[Android] Graphic Buffer 的申请

前言：

MediaCodec 支持 texture mode，即MediaCodec解码video完毕后把 yuv 数据填入 GPU 共享出来的 graphic buffer 里面，app 会把 video 的 yuv数据和 ui 的数据通过通过软件渲染组件(opengl等)发送给GPU 进行一并渲染。这样做的效率较低，但是稳定性较好。且性能取决于GPU的性能。

另外一种是 surface mode，就是常用的模式，这种模式下 app 不再获取 yuv 数据，而是只负责输送 es 数据并决定 render 还是 discard 解码后的数据，且是根据 bufferid 来判断的，根本没有机会接触解码后的 yuv 数据，所有的一切都在平台完成。这种办法效率搞且功耗也很低，因为不需要GPU的参与。大部分情况下都是这种模式。

本文简单理清一下 Graphic Buffer 的框架，以对 texture mode 中的 Graphic Buffer 申请流程有一个大致的概念。

Graphic Buffer 的申请

始于 ANativeWindow 类

frameworks/native/libs/nativewindow/include/system/window.h


struct ANativeWindow
{
#ifdef __cplusplus
    ANativeWindow()
        : flags(0), minSwapInterval(0), maxSwapInterval(0), xdpi(0), ydpi(0)
    {
        common.magic = ANDROID_NATIVE_WINDOW_MAGIC;
        common.version = sizeof(ANativeWindow);
        memset(common.reserved, 0, sizeof(common.reserved));
    }

    /* Implement the methods that sp<ANativeWindow> expects so that it
       can be used to automatically refcount ANativeWindow's. */
    void incStrong(const void* /*id*/) const {
        common.incRef(const_cast<android_native_base_t*>(&common));
    }
    void decStrong(const void* /*id*/) const {
        common.decRef(const_cast<android_native_base_t*>(&common));
    }
#endif

    struct android_native_base_t common;

    /* flags describing some attributes of this surface or its updater */
    const uint32_t flags;

    /* min swap interval supported by this updated */
    const int   minSwapInterval;

    /* max swap interval supported by this updated */
    const int   maxSwapInterval;

    /* horizontal and vertical resolution in DPI */
    const float xdpi;
    const float ydpi;

    /* Some storage reserved for the OEM's driver. */
    intptr_t    oem[4];

    /*
     * Set the swap interval for this surface.
     *
     * Returns 0 on success or -errno on error.
     */
    int     (*setSwapInterval)(struct ANativeWindow* window,
                int interval);

    /*
     * Hook called by EGL to acquire a buffer. After this call, the buffer
     * is not locked, so its content cannot be modified. This call may block if
     * no buffers are available.
     *
     * The window holds a reference to the buffer between dequeueBuffer and
     * either queueBuffer or cancelBuffer, so clients only need their own
     * reference if they might use the buffer after queueing or canceling it.
     * Holding a reference to a buffer after queueing or canceling it is only
     * allowed if a specific buffer count has been set.
     *
     * Returns 0 on success or -errno on error.
     *
     * XXX: This function is deprecated.  It will continue to work for some
     * time for binary compatibility, but the new dequeueBuffer function that
     * outputs a fence file descriptor should be used in its place.
     */
    int     (*dequeueBuffer_DEPRECATED)(struct ANativeWindow* window,
                struct ANativeWindowBuffer** buffer);

    /*
     * hook called by EGL to lock a buffer. This MUST be called before modifying
     * the content of a buffer. The buffer must have been acquired with
     * dequeueBuffer first.
     *
     * Returns 0 on success or -errno on error.
     *
     * XXX: This function is deprecated.  It will continue to work for some
     * time for binary compatibility, but it is essentially a no-op, and calls
     * to it should be removed.
     */
    int     (*lockBuffer_DEPRECATED)(struct ANativeWindow* window,
                struct ANativeWindowBuffer* buffer);

    /*
     * Hook called by EGL when modifications to the render buffer are done.
     * This unlocks and post the buffer.
     *
     * The window holds a reference to the buffer between dequeueBuffer and
     * either queueBuffer or cancelBuffer, so clients only need their own
     * reference if they might use the buffer after queueing or canceling it.
     * Holding a reference to a buffer after queueing or canceling it is only
     * allowed if a specific buffer count has been set.
     *
     * Buffers MUST be queued in the same order than they were dequeued.
     *
     * Returns 0 on success or -errno on error.
     *
     * XXX: This function is deprecated.  It will continue to work for some
     * time for binary compatibility, but the new queueBuffer function that
     * takes a fence file descriptor should be used in its place (pass a value
     * of -1 for the fence file descriptor if there is no valid one to pass).
     */
    int     (*queueBuffer_DEPRECATED)(struct ANativeWindow* window,
                struct ANativeWindowBuffer* buffer);

    /*
     * hook used to retrieve information about the native window.
     *
     * Returns 0 on success or -errno on error.
     */
    int     (*query)(const struct ANativeWindow* window,
                int what, int* value);

    /*
     * hook used to perform various operations on the surface.
     * (*perform)() is a generic mechanism to add functionality to
     * ANativeWindow while keeping backward binary compatibility.
     *
     * DO NOT CALL THIS HOOK DIRECTLY.  Instead, use the helper functions
     * defined below.
     *
     * (*perform)() returns -ENOENT if the 'what' parameter is not supported
     * by the surface's implementation.
     *
     * See above for a list of valid operations, such as
     * NATIVE_WINDOW_SET_USAGE or NATIVE_WINDOW_CONNECT
     */
    int     (*perform)(struct ANativeWindow* window,
                int operation, ... );

    /*
     * Hook used to cancel a buffer that has been dequeued.
     * No synchronization is performed between dequeue() and cancel(), so
     * either external synchronization is needed, or these functions must be
     * called from the same thread.
     *
     * The window holds a reference to the buffer between dequeueBuffer and
     * either queueBuffer or cancelBuffer, so clients only need their own
     * reference if they might use the buffer after queueing or canceling it.
     * Holding a reference to a buffer after queueing or canceling it is only
     * allowed if a specific buffer count has been set.
     *
     * XXX: This function is deprecated.  It will continue to work for some
     * time for binary compatibility, but the new cancelBuffer function that
     * takes a fence file descriptor should be used in its place (pass a value
     * of -1 for the fence file descriptor if there is no valid one to pass).
     */
    int     (*cancelBuffer_DEPRECATED)(struct ANativeWindow* window,
                struct ANativeWindowBuffer* buffer);

    /*
     * Hook called by EGL to acquire a buffer. This call may block if no
     * buffers are available.
     *
     * The window holds a reference to the buffer between dequeueBuffer and
     * either queueBuffer or cancelBuffer, so clients only need their own
     * reference if they might use the buffer after queueing or canceling it.
     * Holding a reference to a buffer after queueing or canceling it is only
     * allowed if a specific buffer count has been set.
     *
     * The libsync fence file descriptor returned in the int pointed to by the
     * fenceFd argument will refer to the fence that must signal before the
     * dequeued buffer may be written to.  A value of -1 indicates that the
     * caller may access the buffer immediately without waiting on a fence.  If
     * a valid file descriptor is returned (i.e. any value except -1) then the
     * caller is responsible for closing the file descriptor.
     *
     * Returns 0 on success or -errno on error.
     */
    int     (*dequeueBuffer)(struct ANativeWindow* window,
                struct ANativeWindowBuffer** buffer, int* fenceFd);

    /*
     * Hook called by EGL when modifications to the render buffer are done.
     * This unlocks and post the buffer.
     *
     * The window holds a reference to the buffer between dequeueBuffer and
     * either queueBuffer or cancelBuffer, so clients only need their own
     * reference if they might use the buffer after queueing or canceling it.
     * Holding a reference to a buffer after queueing or canceling it is only
     * allowed if a specific buffer count has been set.
     *
     * The fenceFd argument specifies a libsync fence file descriptor for a
     * fence that must signal before the buffer can be accessed.  If the buffer
     * can be accessed immediately then a value of -1 should be used.  The
     * caller must not use the file descriptor after it is passed to
     * queueBuffer, and the ANativeWindow implementation is responsible for
     * closing it.
     *
     * Returns 0 on success or -errno on error.
     */
    int     (*queueBuffer)(struct ANativeWindow* window,
                struct ANativeWindowBuffer* buffer, int fenceFd);

    /*
     * Hook used to cancel a buffer that has been dequeued.
     * No synchronization is performed between dequeue() and cancel(), so
     * either external synchronization is needed, or these functions must be
     * called from the same thread.
     *
     * The window holds a reference to the buffer between dequeueBuffer and
     * either queueBuffer or cancelBuffer, so clients only need their own
     * reference if they might use the buffer after queueing or canceling it.
     * Holding a reference to a buffer after queueing or canceling it is only
     * allowed if a specific buffer count has been set.
     *
     * The fenceFd argument specifies a libsync fence file decsriptor for a
     * fence that must signal before the buffer can be accessed.  If the buffer
     * can be accessed immediately then a value of -1 should be used.
     *
     * Note that if the client has not waited on the fence that was returned
     * from dequeueBuffer, that same fence should be passed to cancelBuffer to
     * ensure that future uses of the buffer are preceded by a wait on that
     * fence.  The caller must not use the file descriptor after it is passed
     * to cancelBuffer, and the ANativeWindow implementation is responsible for
     * closing it.
     *
     * Returns 0 on success or -errno on error.
     */
    int     (*cancelBuffer)(struct ANativeWindow* window,
                struct ANativeWindowBuffer* buffer, int fenceFd);
};

上述代码中有很多函数指针，比如

    int     (*dequeueBuffer)(struct ANativeWindow* window,
                struct ANativeWindowBuffer** buffer, int* fenceFd);

这些指针的具体实现在其子类中有实现和设置，这里是一种 C 的 hook 写法，而不是 c++ 的虚函数写法，可能出于兼容性考虑。

\frameworks\native\libs\nativewindow\ANativeWindow.cpp 里面并不是 ANativeWindow的实现，而是对其封装，比如：

int ANativeWindow_dequeueBuffer(ANativeWindow* window, ANativeWindowBuffer** buffer, int* fenceFd) {
    return window->dequeueBuffer(window, buffer, fenceFd);
}

可以看到，这里调用了 ANativeWindow的dequeueBuffer函数，而这个函数在 ANativeWindow 结构里是一个函数指针，而这个函数指针又指向哪里呢？

Hook的实现者Surface

\frameworks\native\libs\gui\include\gui\Surface.h

\frameworks\native\libs\gui\Surface.cpp

class Surface
    : public ANativeObjectBase<ANativeWindow, Surface, RefBase>
{

}

Surface 是 ANativeWindow的子类，但是考虑到 ANativeWindow通过函数指针的方式实现多态，所以Surface里面一定有设置Hook的地方，那就是在构造函数里。

Surface::Surface(const sp<IGraphicBufferProducer>& bufferProducer, bool controlledByApp)
      : mGraphicBufferProducer(bufferProducer),
        mCrop(Rect::EMPTY_RECT),
        mBufferAge(0),
        mGenerationNumber(0),
        mSharedBufferMode(false),
        mAutoRefresh(false),
        mSharedBufferSlot(BufferItem::INVALID_BUFFER_SLOT),
        mSharedBufferHasBeenQueued(false),
        mQueriedSupportedTimestamps(false),
        mFrameTimestampsSupportsPresent(false),
        mEnableFrameTimestamps(false),
        mFrameEventHistory(std::make_unique<ProducerFrameEventHistory>()) {
    // Initialize the ANativeWindow function pointers.
    ANativeWindow::setSwapInterval  = hook_setSwapInterval;
    ANativeWindow::dequeueBuffer    = hook_dequeueBuffer;
    ANativeWindow::cancelBuffer     = hook_cancelBuffer;
    ANativeWindow::queueBuffer      = hook_queueBuffer;
    ANativeWindow::query            = hook_query;
    ANativeWindow::perform          = hook_perform;

    ANativeWindow::dequeueBuffer_DEPRECATED = hook_dequeueBuffer_DEPRECATED;
    ANativeWindow::cancelBuffer_DEPRECATED  = hook_cancelBuffer_DEPRECATED;
    ANativeWindow::lockBuffer_DEPRECATED    = hook_lockBuffer_DEPRECATED;
    ANativeWindow::queueBuffer_DEPRECATED   = hook_queueBuffer_DEPRECATED;

至此，所有调用 ANativeWindow_xxx 的入口都会被导入 Surface 里的相应接口中，进入 hook_xxx。

int Surface::hook_dequeueBuffer(ANativeWindow* window,
        ANativeWindowBuffer** buffer, int* fenceFd) {
    Surface* c = getSelf(window);
    return c->dequeueBuffer(buffer, fenceFd);
}

进而在进入 Surface 的 xxx 函数。从 xxx 函数开始，和 graphic service 打交道就正式开始了。

int Surface::dequeueBuffer(android_native_buffer_t** buffer, int* fenceFd) {
    ATRACE_CALL();

    status_t result = mGraphicBufferProducer->dequeueBuffer(&buf, &fence, reqWidth, reqHeight,
                                                            reqFormat, reqUsage, &mBufferAge,
                                                            enableFrameTimestamps ? &frameTimestamps
                                                                                  : nullptr);


}

Graphic Buffer 服务

Surface 的所有和 Graphic Buffer 相关的接口都会使用到成员 mGraphicBufferProducer

sp<IGraphicBufferProducer> mGraphicBufferProducer;

这个成员的类型一看就是一个 binder Interface，继续看 IGraphicBufferProducer。

\frameworks\native\libs\gui\include\gui\IGraphicBufferProducer.h

class IGraphicBufferProducer : public IInterface
{
public:
    using HGraphicBufferProducerV1_0 =
            ::android::hardware::graphics::bufferqueue::V1_0::
            IGraphicBufferProducer;
    using HGraphicBufferProducerV2_0 =
            ::android::hardware::graphics::bufferqueue::V2_0::
            IGraphicBufferProducer;

。。。

    virtual status_t dequeueBuffer(int* slot, sp<Fence>* fence, uint32_t w, uint32_t h,
                                   PixelFormat format, uint64_t usage, uint64_t* outBufferAge,
                                   FrameEventHistoryDelta* outTimestamps) = 0;

。。。

}

至此，一个binder 模型已经形成。

Surface::mGraphicBufferProducer as Bp <--IGraphicBufferProducer BINDER--> ? as Bn

那么这里的Bn是什么呢？只需要在源码里搜一下哪些类继承IGraphicBufferProducer 即可。其中一个常用的类就是 BufferQueueProducer。

\frameworks\native\libs\gui\include\gui\BufferQueueProducer.h

class BufferQueueProducer : public BnGraphicBufferProducer,
                            private IBinder::DeathRecipient {

。。。
    virtual status_t queueBuffer(int slot,
            const QueueBufferInput& input, QueueBufferOutput* output);
。。。

}

继承自BnGraphicBufferProducer，Bn开头，一看就知道是server端。

status_t BufferQueueProducer::dequeueBuffer(int* outSlot, sp<android::Fence>* outFence,
                                            uint32_t width, uint32_t height, PixelFormat format,
                                            uint64_t usage, uint64_t* outBufferAge,
                                            FrameEventHistoryDelta* outTimestamps) {

        sp<GraphicBuffer> graphicBuffer = new GraphicBuffer(
                width, height, format, BQ_LAYER_COUNT, usage,
                {mConsumerName.string(), mConsumerName.size()});

        status_t error = graphicBuffer->initCheck();


}

在上面的代码里有 new GraphicBuffer 和 initCheck() 这两个动作，这里就是在创建管理对象并进行初始化检查。

内存管理

\frameworks\native\libs\ui\include\ui\GraphicBuffer.h

class GraphicBuffer
    : public ANativeObjectBase<ANativeWindowBuffer, GraphicBuffer, RefBase>,
      public Flattenable<GraphicBuffer>
{

    GraphicBuffer(uint32_t inWidth, uint32_t inHeight, PixelFormat inFormat,
            uint32_t inUsage, std::string requestorName = "<Unknown>");

}

\frameworks\native\libs\ui\GraphicBuffer.cpp

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

上面创建 Graphic Buffer 的过程中会有一个 initWithSize ，这是初始化的过程，在初始化过程里又又 allocator.allocate，可见是一个内存分配的过程。

内存分配 GraphicBufferAllocator

\frameworks\native\libs\ui\include\ui\GraphicBufferAllocator.h

\frameworks\native\libs\ui\GraphicBufferAllocator.cpp

class GraphicBufferAllocator : public Singleton<GraphicBufferAllocator>
{

}

allocate 方法如下

status_t GraphicBufferAllocator::allocate(uint32_t width, uint32_t height,
        PixelFormat format, uint32_t layerCount, uint64_t usage,
        buffer_handle_t* handle, uint32_t* stride,
        uint64_t /*graphicBufferId*/, std::string requestorName)
{

。。。
    status_t error =
            mAllocator->allocate(width, height, format, layerCount, usage, 1, stride, handle);
。。。

}

mAllocator 是什么？

sp<hardware::graphics::allocator::V2_0::IAllocator> mAllocator;

又是一个 Interface ，这次是 hardware 的，因此其实现是由硬件完成的。具体细节参考 /hardware/interfaces/graphics/allocator/2.0/