前文僅了解了overlay HAL的架構，下面繼續看看系統層是如何調用Overlay模塊。
1、 測試代碼
frameworks/base/libs/surfaceflinger/tests/overlays/overlays.cpp提供了一個簡單的overlay調用流程，可惜這個測試程序有錯誤，
在sp<Surface> surface = client->createSurface(getpid(), 0, 320, 240, PIXEL_FORMAT_UNKNOWN, ISurfaceComposer::ePushBuffers);
這句話編譯不過去，錯誤在Surface的申請，和overlay無關。
我們來看看這段代碼：
int main(int argc, char** argv)
{
// set up the thread-pool 建立線程池
sp<ProcessState> proc(ProcessState::self());
ProcessState::self()->startThreadPool();
// create a client to surfaceflinger 創建一個SurfaceFlinger client
sp<SurfaceComposerClient> client = new SurfaceComposerClient();
// create pushbuffer surface 創建一個surface，最后那個參數是類型？
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 創建一個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 創建好overlay后，即可使用overlay的API，這些都對應到overlay HAL的具體實現
*/
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過queueBuffer將buffer列隊
return 0;
}
2、Android系統創建中Overlay（調用createOverlay）
1）攝像頭相關 CameraService.cpp (frameworks/base/camera/libcameraservice)
setPreviewDisplay()、startPreviewMode()
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setOverlay()
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creatOverlay()
2）界面相關 ISurface.cpp (frameworks/base/libs/ui)
LayerBaseClient::Surface::onTransact() <--該函數位於LayerBase.cpp，好像是用於ibind進程通訊的函數
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BnSurface::onTransact() //有5種方式，只有確定有overlay硬件支持時才會調用case CREATE_OVERLAY
|
... ...
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) 這兒其實是createOverlay的實現
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); //通過OverlaySource來創建overlay
LayerBuffer::OverlaySource::OverlaySource(）//該函數調用了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);
//設置參數，初始化OverlayRef類，OverlayRef的構造函數在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模塊管理
Overlay.cpp (frameworks/base/libs/ui)負責管理overlay HAL，並對HAL的API進行封裝
1）打開Overlay HAL模塊
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
參考上一段，overlayRef = new OverlayRef(mOverlayHandle, channel,mWidth, mHeight, mFormat, mWidthStride, mHeightStride);
構造函數位於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）封裝了很多的API，但是沒有查到那兒有調用，看來還需要大改框架才能真正將overlay利用起來
比如TI自己寫的opencore函數中到時有用到，主要負責視頻輸出。
Android_surface_output_omap34xx.cpp (hardware/ti/omap3/libopencorehw)
4、總結
Overlay的輸出對象有兩種，一種是視頻（主要是YUV格式,調用系統的V4L2），另外一個是ISurface的一些圖像數據（RGB格式，直接寫framebuffer）
從代碼實現角度看，目前Android系統默認並沒有使用Overlay功能，雖然提供了Skeleton的Overlay HAL，並對其進行封裝，但是上層幾乎沒有調用到封裝的API。
如果要用好Overlay HAL，需要大量修改上層框架，這對視屏播放可能比較重要，可參考TI的Android_surface_output_omap34xx.cpp。
此外Surface實現的Overlay功能和Copybit的功能有部分重復，從TI的代碼看主要是實現V4L2的Overlay功能。
http://hi.baidu.com/aokikyon/blog/item/6e4e622c4fdaaae18a1399d4.html