前文仅了解了overlay HAL的架构，下面继续看看系统层是如何调用Overlay模块。
1、 测试代码
frameworks/base/libs/surfaceflinger/tests/overlays/overlays.cpp提供了一个简单的overlay调用流程，可惜这个测试程序有错误，
??? 在sp<Surface> surface = client->createSurface(getpid(), 0, 320, 240, PIXEL_FORMAT_UNKNOWN, ISurfaceComposer::ePushBuffers);
这句话编译不过去，错误在Surface的申请，和overlay无关。
我们来看看这段代码：
int main(int argc, char** argv)
{
??? // set up the thread-pool 建立线程池
??? sp<ProcessState> proc(ProcessState::self());
??? ProcessState::self()->startThreadPool();
??? // create a client to surfaceflinger 创建一个SurfaceFlinger client
??? sp<SurfaceComposerClient> client = new SurfaceComposerClient();
???
??? // create pushbuffer surface 创建一个surface，最后那个参数是类型？
??? sp<Surface> surface = client->createSurface(getpid(), 0, 320, 240,
??????????? PIXEL_FORMAT_UNKNOWN, ISurfaceComposer::ePushBuffers);
??? // get to the isurface 取得isurface接口
??? sp<ISurface> isurface = Test::getISurface(surface);
??? printf("isurface = %p\n", isurface.get());
???
??? // now request an overlay 创建一个overlay
??? sp<OverlayRef> ref = isurface->createOverlay(320, 240, PIXEL_FORMAT_RGB_565);
??? sp<Overlay> overlay = new Overlay(ref);
???
??? /*
???? * here we can use the overlay API 创建好overlay后，即可使用overlay的API，这些都对应到overlay HAL的具体实现
???? */
???
??? overlay_buffer_t buffer;
??? overlay->dequeueBuffer(&buffer);
??? printf("buffer = %p\n", buffer);
???
??? void* address = overlay->getBufferAddress(buffer);
??? printf("address = %p\n", address);
??? overlay->queueBuffer(buffer);//最重要的操作就是通过queueBuffer将buffer列队
??? return 0;
}
2、Android系统创建中Overlay（调用createOverlay）
1）摄像头相关 CameraService.cpp (frameworks\base\camera\libcameraservice)
setPreviewDisplay()、startPreviewMode()
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setOverlay()
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creatOverlay()
2）界面相关 ISurface.cpp (frameworks\base\libs\ui)
LayerBaseClient::Surface::onTransact() <--该函数位于LayerBase.cpp，好像是用于ibind进程通讯的函数
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BnSurface::onTransact() //有5种方式，只有确定有overlay硬件支持时才会调用case CREATE_OVERLAY
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... ...
switch(code) {
??????? case REQUEST_BUFFER: {
??????????? CHECK_INTERFACE(ISurface, data, reply);
??????????? int bufferIdx = data.readInt32();
??????????? int usage = data.readInt32();
??????????? sp<GraphicBuffer> buffer(requestBuffer(bufferIdx, usage));
??????????? return GraphicBuffer::writeToParcel(reply, buffer.get());
??????? }
??????? case REGISTER_BUFFERS: {
??????????? CHECK_INTERFACE(ISurface, data, reply);
??????????? BufferHeap buffer;
??????????? buffer.w = data.readInt32();
??????????? buffer.h = data.readInt32();
??????????? buffer.hor_stride = data.readInt32();
??????????? buffer.ver_stride= data.readInt32();
??????????? buffer.format = data.readInt32();
??????????? buffer.transform = data.readInt32();
??????????? buffer.flags = data.readInt32();
??????????? buffer.heap = interface_cast<IMemoryHeap>(data.readStrongBinder());
??????????? status_t err = registerBuffers(buffer);
??????????? reply->writeInt32(err);
??????????? return NO_ERROR;
??????? } break;
??????? case UNREGISTER_BUFFERS: {
??????????? CHECK_INTERFACE(ISurface, data, reply);
??????????? unregisterBuffers();
??????????? return NO_ERROR;
??????? } break;
??????? case POST_BUFFER: {
??????????? CHECK_INTERFACE(ISurface, data, reply);
??????????? ssize_t offset = data.readInt32();
??????????? postBuffer(offset);
??????????? return NO_ERROR;
??????? } break;
??????? case CREATE_OVERLAY: {
??????????? CHECK_INTERFACE(ISurface, data, reply);
??????????? int w = data.readInt32();
??????????? int h = data.readInt32();
??????????? int f = data.readInt32();
??????????? sp<OverlayRef> o = createOverlay(w, h, f);
??????????? return OverlayRef::writeToParcel(reply, o);
??????? } break;
??????? default:
??????????? return BBinder::onTransact(code, data, reply, flags);
... ...
3）LayerBuffer.cpp (frameworks\base\libs\surfaceflinger) 这儿其实是createOverlay的实现
sp<OverlayRef> LayerBuffer::SurfaceLayerBuffer::createOverlay(uint32_t w, uint32_t h, int32_t format)
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sp<OverlayRef> LayerBuffer::createOverlay(uint32_t w, uint32_t h, int32_t f)
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sp<OverlaySource> source = new OverlaySource(*this, &result, w, h, f); //通过OverlaySource来创建overlay
LayerBuffer::OverlaySource::OverlaySource(）//该函数调用了Overlay HAL的API createOverlay
{
??? overlay_control_device_t* overlay_dev = mLayer.mFlinger->getOverlayEngine();//get HAL
??? overlay_t* overlay = overlay_dev->createOverlay(overlay_dev, w, h, format);//HAL API
??? // enable dithering...
??? overlay_dev->setParameter(overlay_dev, overlay, OVERLAY_DITHER, OVERLAY_ENABLE);
//设置参数，初始化OverlayRef类，OverlayRef的构造函数在Overlay.cpp中
??? mOverlay = overlay;
??? mWidth = overlay->w;
??? mHeight = overlay->h;
??? mFormat = overlay->format;
??? mWidthStride = overlay->w_stride;
??? mHeightStride = overlay->h_stride;
??? mInitialized = false;
... ...
??? *overlayRef = new OverlayRef(mOverlayHandle, channel,mWidth, mHeight, mFormat, mWidthStride, mHeightStride);
}
3、Overlay HAL模块管理
Overlay.cpp (frameworks\base\libs\ui)负责管理overlay HAL，并对HAL的API进行封装
1）打开Overlay HAL模块
Overlay::Overlay(const sp<OverlayRef>& overlayRef)
??? : mOverlayRef(overlayRef), mOverlayData(0), mStatus(NO_INIT)
{
??? mOverlayData = NULL;
??? hw_module_t const* module;
??? if (overlayRef != 0) {
??????? if (hw_get_module(OVERLAY_HARDWARE_MODULE_ID, &module) == 0) {
??????????? if (overlay_data_open(module, &mOverlayData) == NO_ERROR) {
??????????????? mStatus = mOverlayData->initialize(mOverlayData,
??????????????????????? overlayRef->mOverlayHandle);
??????????? }
??????? }
??? }
}
2）Overlay HAL的初始化
参考上一段，overlayRef = new OverlayRef(mOverlayHandle, channel,mWidth, mHeight, mFormat, mWidthStride, mHeightStride);
构造函数位于Overlay.cpp
OverlayRef::OverlayRef(overlay_handle_t handle, const sp<IOverlay>& channel,
???????? uint32_t w, uint32_t h, int32_t f, uint32_t ws, uint32_t hs)
??? : mOverlayHandle(handle), mOverlayChannel(channel),
??? mWidth(w), mHeight(h), mFormat(f), mWidthStride(ws), mHeightStride(hs),
??? mOwnHandle(false)
{
}
3）封装了很多的API，但是没有查到那儿有调用，看来还需要大改框架才能真正将overlay利用起来
比如TI自己写的opencore函数中到时有用到，主要负责视频输出。
Android_surface_output_omap34xx.cpp (hardware\ti\omap3\libopencorehw)
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4、总结
Overlay的输出对象有两种，一种是视频（主要是YUV格式,调用系统的V4L2），另外一个是ISurface的一些图像数据（RGB格式，直接写framebuffer）
从代码实现角度看，目前Android系统默认并没有使用Overlay功能，虽然提供了Skeleton的Overlay HAL，并对其进行封装，但是上层几乎没有调用到封装的API。
如果要用好Overlay HAL，需要大量修改上层框架，这对视屏播放可能比较重要，可参考TI的Android_surface_output_omap34xx.cpp。
此外Surface实现的Overlay功能和Copybit的功能有部分重复，从TI的代码看主要是实现V4L2的Overlay功能。
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