Android BufferQueue原理分析
在Android中,BufferQueue是Surface实现本地窗口的关键,驻留在SurfaceFlinger进程中进行服务,下面从BufferQueue的结构开始分析,
class BufferQueue : public BnGraphicBufferProducer, public BnGraphicBufferConsumer, private IBinder::DeathRecipient {
可见BufferQueue拥有producer和consumer两端。再看看createBufferQueue的实现,这里创建了一个BufferQueueCore,然后以这个为参数依次创建了BufferQueueProducer和BufferQueueConsumer。
void BufferQueue::createBufferQueue(sp* outProducer, sp* outConsumer, const sp& allocator) { sp core(new BufferQueueCore(allocator)); sp producer(new BufferQueueProducer(core)); sp consumer(new BufferQueueConsumer(core)); *outProducer = producer; *outConsumer = consumer;}
这里的producer和consumer是要设置的,而allocator是从外面传进来的,如果传NULL,则在BufferQueueCore中会初始化,如下:
BufferQueueCore::BufferQueueCore(const sp& allocator) { if (allocator == NULL) { sp composer(ComposerService::getComposerService()); mAllocator = composer->createGraphicBufferAlloc(); }}
这里的ISurfaceComposer实现在SurfaceFlinger中,再看createGraphicBufferAlloc的实现:
sp SurfaceFlinger::createGraphicBufferAlloc() { sp gba(new GraphicBufferAlloc()); return gba;}
这个GraphicBufferAlloc构造函数是个空壳,看看createGraphicBuffer的实现,
sp GraphicBufferAlloc::createGraphicBuffer(uint32_t width, uint32_t height, PixelFormat format, uint32_t usage, status_t* error) { sp graphicBuffer( new GraphicBuffer(width, height, format, usage)); status_t err = graphicBuffer->initCheck(); return graphicBuffer;}
GraphicBuffer继承自ANativeWindowBuffer,这个定义在window.h中,值得注意的是里面有个buffer_handle_t句柄,用于共享内存映射的文件句柄就保存在里面了。GraphicBuffer实现了Flattenable接口从而可以跨进程传输。GraphicBuffer构造函数中调用initSize开辟内存,
status_t GraphicBuffer::initSize(uint32_t inWidth, uint32_t inHeight, PixelFormat inFormat, uint32_t inUsage) { GraphicBufferAllocator& allocator = GraphicBufferAllocator::get(); status_t err = allocator.alloc(inWidth, inHeight, inFormat, inUsage, &handle, &outStride); ...... return err;}
这里又冒出来一个GraphicBufferAllocator类,注意和之前的GraphicBufferAlloc是两回事,这里是真正创建内存了。这个allocator是个单例,构造函数中加载Gralloc模块。具体的alloc应该会跟平台相关了。
GraphicBufferAllocator::GraphicBufferAllocator() : mAllocDev(0) { hw_module_t const* module; int err = hw_get_module(GRALLOC_HARDWARE_MODULE_ID, &module); if (err == 0) { gralloc_open(module, &mAllocDev); }}status_t GraphicBufferAllocator::alloc(uint32_t width, uint32_t height, PixelFormat format, uint32_t usage, buffer_handle_t* handle, uint32_t* stride) { err = mAllocDev->alloc(mAllocDev, static_cast(width), static_cast(height), format, static_cast(usage), handle, &outStride); ...... return err;}
到这里整个BufferQueue的创建就大致清楚了,其核心是GraphicBuffer的Allocator,最终是调用的Gralloc模块来开辟GraphicBuffer。
搜索一下BufferQueue在哪些地方被创建的,有以下几处,
- SurfaceTexture初始化时
- SurfaceFlinger初始化时,为每个显示器创建一个BufferQueue
- Layer创建时的onFirstRef中
我们重点关注Surface,这是应用端和SurfaceFlinger交互的关键了,SurfaceTexture和Layer都和Surface有千丝万缕的联系。
接下来分析BufferQueueProducer,下面是dequeueBuffer函数,首先找空闲的slot,如果buffer需要重新分配则创建GraphicBuffer设置到mSlots中。
status_t BufferQueueProducer::dequeueBuffer(int *outSlot, ...) { int found = BufferItem::INVALID_BUFFER_SLOT; while (found == BufferItem::INVALID_BUFFER_SLOT) { status_t status = waitForFreeSlotThenRelock(FreeSlotCaller::Dequeue, &found); const sp& buffer(mSlots[found].mGraphicBuffer); } const sp& buffer(mSlots[found].mGraphicBuffer); *outSlot = found; ...... if (returnFlags & BUFFER_NEEDS_REALLOCATION) { sp graphicBuffer = new GraphicBuffer(...); ...... mSlots[*outSlot].mGraphicBuffer = graphicBuffer; } return returnFlags;}
我们再来看Surface中的dequeueBuffer,Surface作为BufferQueue的client端,其mGraphicBufferProducer必定是IGraphicBufferProducer的Bp端,对应的Bn端在BufferQueue中。这里通过dequeueBuffer后,先判断Buffer是否在BufferQueue端重新分配过了,如果是则需要调用requestBuffer刷新本地的GraphicBuffer。
int Surface::dequeueBuffer(android_native_buffer_t** buffer, int* fenceFd) { status_t result = mGraphicBufferProducer->dequeueBuffer(&buf, &fence, reqWidth, reqHeight, reqFormat, reqUsage, enableFrameTimestamps ? &frameTimestamps : nullptr); sp& gbuf(mSlots[buf].buffer); if ((result & IGraphicBufferProducer::BUFFER_NEEDS_REALLOCATION) || gbuf == nullptr) { result = mGraphicBufferProducer->requestBuffer(buf, &gbuf); } *buffer = gbuf.get(); return OK;}
这里的问题是Surface和BufferQueue运行于不同的进程,其GraphicBuffer是否指向同一块物理内存。我们分析requestBuffer函数,我们首先看BpGraphicBufferProducer的transact,这里首先IPC调用REQUEST_BUFFER,获取到reply,然后本地先创建一个GraphicBuffer空壳,再从reply中将真正的数据读进来。
// IGraphicBufferProducer.cpp virtual status_t requestBuffer(int bufferIdx, sp* buf) { Parcel data, reply; data.writeInterfaceToken(IGraphicBufferProducer::getInterfaceDescriptor()); data.writeInt32(bufferIdx); status_t result =remote()->transact(REQUEST_BUFFER, data, &reply); bool nonNull = reply.readInt32(); if (nonNull) { *buf = new GraphicBuffer(); result = reply.read(**buf); } result = reply.readInt32(); return result;}
再来看BnGraphicBufferProducer的onTransact,
case REQUEST_BUFFER: { CHECK_INTERFACE(IGraphicBufferProducer, data, reply); int bufferIdx = data.readInt32(); sp buffer; int result = requestBuffer(bufferIdx, &buffer); reply->writeInt32(buffer != 0); if (buffer != 0) { reply->write(*buffer); } reply->writeInt32(result); return NO_ERROR;}
这里会调用GraphicBufferProducer中的requestBuffer,然后将buffer写入reply中。可见GraphicBuffer可以通过Binder传递,
class GraphicBuffer : public ANativeObjectBase, public Flattenable
原来这里实现了Flattenable,实现这个接口的对象可以序列化到buffer中,包括其中的文件描述符。我们看其unflatten实现,这里的fds很可能已经变了,这里关键是registerBuffer,很可能是要开始映射内存了。这个mBufferMapper是GraphicBufferMapper,
status_t GraphicBuffer::unflatten( void const*& buffer, size_t& size, int const*& fds, size_t& count) { ...... if (handle != 0) { status_t err = mBufferMapper.registerBuffer(handle); ...... } ...... return NO_ERROR;}
再来看GraphicBufferMapper的实现,在构造函数中加载了Gralloc模块,看来是平台相关的。到这里有点眼熟,之前提到的GraphicBufferAllocator初始化时也会加载Gralloc模块,那是在SurfaceFlinger中真正创建GraphicBuffer用的,而这里应用端只需将句柄映射到自己的内存空间即可,所以有了这个GraphicBufferMapper。
GraphicBufferMapper::GraphicBufferMapper() : mAllocMod(0) { hw_module_t const* module; int err = hw_get_module(GRALLOC_HARDWARE_MODULE_ID, &module); if (err == 0) { mAllocMod = (gralloc_module_t const *) module; }}status_t GraphicBufferMapper::registerBuffer(buffer_handle_t handle) { status_t err; err = mAllocMod->registerBuffer(mAllocMod, handle); return err;}
这里的registerBuffer是和平台相关的,我们看msm8960平台的实现:
gralloc_register_buffer(gralloc_module_t const* module, buffer_handle_t handle) { private_handle_t* hnd = (private_handle_t*)handle; hnd->base = 0; hnd->base_metadata = 0; int err = gralloc_map(module, handle); return 0;}
这里调用了gralloc_map将handle句柄映射到自己的进程空间,而这块区域与BufferQueue中指向的物理空间是一致的,从而实现了两者跨进程缓冲区共享。
到这里我们大致明白了本地Surface和对端BufferQueue之间的dequeueBuffer时的过程,接下来我们了解一下Surface是怎么和BufferQueue建立联系的。
我们在本文开头提到,在Layer创建时的onFirstRef中会createBufferQueue,而Layer的创建在SurfaceFlinger中的createLayer函数,而这个函数被Client的createSurface调用,这个Client是BnSurfaceComposerClient的子类,因此其中包含createSurface的实现,我们注意到里面会将所有Bp端发起的createSurface请求串行化后再丢给SurfaceFlinger。
再看BpSurfaceComposerClient中的createSurface是被谁调到的,我们发现SurfaceControl中的nativeCreate时会调到,这是SurfaceControl的构造函数中调到的。
// android_view_SurfaceControl.cppstatic jint nativeCreate(JNIEnv* env, jclass clazz, jobject sessionObj, jstring nameStr, jint w, jint h, jint format, jint flags) { sp client(android_view_SurfaceSession_getClient(env, sessionObj)); sp surface = client->createSurface( String8(name.c_str()), w, h, format, flags); surface->incStrong((void *)nativeCreate); return int(surface.get());}
这里createSurface返回的是SurfaceControl,这个其实是SurfaceComposerClient封装了一层,将远端返回的Binder句柄和GraphicBufferProducer都封装起来了,所以通过SurfaceControl可以和SurfaceFlinger通信。再看nativeCreate中通过android_view_SurfaceSession_getClient获取SurfaceComposerClient,看看SurfaceSession是什么,这个是连接到SurfaceFlinger的,Java类中保存了一个long型的mNativeClient,这在Native层对应着SurfaceComposerClient。
public final class SurfaceSession { // Note: This field is accessed by native code. private long mNativeClient; // SurfaceComposerClient* private static native long nativeCreate(); private static native long nativeCreateScoped(long surfacePtr); private static native void nativeDestroy(long ptr); private static native void nativeKill(long ptr); /** Create a new connection with the surface flinger. */ public SurfaceSession() { mNativeClient = nativeCreate(); } ......}
再看Native层实现:
static jlong nativeCreate(JNIEnv* env, jclass clazz) { SurfaceComposerClient* client = new SurfaceComposerClient(); client->incStrong((void*)nativeCreate); return reinterpret_cast(client);}
这里创建了一个SurfaceComposerClient,并没有看到连到SurfaceFlinger端,看其构造函数也没有,我们想到onFirstRef,
void SurfaceComposerClient::onFirstRef() { sp sm(ComposerService::getComposerService()); if (sm != 0) { auto rootProducer = mParent.promote(); sp conn; conn = sm->createConnection(); if (conn != 0) { mClient = conn; mStatus = NO_ERROR; } }}
到这里就清楚了,ISurfaceComposer是个与SurfaceFlinger通信的Binder句柄,通过该句柄的createConnection可以获取ISurfaceComposerClient,而SurfaceComposerClient是在其基础上包装了一层而已。
所以本地应用需要通过SurfaceSession建立和SurfaceFlinger连接,获取SurfaceComposerClient,然后再通过这个句柄调用createSurface,返回GraphicBufferProducer,封装到SurfaceControl中,有了这个GraphicBufferProducer接下来就好办了。
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