Android显示系统SurfaceFlinger分析

一 Surfaceflinger介绍

surfaceflinger作用是接受多个来源的图形显示数据，将他们合成，然后发送到显示设备。比如打开应用，常见的有三层显示，顶部的statusbar底部或者侧面的导航栏以及应用的界面，每个层是单独更新和渲染，这些界面都是有surfaceflinger合成一个刷新到硬件显示。在显示过程中使用到了bufferqueue，surfaceflinger作为consumer方，比如windwomanager管理的surface作为生产方产生页面，交由surfaceflinger进行合成。

二 bufferqueue 原理

bufferqueue分为生产者和消费者

比如应用通过windowsmanager分配一个surface，需要分配(dequeueBuffer)显示空间在上面进行绘图，在图形绘制完成后需要推送（queueBuffer）到surfaceflinger进行合成显示。

surfaceflinger作为消费者，通过acquireBuffer（）得到一个要合成的图形，在合成完毕后再releaseBuffer（）将图形释放。

bufferqueue类图关系如下：

三 surfaceflinger 关系图

ComposerService 为单例模式负责与surfaceflinger建立binder连接代码如下：

class ComposerService : public Singleton<ComposerService>
{
    sp<ISurfaceComposer> mComposerService;
    sp<IBinder::DeathRecipient> mDeathObserver;
    Mutex mLock;

    ComposerService();
    void connectLocked();
    void composerServiceDied();
    friend class Singleton<ComposerService>;
public:

    // Get a connection to the Composer Service.  This will block until
    // a connection is established.
    static sp<ISurfaceComposer> getComposerService();
};

void ComposerService::connectLocked() {
    const String16 name("SurfaceFlinger");
    while (getService(name, &mComposerService) != NO_ERROR) {
        usleep(250000);
    }
    assert(mComposerService != NULL);

    // Create the death listener.
    class DeathObserver : public IBinder::DeathRecipient {
        ComposerService& mComposerService;
        virtual void binderDied(const wp<IBinder>& who) {
            ALOGW("ComposerService remote (surfaceflinger) died [%p]",
                  who.unsafe_get());
            mComposerService.composerServiceDied();
        }
     public:
        DeathObserver(ComposerService& mgr) : mComposerService(mgr) { }
    };

    mDeathObserver = new DeathObserver(*const_cast<ComposerService*>(this));
    mComposerService->asBinder()->linkToDeath(mDeathObserver);
}
/*static*/ sp<ISurfaceComposer> ComposerService::getComposerService() {
    ComposerService& instance = ComposerService::getInstance();
    Mutex::Autolock _l(instance.mLock);
    if (instance.mComposerService == NULL) {
        ComposerService::getInstance().connectLocked();
        assert(instance.mComposerService != NULL);
        ALOGD("ComposerService reconnected");
    }
    return instance.mComposerService;
}

SurfaceComposerClient则在于surfaceflinger建立连接后建立与Client的连接，通过client调用createSurface，然后返回SurfaceControl

sp<SurfaceControl> SurfaceComposerClient::createSurface(
        const String8& name,
        uint32_t w,
        uint32_t h,
        PixelFormat format,
        uint32_t flags)
{
    sp<SurfaceControl> sur;
    if (mStatus == NO_ERROR) {
        sp<IBinder> handle;
        sp<IGraphicBufferProducer> gbp;
        status_t err = mClient->createSurface(name, w, h, format, flags,
                &handle, &gbp);
        ALOGE_IF(err, "SurfaceComposerClient::createSurface error %s", strerror(-err));
        if (err == NO_ERROR) {
            sur = new SurfaceControl(this, handle, gbp);
        }
    }
    return sur;
}

SurfaceControl负责这个显示层的控制。

sp<Surface> SurfaceControl::getSurface() const
{
    Mutex::Autolock _l(mLock);
    if (mSurfaceData == 0) {
        // This surface is always consumed by SurfaceFlinger, so the
        // producerControlledByApp value doesn't matter; using false.
        mSurfaceData = new Surface(mGraphicBufferProducer, false);
    }
    return mSurfaceData;
}

通过SurfaceControl::getSurface()，得到的真正的显示层，这样之后可以通过Lock和unlock将surface空间分配绘图，再返回给surfaceflinger

上面只是cpp侧的分析，上层比如WMS是java层，他的管理也是同底层一样，只不过是有层JNI的封装。

四 layer显示内存分配

surface创建后得到 mGraphicBufferProducer,通过mGraphicBufferProducer dequeubuffer在surfaceflinger的BnGraphicBufferProducer dequeuebuffer

int Surface::dequeueBuffer(android_native_buffer_t** buffer, int* fenceFd) {
    status_t result = mGraphicBufferProducer->dequeueBuffer(&buf, &fence, mSwapIntervalZero,
            reqW, reqH, mReqFormat, mReqUsage);

    sp<GraphicBuffer>& gbuf(mSlots[buf].buffer);

    if ((result & IGraphicBufferProducer::BUFFER_NEEDS_REALLOCATION) || gbuf == 0) {
        result = mGraphicBufferProducer->requestBuffer(buf, &gbuf);
        if (result != NO_ERROR) {
            ALOGE("dequeueBuffer: IGraphicBufferProducer::requestBuffer failed: %d", result);
            return result;
        }
       *buffer = gbuf.get();
    }

}

在producer的server侧,new GraphicBuffer分配一个GraphicBuffer

    if (returnFlags & BUFFER_NEEDS_REALLOCATION) {
        BQ_LOGV("dequeueBuffer: allocating a new buffer for slot %d", *outSlot);
        sp<GraphicBuffer> graphicBuffer = new GraphicBuffer(
                width, height, format, BQ_LAYER_COUNT, usage,
                {mConsumerName.string(), mConsumerName.size()});

在graphicbuffer中就是分配一个共享内存

GraphicBuffer::GraphicBuffer(uint32_t inWidth, uint32_t inHeight,
        PixelFormat inFormat, uint32_t inLayerCount, uint64_t usage, std::string requestorName)
    : GraphicBuffer()
{
    mInitCheck = initWithSize(inWidth, inHeight, inFormat, inLayerCount,
            usage, std::move(requestorName));
}

status_t GraphicBuffer::initWithSize(uint32_t inWidth, uint32_t inHeight,
        PixelFormat inFormat, uint32_t inLayerCount, uint64_t inUsage,
        std::string requestorName)
{
    GraphicBufferAllocator& allocator = GraphicBufferAllocator::get();
    uint32_t outStride = 0;
    status_t err = allocator.allocate(inWidth, inHeight, inFormat, inLayerCount,
            inUsage, &handle, &outStride, mId,
            std::move(requestorName));
    if (err == NO_ERROR) {
        mBufferMapper.getTransportSize(handle, &mTransportNumFds, &mTransportNumInts);

        width = static_cast<int>(inWidth);
        height = static_cast<int>(inHeight);
        format = inFormat;
        layerCount = inLayerCount;
        usage = inUsage;
        usage_deprecated = int(usage);
        stride = static_cast<int>(outStride);
    }
    return err;
}

GraphicBufferAllocator::get() 使用gralloc进行内存分配，分配完成后，得到bufferIdx 将他发给client端也就是surface端

    virtual status_t requestBuffer(int bufferIdx, sp<GraphicBuffer>* buf) {
        Parcel data, reply;
        data.writeInterfaceToken(IGraphicBufferProducer::getInterfaceDescriptor());
        data.writeInt32(bufferIdx);
        status_t result =remote()->transact(REQUEST_BUFFER, data, &reply);
        if (result != NO_ERROR) {
            return result;
        }
        bool nonNull = reply.readInt32();
        if (nonNull) {
            *buf = new GraphicBuffer();
            result = reply.read(**buf);
            if(result != NO_ERROR) {
                (*buf).clear();
                return result;
            }
        }
        result = reply.readInt32();
        return result;

返回虚拟地址给上层

        void* vaddr;
        status_t res = backBuffer->lockAsync(
                GRALLOC_USAGE_SW_READ_OFTEN | GRALLOC_USAGE_SW_WRITE_OFTEN,
                newDirtyRegion.bounds(), &vaddr, fenceFd);

五 surfaceflinger Layer

上面创建一个surface后，surfaceflinger对应的是一个layer，当上层layer调用刷新后，onFrameAvailable被调用，通知surfaceflinger有layer更新

void BufferLayer::onFrameAvailable(const BufferItem& item) {
    mFlinger->signalLayerUpdate();
}

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