stagefright 架构分析(五) 初始化 VideoDecoder

VideoDecoder的初始化实际就是OMX的创建，Stagefright提供了OMX的封装OMXCodec

在平台上stagefright -l 可以看到所有的组件

VideoDecoder的创建流程

status_t AwesomePlayer::initVideoDecoder(uint32_t flags) {

//最重要的创建mVideoSource

    sp<MediaSource> decoder = OMXCodec::Create(
            mClient.interface(), mVideoTrack->getFormat(),
            false, // createEncoder
            mVideoTrack,
            NULL, flags, USE_SURFACE_ALLOC ? mNativeWindow : NULL);

        status_t err = mVideoSource->start();

}

sp<MediaSource> OMXCodec::Create(
        const sp<IOMX> &omx,
        const sp<MetaData> &meta, bool createEncoder,
        const sp<MediaSource> &source,
        const char *matchComponentName,
        uint32_t flags,
        const sp<ANativeWindow> &nativeWindow) {

//得到MediaExtractor解析的MIME

    bool success = meta->findCString(kKeyMIMEType, &mime);

//找到对应的decoder name

    findMatchingCodecs(
            mime, createEncoder, matchComponentName, flags,
            &matchingCodecs, &matchingCodecQuirks);

//可能会找到多个匹配的codec

    for (size_t i = 0; i < matchingCodecs.size(); ++i) {

//得到对应的component name

        const char *componentNameBase = matchingCodecs[i].string();

//创建软件codec

            softwareCodec = InstantiateSoftwareDecoder(componentName, source);

//根据component name创建OMX NODE

        status_t err = omx->allocateNode(componentName, observer, &node);

//创建OMXCodec

            sp<OMXCodec> codec = new OMXCodec(
                    omx, node, quirks, flags,
                    createEncoder, mime, componentName,
                    source, nativeWindow);

            observer->setCodec(codec);

//根据meta配置codec
            err = codec->configureCodec(meta);

//最终返回的是OMXCodec

return codec;

}

如何找到对应的codec name？

void OMXCodec::findMatchingCodecs(
        const char *mime,
        bool createEncoder, const char *matchComponentName,
        uint32_t flags,
        Vector<String8> *matchingCodecs,
        Vector<uint32_t> *matchingCodecQuirks) {
//得到codec list

    const MediaCodecList *list = MediaCodecList::getInstance();

    for (;;) {

//从list中找到一个对应的codec name
        ssize_t matchIndex =
            list->findCodecByType(mime, createEncoder, index);

        if (matchIndex < 0) {
            break;
        }

//找到后，就从找到的index的下一个继续查找
        index = matchIndex + 1;

//得到ComponentName，并把其放到matchingCodecs中
        const char *componentName = list->getCodecName(matchIndex);

            matchingCodecs->push(String8(componentName));

    }

}

MediaCodecList是怎么创建的？

//这是SINGLETON模型

const MediaCodecList *MediaCodecList::getInstance() {
    Mutex::Autolock autoLock(sInitMutex);
    if (sCodecList == NULL) {
        sCodecList = new MediaCodecList;
    }
    return sCodecList->initCheck() == OK ? sCodecList : NULL;
}

//这个list是从/etc/media_codecs.xml中解析读到的

MediaCodecList::MediaCodecList()
    : mInitCheck(NO_INIT) {
    FILE *file = fopen("/etc/media_codecs.xml", "r");
    if (file == NULL) {
        ALOGW("unable to open media codecs configuration xml file.");
        return;
    }

//解析的协议遵从xml标准，具体可以看device中对应的设备media_codecs.xml
    parseXMLFile(file);
    if (mInitCheck == OK) {
        // These are currently still used by the video editing suite.
        addMediaCodec(true /* encoder */, "AACEncoder", "audio/mp4a-latm");
        addMediaCodec(
                false /* encoder */, "OMX.google.raw.decoder", "audio/raw");
        Vector<AString> QcomAACQuirks;
        QcomAACQuirks.push(AString("requires-allocate-on-input-ports"));
        QcomAACQuirks.push(AString("requires-allocate-on-output-ports"));
        addMediaCodec(false, "OMX.qcom.audio.decoder.multiaac",
            "audio/mp4a-latm", getCodecSpecificQuirks(QcomAACQuirks));
    }

}

OMXCodec::Create中的OMX是哪来的？

在AwesomePlayer中调用OMXCodec::Create时，传入的OMX是mClient.interface()

OMXClient 的结构：

class OMXClient {
public:
    OMXClient();
    status_t connect();
    void disconnect();
    sp<IOMX> interface() {
        return mOMX;
    }
private:
    sp<IOMX> mOMX;
    OMXClient(const OMXClient &);
    OMXClient &operator=(const OMXClient &);
};

mClient的初始化是在AwesomePlayer构造时完成的，connect实际是其初始化。

AwesomePlayer::AwesomePlayer() {

    CHECK_EQ(mClient.connect(), (status_t)OK);

}

status_t OMXClient::connect() {

//OMX实际上是从MediaPlayerService中得到的

    sp<IMediaPlayerService> service = interface_cast<IMediaPlayerService>(binder);
    mOMX = service->getOMX();

}

//MediaPlayerService只有一个OMX实例

sp<IOMX> MediaPlayerService::getOMX() {
    Mutex::Autolock autoLock(mLock);
    if (mOMX.get() == NULL) {
        mOMX = new OMX;
    }
    return mOMX;
}

OMX如何创建codec对应的component

status_t OMX::allocateNode(
        const char *name, const sp<IOMXObserver> &observer, node_id *node) {

//name是通过findMatchingCodecs找到的component name

    OMXNodeInstance *instance = new OMXNodeInstance(this, observer);
//得到实际的ComponentInstance

    OMX_ERRORTYPE err = mMaster->makeComponentInstance(
            name, &OMXNodeInstance::kCallbacks,
            instance, &handle);

}

OMX_ERRORTYPE OMXMaster::makeComponentInstance(
        const char *name,
        const OMX_CALLBACKTYPE *callbacks,
        OMX_PTR appData,
        OMX_COMPONENTTYPE **component) {
//根据name得到list中的index
    ssize_t index = mPluginByComponentName.indexOfKey(String8(name));
//得到对应的plugin
    OMXPluginBase *plugin = mPluginByComponentName.valueAt(index);
//创建ComponentInstance

    OMX_ERRORTYPE err =
        plugin->makeComponentInstance(name, callbacks, appData, component);
    if (err != OMX_ErrorNone) {
        return err;
    }
    mPluginByInstance.add(*component, plugin);
    return err;
}

codec 参数配置

//通过meta进行配置

err = codec->configureCodec(meta);

status_t OMXCodec::configureCodec(const sp<MetaData> &meta) {

//得到ESDS，解析出来得到codec_specific_data

        if (meta->findData(kKeyESDS, &type, &data, &size)) {
            ESDS esds((const char *)data, size);
            CHECK_EQ(esds.InitCheck(), (status_t)OK);


            const void *codec_specific_data;
            size_t codec_specific_data_size;
            esds.getCodecSpecificInfo(
                    &codec_specific_data, &codec_specific_data_size);


            meta->findCString(kKeyMIMEType, &mime_type);
            if (strncmp(mime_type, MEDIA_MIMETYPE_AUDIO_MPEG, 10)) {
            addCodecSpecificData(
                    codec_specific_data, codec_specific_data_size);
            }

//后面都是类似的操作，根据不同的codec，配置不同的信息，例如：

else if (!strcasecmp(MEDIA_MIMETYPE_AUDIO_AC3, mMIME)) {
        int32_t numChannels, sampleRate;
        CHECK(meta->findInt32(kKeyChannelCount, &numChannels));
        CHECK(meta->findInt32(kKeySampleRate, &sampleRate));
        setAC3Format(numChannels, sampleRate);
    }

//初始化OutputFormat

    initOutputFormat(meta);

//初始化NativeWindow

    if (mNativeWindow != NULL
        && !mIsEncoder
        && !strncasecmp(mMIME, "video/", 6)
        && !strncmp(mComponentName, "OMX.", 4)) {
        status_t err = initNativeWindow();
        if (err != OK) {
            return err;
        }
    }