesomeNativeWindowRenderer(            const sp<ANativeWindow> &nativeWindow,            int32_t rotationDegrees)        : mNativeWindow(nativeWindow) {        applyRotation(rotationDegrees);    }    virtual void render(MediaBuffer *buffer) {        ATRACE_CALL();        int64_t timeUs;        CHECK(buffer->meta_data()->findInt64(kKeyTime, &timeUs));        native_window_set_buffers_timestamp(mNativeWindow.get(), timeUs * 1000);        status_t err = mNativeWindow->queueBuffer(                mNativeWindow.get(), buffer->graphicBuffer().get(), -1);//直接使用queuebuffer进行渲染显示        if (err != 0) {            ALOGE("queueBuffer failed with error %s (%d)", strerror(-err),                    -err);            return;        }        sp<MetaData> metaData = http://www.mamicode.com/buffer->meta_data();>

不是很复杂,只是实现了AwesomeRenderer渲染接口render。最终调用这个函数来实现对buffer的显示。这里看到很熟悉的queueBuffer,大家可以回看我的博文Android4.2.2 SurfaceFlinger之图形渲染queueBuffer实现和VSYNC的存在感 ,这是通过应用程序的本地窗口mNativeWindow(因为播放器videoview继承了sufaceview,surfaceview类会创建一个本地的surface,其继承了本地窗口类)将当前buffer提交给SurfaceFlinger服务进行显示,具体内容不在展开。

至此我们完成了stagefright下的编解码的数据流的相关操作,程序上复杂主要体现在emptybuffer和fillbuffer为主。当然由于能力有限,在很多细节上也没有进行很详细的分析,也希望大家多交流,多学习。