一文搞懂Codec2解码组件

1 前言

在本篇中,我们将关注Codec 2.0以下几个问题:

1.从顶而下,一个解码组件是如何创建的

2.组件的接口有哪些,分别是什么含义

3.组件是如何运行的,输入与输出的数据流是怎样的

2 组件的创建

CCodec在allocate中,通过CreateComponentByName创建了具体的解码组件。

//android/frameworks/av/media/codec2/sfplguin/CCodec.cpp
void CCodec::allocate(const sp &codecInfo) {
    ...
    AString componentName = codecInfo->getCodecName();
    std::shared_ptr client;

    // set up preferred component store to access vendor store parameters
    //从CCodec调用到component是通过HAL层服务的,默认谷歌的原生服务为 
    //android.hardware.media.c2@IComponentStore/software,默认厂商的服务为 	
    //android.hardware.media.c2@IComponentStore/default,在android小机shell中通过lshal|grep media可以查询 
    //到正在运行的codec2服务,如果厂商已支持codec2,则可以查询到default服务。如果CCodec中能够创建到default 
    //服务,则可以将该服务设置为Preferred Codec2 ComponentStore,也就是将其作为目标组件。
    client = Codec2Client::CreateFromService("default");
    if (client) {
        ALOGI("setting up '%s' as default (vendor) store", client->getServiceName().c_str());
        SetPreferredCodec2ComponentStore(
                std::make_shared(client));
    }
	//创建具体的解码组件或者编码组件,譬如c2.android.avc.decoder
	//所有omx与codec2的编解码组件支持列表可以在libstagefright/data目录下的xml中查询得到,它们的加载与
	//排序情况可以在libstagefright/MediaCodecList.cpp中追踪
    std::shared_ptr comp =
            Codec2Client::CreateComponentByName(
            componentName.c_str(),
            mClientListener,
            &client);
    ...
    ALOGI("Created component [%s]", componentName.c_str());
    mChannel->setComponent(comp);
    auto setAllocated = [this, comp, client] {
        Mutexed::Locked state(mState);
        if (state->get() != ALLOCATING) {
            state->set(RELEASED);
            return UNKNOWN_ERROR;
        }
        state->set(ALLOCATED);
        state->comp = comp;
        mClient = client;
        return OK;
    };
    ...

    // initialize config here in case setParameters is called prior to configure
    Mutexed::Locked config(mConfig);
    status_t err = config->initialize(mClient, comp);
    ...
    config->queryConfiguration(comp);

    mCallback->onComponentAllocated(componentName.c_str());
}

继续追踪Codec2Client::CreateComponentByName接口。

//android/frameworks/av/media/codec2/hidl/client/client.cpp
std::shared_ptr
        Codec2Client::CreateComponentByName(
        const char* componentName,
        const std::shared_ptr& listener,
        std::shared_ptr* owner,
        size_t numberOfAttempts) {
    std::string key{"create:"};
    key.append(componentName);
    std::shared_ptr component;
    c2_status_t status = ForAllServices(
            key,
            numberOfAttempts,
            [owner, &component, componentName, &listener](
                    const std::shared_ptr &client)
                        -> c2_status_t {
                //调用Codec2Client类的createComponent接口,获取component
                c2_status_t status = client->createComponent(componentName,
                                                             listener,
                                                             &component);
                ...
                return status;
            });
    ...
    return component;
}

追踪Codec2Client类的createComponent接口。

\\av\media\codec2\hidl\client\client.cpp
c2_status_t Codec2Client::createComponent(
        const C2String& name,
        const std::shared_ptr& listener,
        std::shared_ptr* const component) {

    c2_status_t status;
    sp hidlListener = new Component::HidlListener{};
    hidlListener->base = listener;
    //这里的mBase是什么?这里调用的是IComponentStore的createComponent接口
    Return transStatus = mBase->createComponent(
            name,
            hidlListener,
            ClientManager::getInstance(),
            [&status, component, hidlListener](
                    Status s,
                    const sp& c) {
                status = static_cast(s);
                if (status != C2_OK) {
                    return;
                }
                *component = std::make_shared(c);
                hidlListener->component = *component;
            });
    ...
    return status;
}

我们先看一下IComponentStore的createComponent接口。

\\av\media\codec2\hidl\1.0\utils\include\codec2\hidl\1.0\ComponentStore.h
struct ComponentStore : public IComponentStore {
    ComponentStore(const std::shared_ptr& store);
    virtual ~ComponentStore() = default;
    // Methods from ::android::hardware::media::c2::V1_0::IComponentStore.
    virtual Return createComponent(
            const hidl_string& name,
            const sp& listener,
            const sp& pool,
            createComponent_cb _hidl_cb) override;
    virtual Return createInterface(
            const hidl_string& name,
            createInterface_cb _hidl_cb) override;、
    ...
}

该接口的实现为:

\\av\media\codec2\hidl\1.0\utils\ComponentStore.cpp
// Methods from ::android::hardware::media::c2::V1_0::IComponentStore
Return ComponentStore::createComponent(
        const hidl_string& name,
        const sp& listener,
        const sp& pool,
        createComponent_cb _hidl_cb) {

    sp component;
    std::shared_ptr c2component;
    //C2PlatformComponentStore的createComponent调用
    //调用C2PlatformComponentStore的createComponent接口,返回的是一个C2Component对象
    //譬如,这个对象可以是C2SoftAvcDec Component对象,也可以是VendorHwAvcDec Component对象
    Status status = static_cast(
            mStore->createComponent(name, &c2component));

    if (status == Status::OK) {
        onInterfaceLoaded(c2component->intf());
        //把前面创建的C2SoftAvcDec“装载”到Component类中,Client调用Component
        //Component内部会调用到C2SoftAvcDec
        //Component相当于对原生编解码组件/厂商编解码组件的统一封装
        component = new Component(c2component, listener, this, pool);
        if (!component) {
            status = Status::CORRUPTED;
        } else {
            reportComponentBirth(component.get());
            if (component->status() != C2_OK) {
                status = static_cast(component->status());
            } else {
                component->initListener(component);
                if (component->status() != C2_OK) {
                    status = static_cast(component->status());
                }
            }
        }
    }
    _hidl_cb(status, component);
    return Void();
}

关于C2PlatformComponentStore的createComponent调用,它的实现在C2Store.cpp中,它继承于C2ComponentStore类,有几个重要成员对象,ComponentModule,ComponentLoader,有几个重要的接口,listComponents(),createComponent(),createInterface()。ComponentLoader包含ComponentModule对象,而ComponentModule主要提供两个接口,createComponent()与createInterface(),内部也包含着C2ComponentFactory成员以及它的创建与销毁接口,分别是C2ComponentFactory::CreateCodec2FactoryFunc,C2ComponentFactory::DestroyCodec2FactoryFunc。

    \\av\media\codec2\vndk\C2Store.cpp
    class C2PlatformComponentStore : public C2ComponentStore {
    public:
        virtual std::vector> listComponents() override;
        ...
        virtual c2_status_t createInterface(
                C2String name, std::shared_ptr *const interface) override;
        virtual c2_status_t createComponent(
                C2String name, std::shared_ptr *const component) override;
        virtual ~C2PlatformComponentStore() override = default;
    
    private:
    
        /**
         * An object encapsulating a loaded component module.
         */
        struct ComponentModule : public C2ComponentFactory,
                public std::enable_shared_from_this {
            virtual c2_status_t createComponent(
                    c2_node_id_t id, std::shared_ptr *component,
                    ComponentDeleter deleter = std::default_delete()) override;
            virtual c2_status_t createInterface(
                    c2_node_id_t id, std::shared_ptr *interface,
                    InterfaceDeleter deleter = std::default_delete()) override;
     		...
        protected:
    		...
            void *mLibHandle; ///< loaded library handle
            C2ComponentFactory::CreateCodec2FactoryFunc createFactory; ///< loaded create function
            C2ComponentFactory::DestroyCodec2FactoryFunc destroyFactory; ///< loaded destroy function
            C2ComponentFactory *mComponentFactory; ///< loaded/created component factory
        };
    
        /**
         * An object encapsulating a loadable component module.
         */
        struct ComponentLoader {
            /**
             * Load the component module.
             *
             * This method simply returns the component module if it is already currently loaded, or
             * attempts to load it if it is not.
             */
            c2_status_t fetchModule(std::shared_ptr *module) {
                c2_status_t res = C2_OK;
                std::lock_guard lock(mMutex);
                std::shared_ptr localModule = mModule.lock();
                if (localModule == nullptr) {
                    localModule = std::make_shared();
                    res = localModule->init(mLibPath);
                    if (res == C2_OK) {
                        mModule = localModule;
                    }
                }
                *module = localModule;
                return res;
            }
    
            /**
             * Creates a component loader for a specific library path (or name).
             */
            ComponentLoader(std::string libPath)
                : mLibPath(libPath) {}
    
        private:
            std::weak_ptr mModule; ///< weak reference to the loaded module
        };
    
        struct Interface : public C2InterfaceHelper {
    	...
        };
    
        /**
         * Retrieves the component module for a component.
         */
        c2_status_t findComponent(C2String name, std::shared_ptr *module);
    
        /**
         * Loads each component module and discover its contents.
         */
        void visitComponents();
        std::map mComponents; ///< path -> component module
        std::map mComponentNameToPath; ///< name -> path
        std::vector> mComponentList;
    	...
    };

C2PlatformComponentStore::createComponent调用findComponent(name, &module)找到拥有component的ComponentModule,再通过module->createComponent(0, component)调用,找到相应的component。

    \\av\media\codec2\vndk\C2Store.cpp
    c2_status_t C2PlatformComponentStore::createComponent(
            C2String name, std::shared_ptr *const component) {
        // This method SHALL return within 100ms.
        component->reset();
        std::shared_ptr module;
        c2_status_t res = findComponent(name, &module);
        if (res == C2_OK) {
            // TODO: get a unique node ID
            res = module->createComponent(0, component);
        }
        return res;
    }

findComponent(name, &module)有两步,先通过visitComponents()列举出所有可用的components,再调用ComponentLoader的fetchModule(),找到拥有component的ComponentModule。module可以看作是组件,加载某个module,也就是加载对应的组件,module提供的 createComponent()接口就是用来创建具体component的,譬如C2SoftAvcDec。

    \\av\media\codec2\vndk\C2Store.cpp
    c2_status_t C2PlatformComponentStore::findComponent(
            C2String name, std::shared_ptr *module) {
        (*module).reset();
        visitComponents();
        auto pos = mComponentNameToPath.find(name);
        if (pos != mComponentNameToPath.end()) {
            return mComponents.at(pos->second).fetchModule(module);
        }
        return C2_NOT_FOUND;
    }

visitComponents()访问mComponents对象(这是一个map对象,将path与component module映射关联,这一映射工作在C2PlatformComponentStore初始化时进行),遍历所有的mComponents,即pathAndLoader对象,如果一个对象的loader能够加载成功,则添加到mComponentNameToPath对象中。

    \\av\media\codec2\vndk\C2Store.cpp
    void C2PlatformComponentStore::visitComponents() {
        std::lock_guard lock(mMutex);
        if (mVisited) {
            return;
        }
        //参考定义 std::map mComponents; ///< path -> component module
        for (auto &pathAndLoader : mComponents) {
            const C2String &path = pathAndLoader.first;
            ComponentLoader &loader = pathAndLoader.second;
            std::shared_ptr module;
            if (loader.fetchModule(&module) == C2_OK) {
                std::shared_ptr traits = module->getTraits();
                if (traits) {
                    mComponentList.push_back(traits);
                    mComponentNameToPath.emplace(traits->name, path);
                    for (const C2String &alias : traits->aliases) {
                        mComponentNameToPath.emplace(alias, path);
                    }
                }
            }
        }
        mVisited = true;
    }

loader.fetchModule(&module)这个函数定义在ComponentLoader类中,在这里再贴一次代码。

    \\av\media\codec2\vndk\C2Store.cpp   
    c2_status_t fetchModule(std::shared_ptr *module) {
        c2_status_t res = C2_OK;
        std::lock_guard lock(mMutex);
        std::shared_ptr localModule = mModule.lock();
        if (localModule == nullptr) {
            localModule = std::make_shared();
            res = localModule->init(mLibPath);
            if (res == C2_OK) {
                mModule = localModule;
            }
        }
        *module = localModule;
        return res;
    }

对于module,会调用初始化函数,初始化成功就算是fetch到了。初始化作了什么工作,参见C2PlatformComponentStore::ComponentModule::init函数,也就是对编解码库dlopen成功,可获得相应的函数地址,譬如,C2SoftAvcDec.cpp中的C2ComponentFactory* CreateCodec2Factory()与void DestroyCodec2Factory()。当然还有其他,不面面俱道了。

    \\av\media\codec2\vndk\C2Store.cpp   
    c2_status_t C2PlatformComponentStore::ComponentModule::init(
            std::string libPath) {
        ALOGV("in %s", __func__);
        ALOGV("loading dll");
        mLibHandle = dlopen(libPath.c_str(), RTLD_NOW|RTLD_NODELETE);
        createFactory =
            (C2ComponentFactory::CreateCodec2FactoryFunc)dlsym(mLibHandle, "CreateCodec2Factory");
        LOG_ALWAYS_FATAL_IF(createFactory == nullptr,
                "createFactory is null in %s", libPath.c_str());
        destroyFactory =
            (C2ComponentFactory::DestroyCodec2FactoryFunc)dlsym(mLibHandle, "DestroyCodec2Factory");
        LOG_ALWAYS_FATAL_IF(destroyFactory == nullptr,
                "destroyFactory is null in %s", libPath.c_str());
        mComponentFactory = createFactory();
    	...
        std::shared_ptr intf;
        c2_status_t res = createInterface(0, &intf);
    	...
        return mInit;
    }

那么问题来了,为什么谷歌对它自己的codec2插件组C2PlatformComponentStore设计得这么复杂,能不能简化一点。

3 组件接口

在codec2/components目录下,有base, avc, aom, hevc, aac等文件夹,base目录下是SimpleC2Component.cpp与SimpleC2Interface.cpp以及对应的头文件,avc目录下是C2SoftAvcDec.cpp,C2SoftAvcEnc.cpp以及对应的头文件,其他编解码器文件夹亦同样道理。C2SoftAvcDec,C2SoftHevcDec等编解码器类都是继承于SimpleC2Component类的,也就是说,SimpleC2Component是components的顶层类,它对接了component类的接口,实现了编解码器的公共流程部分,C2SoftAvcDec,C2SoftHevcDec等子类继承SimpleC2Component的一些接口,实现各自的编解码操作。

SimpleC2Component实现的component的接口如下:

    \\av\media\codec2\components\base\include\SimpleC2Component.h
    // C2Component
    // From C2Component
    //设置回调
    virtual c2_status_t setListener_vb(
    	const std::shared_ptr &listener, c2_blocking_t mayBlock) override;
    //送数据到component,数据打包成某种对象,叫C2Work,这个对象很关键,它包含input与output
    virtual c2_status_t queue_nb(std::list>* const items) override;
    //暂时没有多大用处,不管它
    virtual c2_status_t announce_nb(const std::vector &items) override;
    //跳播使用,将当前数据冲刷掉
    virtual c2_status_t flush_sm(
    	flush_mode_t mode, std::list>* const flushedWork) override;
    //渲染可用的帧
    virtual c2_status_t drain_nb(drain_mode_t mode) override;
    virtual c2_status_t start() override;
    virtual c2_status_t stop() override;
    virtual c2_status_t reset() override;
    virtual c2_status_t release() override;
    virtual std::shared_ptr intf() override;

而C2SoftAvcDec,C2SoftHevcDec等子类继承SimpleC2Component的接口如下:

    \\av\media\codec2\components\base\include\SimpleC2Component.h
    virtual c2_status_t onInit() = 0;
    virtual c2_status_t onStop() = 0;
    virtual void onReset() = 0;
    virtual void onRelease() = 0;
    virtual c2_status_t onFlush_sm() = 0;
    //最重要的处理函数,处理的对象是C2Work,它包含着输入输出,交互配置方面的类。
    virtual void process(
        const std::unique_ptr &work,
        const std::shared_ptr &pool) = 0;
    virtual c2_status_t drain(
        uint32_t drainMode,
        const std::shared_ptr &pool) = 0;

4 组件运行原理

SimpleC2Component有一个成员对象WorkHandler,这个类继承于AHandler,也就是说,SimpleC2Component内部运行一个线程,来自上层的接口调用,都可以发送消息到onMessageReceived中排队处理,譬如初始化、停止、重置、释放以及数据处理等工作,都在队列中排队处理,相应的处理都是调用到子类的实现,譬如,onInit(),onStop(),onReset(),onRelease(),以及processQueue()。

我们可以看一下onMessageReceived的实现。

\\av\media\codec2\components\base\SimpleC2Component.cpp
void SimpleC2Component::WorkHandler::onMessageReceived(const sp &msg) {
    std::shared_ptr thiz = mThiz.lock();
	...
    switch (msg->what()) {
        case kWhatProcess: {
            if (mRunning) {
                if (thiz->processQueue()) {
                    (new AMessage(kWhatProcess, this))->post();
                }
            } else {
                ALOGV("Ignore process message as we're not running");
            }
            break;
        }
        case kWhatInit: {
            int32_t err = thiz->onInit();
            Reply(msg, &err);
            [[fallthrough]];
        }
        case kWhatStart: {
            mRunning = true;
            break;
        }
        case kWhatStop: {
            int32_t err = thiz->onStop();
            Reply(msg, &err);
            break;
        }
        case kWhatReset: {
            thiz->onReset();
            mRunning = false;
            Reply(msg);
            break;
        }
        case kWhatRelease: {
            thiz->onRelease();
            mRunning = false;
            Reply(msg);
            break;
        }
        default: {
            ALOGD("Unrecognized msg: %d", msg->what());
            break;
        }
    }
}

我们看一下AVC解码器内部是如何处理输入与输出数据的,在这个process中,处理完输入,解码,处理输出,在处理output buffer时,process的思路是这样的:从内存池申请一个GraphicBlock,对应地设置给解码器Buffer地址以供解码输出,如果解码后有帧输出,则将当前的GraphicBlock转换为C2Buffer对象,返回给上层。类似于FFMPEG,你给它一个output frame,它就将解码图片填充到frame,你取走显示。可以推断,软解码器内部应该也有申请一个队列的buffer,这个队列维护着解码所需要的参考图像。

\\av\media\codec2\components\avc\C2SoftAvcDec.cpp
//省略了部分不影响理解主要流程的代码
void C2SoftAvcDec::process(
        const std::unique_ptr &work,
        const std::shared_ptr &pool) {
    // Initialize output work
    work->result = C2_OK;
    work->workletsProcessed = 0u;
    work->worklets.front()->output.flags = work->input.flags;
    size_t inOffset = 0u;
    size_t inSize = 0u;
    uint32_t workIndex = work->input.ordinal.frameIndex.peeku() & 0xFFFFFFFF;
    C2ReadView rView = mDummyReadView;
    if (!work->input.buffers.empty()) {
    	//为了得到输入数据,层层访问,真正放数据的地址在rView.data()[]中
    	//把work这个对象用思维导图画出来,我们可以更容易的理解work,到底拥有哪些成员,如何访问
        rView = work->input.buffers[0]->data().linearBlocks().front().map().get();
        inSize = rView.capacity();
        ...
    }
    bool eos = ((work->input.flags & C2FrameData::FLAG_END_OF_STREAM) != 0);
    bool hasPicture = false;

    ALOGV("in buffer attr. size %zu timestamp %d frameindex %d, flags %x",
          inSize, (int)work->input.ordinal.timestamp.peeku(),
          (int)work->input.ordinal.frameIndex.peeku(), work->input.flags);
    size_t inPos = 0;
    while (inPos < inSize) {
    	//ensureDecoderState会从内存池中fetch一个GraphicBlock
    	//实质上也就是调用Gralloc接口取得一个output buffer
        if (C2_OK != ensureDecoderState(pool)) {
            mSignalledError = true;
            work->workletsProcessed = 1u;
            work->result = C2_CORRUPTED;
            return;
        }

        ivd_video_decode_ip_t s_decode_ip;
        ivd_video_decode_op_t s_decode_op;
        {
        	//mOutBlock即是上述fetch到的output buffer,通过map映射可以得到一个wView,类似于rView
        	//wView.data()[]指向out buffer的真正地址
        	//wView.data()[C2PlanarLayout::PLANE_Y]就是要存在Y变量的地址
        	//wView.data()[C2PlanarLayout::PLANE_U]就是要存在U变量的地址
            C2GraphicView wView = mOutBlock->map().get();
            ...
            //setDecodeArgs所作的主要工作是,告诉解码器,输入数据的地址是什么,输出地址包括Y/U/V
            //分量的地址是什么,输入数据的长度是多少
            if (!setDecodeArgs(&s_decode_ip, &s_decode_op, &rView, &wView,
                               inOffset + inPos, inSize - inPos, workIndex)) {
                mSignalledError = true;
                work->workletsProcessed = 1u;
                work->result = C2_CORRUPTED;
                return;
            }

            if (false == mHeaderDecoded) {
                /* Decode header and get dimensions */
                setParams(mStride, IVD_DECODE_HEADER);
            }
            //解码器库是用了第三方的,已经被谷歌收购
            (void) ivdec_api_function(mDecHandle, &s_decode_ip, &s_decode_op);
        }
        if (s_decode_op.i4_reorder_depth >= 0 && mOutputDelay != s_decode_op.i4_reorder_depth) {
        	//目前不清楚把这个重排序长度告诉上层有什么作用,TODO
            mOutputDelay = s_decode_op.i4_reorder_depth;
            ALOGV("New Output delay %d ", mOutputDelay);

            C2PortActualDelayTuning::output outputDelay(mOutputDelay);
            std::vector> failures;
            c2_status_t err =
                mIntf->config({&outputDelay}, C2_MAY_BLOCK, &failures);
            if (err == OK) {
                work->worklets.front()->output.configUpdate.push_back(
                    C2Param::Copy(outputDelay));
            } 
            continue;
        }
        if (0 < s_decode_op.u4_pic_wd && 0 < s_decode_op.u4_pic_ht) {
            if (mHeaderDecoded == false) {
                mHeaderDecoded = true;
                setParams(ALIGN64(s_decode_op.u4_pic_wd), IVD_DECODE_FRAME);
            }
            if (s_decode_op.u4_pic_wd != mWidth || s_decode_op.u4_pic_ht != mHeight) {
                mWidth = s_decode_op.u4_pic_wd;
                mHeight = s_decode_op.u4_pic_ht;
                CHECK_EQ(0u, s_decode_op.u4_output_present);

                C2StreamPictureSizeInfo::output size(0u, mWidth, mHeight);
                std::vector> failures;
                c2_status_t err = mIntf->config({&size}, C2_MAY_BLOCK, &failures);
                if (err == OK) {
                    work->worklets.front()->output.configUpdate.push_back(
                        C2Param::Copy(size));
                } 
                continue;
            }
        }
        (void)getVuiParams();
        hasPicture |= (1 == s_decode_op.u4_frame_decoded_flag);
        if (s_decode_op.u4_output_present) {
        	//通过createGraphicBuffer调用,将mOutBlock"转换"成C2Buffer对象
        	//把C2Buffer添加到work对象的输出队列中
        	//通过listener->onWorkDone_nb回调,可以将work返回到CCodec层
        	//以上是这个函数以及其内部调用的主要实现内容,内部调用的finish()函数属于SimpleC2Component
            finishWork(s_decode_op.u4_ts, work);
        }
        inPos += s_decode_op.u4_num_bytes_consumed;
    }
    if (eos) {
        drainInternal(DRAIN_COMPONENT_WITH_EOS, pool, work);
        mSignalledOutputEos = true;
    } else if (!hasPicture) {
        fillEmptyWork(work);
    }

    work->input.buffers.clear();
}

在Component中,输入与输出对象都封装在work对象中,甚至上下层的配置交互对象也包括在work对象中,与OMX是不一样的,OMX的数据对象是BufferHeader,输入是一个Input BufferHeader,输出是一个Output BufferHeader,对象中包括buffer地址,分配的buffer大小,有效数据长度,有效数据长度的偏移量,buffer标志等。 那么,work对象也应该会包括类似的成员。

我们来看两张思维导图,全局观察work对象。

一文搞懂Codec2解码组件_第1张图片

一文搞懂Codec2解码组件_第2张图片

C2SoftAvcDec::process中有一句代码,从work中访问rView。

rView = work->input.buffers[0]->data().linearBlocks().front().map().get();

从上述两图中,我们可以追踪这一条访问线路,访问C2Work对象的成员C2FrameData,继续访问C2FrameData对外的成员vector linearBlocks(),C2ConstLinearBlock有一个方法C2Acquirable map(),这个映射方法返回一个C2ReadView对象,这个C2ReadView对象有一个data()[]数组,指向了Y/U/V的向量地址,也就是真正存放解码数据的内存地址。而Input与Output都是以C2FrameData来描述,Output并非像Input一样,直接作为C2Work的成员,而是作为C2Work->worklets的成员。worklet是一个list类型,C2SoftAvcDec在存放output buffer的时候,总是存放在第一个worklets的output中,参见思维导图,output是C2FrameData类型,它拥有一个C2Buffer容器,C2SoftAvcDec总是将新的output buffer丢进容器中,它可以一次丢很多个output buffer,然后一次性通过work回送到上层,上层可以一次性从work中取到多个output buffer去作渲染。C2WorkOrdinalStruct ordinal包括着buffer的pts与frameIndex信息。这里有个疑问待解决,为什么output buffer总是存放在第一个worklets的output中,worklets作为一个队列对象,有什么其他的意义?

上面我们分析了两个点,一个是模块的消息处理机制,另一个是如何送数据到解码器再取出帧数据回送到上层,接下来看第三点,CCodec每次送多少输入数据下来,component每次处理多少数据,回送输出数据给CCodec作渲染在哪些地方。

上层是调用SimpleC2Component::queue_nb接口送数据下来的。

\\av\media\codec2\components\base\SimpleC2Component.cpp
c2_status_t SimpleC2Component::queue_nb(std::list> * const items) {
    {
        Mutexed::Locked state(mExecState);
        if (state->mState != RUNNING) {
            return C2_BAD_STATE;
        }
    }
    bool queueWasEmpty = false;
    {
        Mutexed::Locked queue(mWorkQueue);
        queueWasEmpty = queue->empty();
        while (!items->empty()) {
            queue->push_back(std::move(items->front()));
            items->pop_front();
        }
    }
    if (queueWasEmpty) {
        (new AMessage(WorkHandler::kWhatProcess, mHandler))->post();
    }
    return C2_OK;
}

观察上面的代码,入参是一个列表对象,也就是说,每次送多个work,一个work可以包括一个C2Buffer容器,码流都是放在容器的第一个元素,虽然一个容器可以放多个C2Buffer,但它就只放了一个C2Buffer。我们可以从下面的代码中发现,每一次的process,都只从work中取一个C2Buffer。

 \\av\media\codec2\components\avc\C2SoftAvcDec.cpp
 void C2SoftAvcDec::process(
         const std::unique_ptr &work,
         const std::shared_ptr &pool) {
 	...
     uint32_t workIndex = work->input.ordinal.frameIndex.peeku() & 0xFFFFFFFF;
     C2ReadView rView = mDummyReadView;
     if (!work->input.buffers.empty()) {
     	//关注buffers[0]
         rView = work->input.buffers[0]->data().linearBlocks().front().map().get();
         inSize = rView.capacity();
     }
 }

SimpleC2Component::processQueue()每次只处理一个work,处理完就把work回送上去。

\\av\media\codec2\components\base\SimpleC2Component.cpp
bool SimpleC2Component::processQueue() {
	....
    ALOGV("start processing frame #%" PRIu64, work->input.ordinal.frameIndex.peeku());
    //处理work
    process(work, mOutputBlockPool);
    ALOGV("processed frame #%" PRIu64, work->input.ordinal.frameIndex.peeku());
    Mutexed::Locked queue(mWorkQueue);
    if (work->workletsProcessed != 0u) {
        queue.unlock();
        Mutexed::Locked state(mExecState);
        ALOGV("returning this work");
        std::shared_ptr listener = state->mListener;
        state.unlock();
        //回送work
        listener->onWorkDone_nb(shared_from_this(), vec(work));
    }
    ...
}

在没有新送下来的work需要处理的时候,processQueue()会调用drain接口作“渲染”操作,它会看解码器是否有帧数据生成,有的话,就填充到work中回送到上层。

\\av\media\codec2\components\base\SimpleC2Component.cpp
bool SimpleC2Component::processQueue() {
	....
    if (!work) {
        c2_status_t err = drain(drainMode, mOutputBlockPool);
        if (err != C2_OK) {
            Mutexed::Locked state(mExecState);
            std::shared_ptr listener = state->mListener;
            state.unlock();
            listener->onError_nb(shared_from_this(), err);
        }
        return hasQueuedWork;
    }
    ...
}

另一个渲染的地方是在process()中,解码完发现有帧数据的时候,就调用finishWork()将work回送。

\\av\media\codec2\components\avc\C2SoftAvcDec.cpp
void C2SoftAvcDec::process(
        const std::unique_ptr &work,
        const std::shared_ptr &pool) {
	...
        if (s_decode_op.u4_output_present) {
            finishWork(s_decode_op.u4_ts, work);
        }
    ...
}

5 小结

Component内部的逻辑还是比较好理解的,重点在于它是如何申请buffer的,如何将buffer“送”给解码器,解码完后是如何取得buffer并返回上层,难点在于work对象层层封装,当你要访问实际内存地址时,如何访问,如果要取得内存的handle,又要如何访问,这一点通过将work对象一层一层的“绘制”出来,就好懂得多。接下来问题来了,在OMX中,上下层的交互配置是通过setParamerter/getParamerter等接口进行的,那么在Codec2中是如何进行的?Codec2中到底有没有像OMX一样的BufferCountActual设计?Codec2在调用nativewindow的setMaxDequeuedBufferCount时是如何确定maxDequeueBufferCount的?GraphicBuffer的生命周期是如何控制的?

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