当有事务的更新或者有Buffer的更新便会触发后面刷新的流程。

onMessageRefresh

文件：frameworks/native/services/surfaceflinger/Surfaceflinger.cpp
void SurfaceFlinger::onMessageRefresh() {
    ATRACE_CALL();

    mRefreshPending = false;

    compositionengine::CompositionRefreshArgs refreshArgs;
    const auto& displays = ON_MAIN_THREAD(mDisplays);
     // display 的数量，把当前displaydevice 记录下来
    refreshArgs.outputs.reserve(displays.size());
    for (const auto& [_, display] : displays) {
        refreshArgs.outputs.push_back(display->getCompositionDisplay());
    }
    // 把当前layer记录下来
    mDrawingState.traverseInZOrder([&refreshArgs](Layer* layer) {
        if (auto layerFE = layer->getCompositionEngineLayerFE())
            refreshArgs.layers.push_back(layerFE);
    });
    // 把当前有queueframe的layer记录下来, 有Buffer的layer
    refreshArgs.layersWithQueuedFrames.reserve(mLayersWithQueuedFrames.size());
    for (sp<Layer> layer : mLayersWithQueuedFrames) {
        if (auto layerFE = layer->getCompositionEngineLayerFE())
            refreshArgs.layersWithQueuedFrames.push_back(layerFE);
    }
 
    ...
    // 主要逻辑在present里面
    mCompositionEngine->present(refreshArgs);
   }

刷新的主要逻辑在present里面。

文件：frameworks/native/services/surfaceflinger/CompositionEngine/src/CompositionEngine.cpp

void CompositionEngine::present(CompositionRefreshArgs& args) {
    ATRACE_CALL();
    ALOGV(__FUNCTION__);
    // 根据是否还有layer没有消费掉，判断再调起一次刷新
    preComposition(args);

    {
        // latchedLayers is used to track the set of front-end layer state that
        // has been latched across all outputs for the prepare step, and is not
        // needed for anything else.
        LayerFESet latchedLayers;

        for (const auto& output : args.outputs) {
            // 计算各个区域大小以及创建hwc layer
            output->prepare(args, latchedLayers);
        }
    }
    // 更新layerCompositionState
    updateLayerStateFromFE(args);

    for (const auto& output : args.outputs) {
        // 逻辑在output.cpp的present里面
        output->present(args);
    }
}

prepare主要的作用是计算各个区域大小以及创建hwc layer，主要逻辑在ensureOutputLayerIfVisible 里面。

ensureOutputLayerIfVisible

文件：frameworks/native/services/surfaceflinger/CompositionEngine/src/Output.cpp

void Output::ensureOutputLayerIfVisible(sp<compositionengine::LayerFE>& layerFE,
                                        compositionengine::Output::CoverageState& coverage) {
    ...
    /*
     * opaqueRegion: area of a surface that is fully opaque.
     */
    Region opaqueRegion;

    /*
     * visibleRegion: area of a surface that is visible on screen and not fully
     * transparent. This is essentially the layer's footprint minus the opaque
     * regions above it. Areas covered by a translucent surface are considered
     * visible.
     */
    Region visibleRegion;

    /*
     * coveredRegion: area of a surface that is covered by all visible regions
     * above it (which includes the translucent areas).
     */
    Region coveredRegion;

    /*
     * transparentRegion: area of a surface that is hinted to be completely
     * transparent. This is only used to tell when the layer has no visible non-
     * transparent regions and can be removed from the layer list. It does not
     * affect the visibleRegion of this layer or any layers beneath it. The hint
     * may not be correct if apps don't respect the SurfaceView restrictions
     * (which, sadly, some don't).
     */
    Region transparentRegion;

    /*
     * shadowRegion: Region cast by the layer's shadow.
     */
    Region shadowRegion;
    ....
    // 创建OutputLayer
    auto result = ensureOutputLayer(prevOutputLayerIndex, layerFE);
    ...
 }

各个layer区域的计算结果可以通过adb shell dumpsys SurfaceFlinger看到，通过区域的计算把区域为空的layer过滤掉

区域.png

ensureOutputLayer将创建Output layer，这些layer最后是要显示到屏幕上的。

文件: frameworks/native/services/surfaceflinger/CompositionEngine/include/compositionengine/impl/Output.h

OutputLayer* ensureOutputLayer(std::optional<size_t> prevIndex,
                                       const sp<LayerFE>& layerFE) {

            // 判断当前的outputlayer 集合里面有没有当前的layer，如果没有则新创建一个
            auto outputLayer = (prevIndex && *prevIndex <= mCurrentOutputLayersOrderedByZ.size())
                    ? std::move(mCurrentOutputLayersOrderedByZ[*prevIndex])
                    : BaseOutput::createOutputLayer(layerFE);
            auto result = outputLayer.get();
            mPendingOutputLayersOrderedByZ.emplace_back(std::move(outputLayer));
            return result;
        }


文件：frameworks/native/services/surfaceflinger/CompositionEngine/src/Display.cpp

std::unique_ptr<compositionengine::OutputLayer> Display::createOutputLayer(
        const sp<compositionengine::LayerFE>& layerFE) const {
    auto result = impl::createOutputLayer(*this, layerFE);

    if (result && mId) {
        auto& hwc = getCompositionEngine().getHwComposer();
        auto displayId = *mId;
        // 创建hwclayer
        auto hwcLayer = std::shared_ptr<HWC2::Layer>(hwc.createLayer(displayId),
                                                     [&hwc, displayId](HWC2::Layer* layer) {
                                                         hwc.destroyLayer(displayId, layer);
                                                     });
        ALOGE_IF(!hwcLayer, "Failed to create a HWC layer for a HWC supported display %s",
                 getName().c_str());
        // 更新state.hwc
        result->setHwcLayer(std::move(hwcLayer));
    }
    return result;
}

可以看到prepare主要的作用是遍历layer计算各个区域大小，过滤掉空区域的layer以及创建hwc layer。

updateLayerStateFromFE

更新OutputlayerState的参数，即layer的状态信息。

文件：frameworks/native/services/surfaceflinger/Layer.cpp

void Layer::prepareCompositionState(compositionengine::LayerFE::StateSubset subset) {
    using StateSubset = compositionengine::LayerFE::StateSubset;

    switch (subset) {
        case StateSubset::BasicGeometry:
            prepareBasicGeometryCompositionState();
            break;

        case StateSubset::GeometryAndContent:
            prepareBasicGeometryCompositionState();
            prepareGeometryCompositionState();
            preparePerFrameCompositionState();
            break;

        case StateSubset::Content:
            preparePerFrameCompositionState();
            break;

        case StateSubset::Cursor:
            prepareCursorCompositionState();
            break;
    }
}

这部分也可以通过adb shell dumpsys SurfaceFlinger看出来有哪些属性。

接下来看下present的逻辑。

文件：frameworks/native/services/surfaceflinger/CompositionEngine/src/Output.cpp

void Output::present(const compositionengine::CompositionRefreshArgs& refreshArgs) {
    ATRACE_CALL();
    ALOGV(__FUNCTION__);
     //设置 Display的dataspace, colormode
    updateColorProfile(refreshArgs);
    // 设置outputlayer的dispFrame和sourceCrop等以及将outputlayer属性设给hwc
    updateAndWriteCompositionState(refreshArgs);
    // 设置display的颜色矩阵，对全屏有效
    setColorTransform(refreshArgs);
    // FrameBuffer里面没做啥操作
    beginFrame();
     // 选择合成策略，判断是device还是GPU合成，如果是device合成，直接present，如果要走GPU合成则需要validate
    prepareFrame();
    // 一般不走
    devOptRepaintFlash(refreshArgs);
    // GPU合成主要逻辑在此，device合成没做啥
    finishFrame(refreshArgs);
    // device合成设置release fence，GPU合成需要present给hwc
    postFramebuffer();
}

先关注device合成，涉及GPU部分单独分析。

updateAndWriteCompositionState

文件：frameworks/native/services/surfaceflinger/CompositionEngine/src/Output.cpp

void Output::updateAndWriteCompositionState(
        const compositionengine::CompositionRefreshArgs& refreshArgs) {
    ATRACE_CALL();
    ALOGV(__FUNCTION__);

    if (!getState().isEnabled) {
        return;
    }

    mLayerRequestingBackgroundBlur = findLayerRequestingBackgroundComposition();
    // 如果是有背景模糊的layer则强制使用GPU合成
    bool forceClientComposition = mLayerRequestingBackgroundBlur != nullptr;

    for (auto* layer : getOutputLayersOrderedByZ()) {
        // 遍历outputlayer，计算DisplayFrame， SourceCrop 等参数
        layer->updateCompositionState(refreshArgs.updatingGeometryThisFrame,
                                      refreshArgs.devOptForceClientComposition ||
                                              forceClientComposition,
                                      refreshArgs.internalDisplayRotationFlags);

        if (mLayerRequestingBackgroundBlur == layer) {
            forceClientComposition = false;
        }

        // 将layer属性设给hwc
        layer->writeStateToHWC(refreshArgs.updatingGeometryThisFrame);
    }

这部分参数也可以通过adb shell dumpsys SurfaceFlinger看到

屏幕显示区域.png

setColorTransform

文件：frameworks/native/services/surfaceflinger/CompositionEngine/src/Display.cpp

void Display::setColorTransform(const compositionengine::CompositionRefreshArgs& args) {
    Output::setColorTransform(args);

    if (!mId || CC_LIKELY(!args.colorTransformMatrix)) {
        return;
    }

    auto& hwc = getCompositionEngine().getHwComposer();
    // 将上层设的colorTransformMatrix 设给hwc，这个效果作用于全屏，比如护眼模式下，改变的就是这个colorTransformMatrix
    status_t result = hwc.setColorTransform(*mId, *args.colorTransformMatrix);
    ALOGE_IF(result != NO_ERROR, "Failed to set color transform on display \"%s\": %d",
             mId ? to_string(*mId).c_str() : "", result);
}

setColorTransform 设置颜色矩阵给屏幕，作用于屏幕显示，所有的layer都受影响，护眼效果就是改变的这个值。

prepareFrame

文件：frameworks/native/services/surfaceflinger/CompositionEngine/src/Output.cpp

void Output::prepareFrame() {
    ATRACE_CALL();
    ALOGV(__FUNCTION__);

    const auto& outputState = getState();
    if (!outputState.isEnabled) {
        return;
    }
    // 选择合成类型，如果是device合成，则跳过validate，直接present送显
    chooseCompositionStrategy();
    // 把合成类型送到frameBufferSurface，没啥逻辑
    mRenderSurface->prepareFrame(outputState.usesClientComposition,
                                 outputState.usesDeviceComposition);
}

void Output::chooseCompositionStrategy() {
    // The base output implementation can only do client composition
    // 默认使用GPU合成,针对没有hwc的设备
    auto& outputState = editState();
    outputState.usesClientComposition = true;
    outputState.usesDeviceComposition = false;
    outputState.reusedClientComposition = false;
}

文件：frameworks/native/services/surfaceflinger/CompositionEngine/src/Display.cpp

void Display::chooseCompositionStrategy() {
    ATRACE_CALL();
    ALOGV(__FUNCTION__);

    // Default to the base settings -- client composition only.
    Output::chooseCompositionStrategy();

    // If we don't have a HWC display, then we are done
    if (!mId) {
        return;
    }

    // Get any composition changes requested by the HWC device, and apply them.
    std::optional<android::HWComposer::DeviceRequestedChanges> changes;
    auto& hwc = getCompositionEngine().getHwComposer();
    // 从HWC device获得合成类型的改变，这个根据hwc能力来选择device还是GPU合成
    if (status_t result = hwc.getDeviceCompositionChanges(*mId, anyLayersRequireClientComposition(),
                                                          &changes);
        result != NO_ERROR) {
        ALOGE("chooseCompositionStrategy failed for %s: %d (%s)", getName().c_str(), result,
              strerror(-result));
        return;
    }
    //如果有变化则设置给对应的layer
    if (changes) {
        applyChangedTypesToLayers(changes->changedTypes);
        applyDisplayRequests(changes->displayRequests);
        applyLayerRequestsToLayers(changes->layerRequests);
        applyClientTargetRequests(changes->clientTargetProperty);
    }

    // Determine what type of composition we are doing from the final state
    // 决定最后的合成类型
    auto& state = editState();
    state.usesClientComposition = anyLayersRequireClientComposition();
    state.usesDeviceComposition = !allLayersRequireClientComposition();
}

文件：frameworks/native/services/surfaceflinger/DisplayHardware/HWComposer.cpp

status_t HWComposer::getDeviceCompositionChanges(
        DisplayId displayId, bool frameUsesClientComposition,
        std::optional<android::HWComposer::DeviceRequestedChanges>* outChanges) {
    ATRACE_CALL();

    ...
    if (!frameUsesClientComposition) {
        sp<Fence> outPresentFence;
        uint32_t state = UINT32_MAX;
         // 如果所有的layer都能走device合成，则在hwc里面直接present，若有不支持device合成的情况，则走GPU合成，会走validate逻辑
        error = hwcDisplay->presentOrValidate(&numTypes, &numRequests, &outPresentFence , &state);
        if (!hasChangesError(error)) {
            RETURN_IF_HWC_ERROR_FOR("presentOrValidate", error, displayId, UNKNOWN_ERROR);
        }
        if (state == 1) { //Present Succeeded.
            // present成功，数据直接提交给了hwc
            std::unordered_map<HWC2::Layer*, sp<Fence>> releaseFences;
            error = hwcDisplay->getReleaseFences(&releaseFences);
            displayData.releaseFences = std::move(releaseFences);
            displayData.lastPresentFence = outPresentFence;
            displayData.validateWasSkipped = true;
            displayData.presentError = error;
            return NO_ERROR;
        }
 // Present failed but Validate ran.
    } else {
        // 若有GPU合成，则走validate逻辑
        error = hwcDisplay->validate(&numTypes, &numRequests);
    }
   // 接收hwc过来的change，对于device合成不走，GPU合成走的逻辑
   ...

prepareFrame 的作用是根据hwc的能力选择合成方式，如果是device合成则直接走hwc present上屏，如果是GPU合成则走hwc validate，然后根据hwc过来的变化改变layer的合成方式。

finishFrame

文件： frameworks/native/services/surfaceflinger/CompositionEngine/src/Output.cpp

void Output::finishFrame(const compositionengine::CompositionRefreshArgs& refreshArgs) {
    ATRACE_CALL();
    ALOGV(__FUNCTION__);

    if (!getState().isEnabled) {
        return;
    }

    // Repaint the framebuffer (if needed), getting the optional fence for when
    // the composition completes.
    // 主要针对GPU合成的逻辑
    auto optReadyFence = composeSurfaces(Region::INVALID_REGION, refreshArgs);
    // device合成直接return
    if (!optReadyFence) {
        return;
    }

    // swap buffers (presentation)
    mRenderSurface->queueBuffer(std::move(*optReadyFence));
}

composeSurfaces主要的逻辑是针对GPU合成，对于device合成，直接return了。

postFramebuffer

文件：frameworks/native/services/surfaceflinger/CompositionEngine/src/Output.cpp

void Output::postFramebuffer() {
   ...
    // device合成获取release fence
    auto frame = presentAndGetFrameFences();
   
    mRenderSurface->onPresentDisplayCompleted();

    for (auto* layer : getOutputLayersOrderedByZ()) {
        // The layer buffer from the previous frame (if any) is released
        // by HWC only when the release fence from this frame (if any) is
        // signaled.  Always get the release fence from HWC first.
        sp<Fence> releaseFence = Fence::NO_FENCE;

        if (auto hwcLayer = layer->getHwcLayer()) {
            if (auto f = frame.layerFences.find(hwcLayer); f != frame.layerFences.end()) {
                releaseFence = f->second;
            }
        }
        if (outputState.usesClientComposition) {
            releaseFence =
                    Fence::merge("LayerRelease", releaseFence, frame.clientTargetAcquireFence);
        }
       // 将releasefence放到mslot里面，这个是dequeueBuffer等的fence
        layer->getLayerFE().onLayerDisplayed(releaseFence);
    }
...
}

文件：frameworks/native/services/surfaceflinger/DisplayHardware/HWComposer.cpp

status_t HWComposer::presentAndGetReleaseFences(DisplayId displayId) {
    ATRACE_CALL();
    
    RETURN_IF_INVALID_DISPLAY(displayId, BAD_INDEX);
    
    auto& displayData = mDisplayData[displayId];
    auto& hwcDisplay = displayData.hwcDisplay;
     // device合成直接return
    if (displayData.validateWasSkipped) {
        // explicitly flush all pending commands
        auto error = static_cast<hal::Error>(mComposer->executeCommands()); 
        RETURN_IF_HWC_ERROR_FOR("executeCommands", error, displayId, UNKNOWN_ERROR);
        RETURN_IF_HWC_ERROR_FOR("present", displayData.presentError, displayId, UNKNOWN_ERROR);
        return NO_ERROR;
    }
    //GPU 合成走present
    auto error = hwcDisplay->present(&displayData.lastPresentFence);
    RETURN_IF_HWC_ERROR_FOR("present", error, displayId, UNKNOWN_ERROR);

    std::unordered_map<HWC2::Layer*, sp<Fence>> releaseFences;
    error = hwcDisplay->getReleaseFences(&releaseFences);
    RETURN_IF_HWC_ERROR_FOR("getReleaseFences", error, displayId, UNKNOWN_ERROR);

    displayData.releaseFences = std::move(releaseFences);

    return NO_ERROR;
}

postComposition

文件： frameworks/native/services/surfaceflinger/SurfaceFlinger.cpp

void SurfaceFlinger::postComposition()
{
    ATRACE_CALL();
    ALOGV("postComposition");

    nsecs_t dequeueReadyTime = systemTime();
     // release Bufferqueuelayer
    for (auto& layer : mLayersWithQueuedFrames) {
        layer->releasePendingBuffer(dequeueReadyTime);
    }
    ...
     // mPreviousPresentFences[0]是这一帧的present fence，mPreviousPresentFences[1]是上一帧的present fence
    mPreviousPresentFences[1] = mPreviousPresentFences[0];
    mPreviousPresentFences[0] =
            display ? getHwComposer().getPresentFence(*display->getId()) : Fence::NO_FENCE;
    ...
     // 这个有关vsync，是校验presentfence释放的时间戳
     if (display && display->isPrimary() && display->getPowerMode() == hal::PowerMode::ON &&
        presentFenceTime->isValid()) {
        mScheduler->addPresentFence(presentFenceTime);
    }
   ....
}

postComposition的作用就是释放上一帧显示的Buffer，然后重新设置当前帧的present fence，以及根据presentfence时间戳判断是否重新打开HW Vsync和软件Vsync进行校准。至此，SurfaceFlinger刷新的主体函数分析完了，可以看出来，如果是走device合成，SurfaceFlinger是直接把layer信息提交给hwc，由hwc去合成，涉及到GPU部分，下篇进行分析。