上篇博客最后分析到MonitoredProducer对象,这个对象只是一个代理,真正实是BufferQueueProducer类,这个对象和BufferQueueCore有关联,可以管理最多达64块的缓冲区。Surface可以理解为一张画布,那么Surface为何要和一个缓冲区队列相关呢?在播放动画时,美妙至少要播放24帧画面才能形成比较真实的动画效果。而这些数据是通过cpu解码得到的,准备他们需要时间。对于图像显示设备而言,刷新周期是固定的,我们必须要在它需要数据的时候把数据准备好。视频播放的每一帧也需要在指定的时间播放,因此解码器会提前准备好一批数据,这些数据保存在解码器内存的缓冲区中,当时间到达是,解码器会把内部缓冲区的图像复制到Surface中,但是显示设备并不是立刻就把数据取走的,因此Surface也需要缓冲区来临时保存数据。
一、BufferQueueCore BufferQueueProducer BufferQueueConsumer
上篇博客在Layer的onFirstRef函数中,调用了下面函数,创建了3个对象BufferQueueCore BufferQueueProducer BufferQueueConsumer。其中BufferCore是核心,把BufferQueueProducer和BufferQueueConsumer对象连接在一起。
- void BufferQueue::createBufferQueue(sp<IGraphicBufferProducer>* outProducer,
- sp<IGraphicBufferConsumer>* outConsumer,
- const sp<IGraphicBufferAlloc>& allocator) {
-
- sp<BufferQueueCore> core(new BufferQueueCore(allocator));
- sp<IGraphicBufferProducer> producer(new BufferQueueProducer(core));
- sp<IGraphicBufferConsumer> consumer(new BufferQueueConsumer(core));
-
- *outProducer = producer;
- *outConsumer = consumer;
- }
1.1 生产者和core的联系
IGraphicBufferProducer 的大致接口如下,BufferQueueProducer类是接口IGraphicBufferProducer 的实现,使用BufferQueueProducer之前先要调用connect函数,使用结束后调用disconnect断开连接。
- class IGraphicBufferProducer : public IInterface
- {
- public:
- virtual status_t requestBuffer(int slot, sp<GraphicBuffer>* buf) = 0;
-
- virtual status_t setBufferCount(int bufferCount) = 0;
-
- virtual status_t dequeueBuffer(int* slot, sp<Fence>* fence, bool async,
- uint32_t w, uint32_t h, PixelFormat format, uint32_t usage) = 0;
-
- virtual status_t detachBuffer(int slot) = 0;
-
- virtual status_t detachNextBuffer(sp<GraphicBuffer>* outBuffer,
- sp<Fence>* outFence) = 0;
-
- virtual status_t attachBuffer(int* outSlot,
- const sp<GraphicBuffer>& buffer) = 0;
-
- virtual status_t queueBuffer(int slot,
- const QueueBufferInput& input, QueueBufferOutput* output) = 0;
- virtual void cancelBuffer(int slot, const sp<Fence>& fence) = 0;
- virtual int query(int what, int* value) = 0;
- virtual status_t connect(const sp<IProducerListener>& listener,
- int api, bool producerControlledByApp, QueueBufferOutput* output) = 0;
-
- virtual status_t disconnect(int api) = 0;
- virtual status_t setSidebandStream(const sp<NativeHandle>& stream) = 0;
- virtual void allocateBuffers(bool async, uint32_t width, uint32_t height,
- PixelFormat format, uint32_t usage) = 0;
- virtual status_t allowAllocation(bool allow) = 0;
-
- virtual status_t setGenerationNumber(uint32_t generationNumber) = 0;
-
- virtual String8 getConsumerName() const = 0;
- };
在BufferQueueCore类中定义了一个64项的数据mSlots。
- BufferQueueDefs::SlotsType mSlots;
- typedef BufferSlot SlotsType[NUM_BUFFER_SLOTS];
每个缓冲区的类型是BufferSlot类型。它有两个重要的成员变量,mGraphicBuffer是指向图像缓冲区GraphicBuffer的指针,mBufferState表示图像缓冲区的状态。
- sp<GraphicBuffer> mGraphicBuffer;
- ......
- BufferState mBufferState;
BufferState的状态有下面几个
- enum BufferState {
- FREE = 0,
- DEQUEUED = 1,
- QUEUED = 2,
- ACQUIRED = 3
- };
BufferQueueProducer的dequeueBuffer函数用来向BufferQueueCore申请一个空闲的slot,这个slot可能已经有缓冲区,也可能没有,如果没有缓冲区,dequeueBuffer函数会分配一块新的缓冲区。得到空闲的slot后,还需要调用requestBuffer函数来取得一块缓冲区。得到缓冲区,如果不需要了,可以使用cancelBuffer函数来释放这个slot。调用dequeueBuffer函数之后,缓冲区的拥有者是生产者,缓冲区处于DEQUEUED状态。一旦缓冲区复制数据完成,通过queueBuffer函数把缓冲区的控制权交还给BufferQueueCore,这时候缓冲区将处于QUEUED状态。
1.2 消费者和core的联系
下面是IGraphicBufferComsumer接口的几个主要函数:
- virtual status_t acquireBuffer(BufferItem* outBuffer,
- nsecs_t expectedPresent, uint64_t maxFrameNumber = 0) override;
- ......
-
- virtual status_t releaseBuffer(int slot, uint64_t frameNumber,
- const sp<Fence>& releaseFence, EGLDisplay display,
- EGLSyncKHR fence);
-
-
- virtual status_t connect(const sp<IConsumerListener>& consumerListener,
- bool controlledByApp);
- virtual status_t disconnect();
BufferQueueConsumer类是接口IGraphicBufferComsumer的实现,在使用它之前,先要调用connect函数建立联系,这里传递的参数是IConsumerListener对象,是一个回调接口,如果BufferQueue中有数据准备好了就会调用它的onFrameAvailable函数来通知消费者取走数据。取走数据的时候,需要调用acquireBuffer函数,将缓冲区状态变成ACQUIRED,使用完之后调用releaseBuffer函数可以吧缓冲区数据归还给BufferQueueCore,这样缓冲区就变成FREE。
1.3 三者联系
对象BufferQueueProducer和BufferQueueConsumer好像没有直接联系,其实都是通过共同的BufferQueueCore对象连接在一起的,很多操作时直接使用BufferQueueCore对象的成员变量而不是函数来完成的。
二、GraphicBuffer对象的创建
对Surface而言,图像缓冲区是一个重要的数据结构,它是用户进程和图像显示器之间的纽带,下面我们来看看Surface的图像缓冲区是如何创建的。
2.1 内存缓冲区的创建
前面介绍了过dequeueBuffer函数,图像缓冲区GraphicBuffer就是在这个函数中创建的,当从BufferQueueCore中获取到空间的slot时,如果这个slot没有缓冲区就要新建一个。
下面是dequeueBuffer函数的部分代码,在从BufferQueueCore中获取到slot的时候,如果需要重新分配图像缓冲区就会调用mCore->mAllocator->createGraphicBuffer函数来重新创建一个图像缓冲区。
- ......
- *outSlot = found;
- ATRACE_BUFFER_INDEX(found);
-
- attachedByConsumer = mSlots[found].mAttachedByConsumer;
-
- mSlots[found].mBufferState = BufferSlot::DEQUEUED;
-
- const sp<GraphicBuffer>& buffer(mSlots[found].mGraphicBuffer);
- if ((buffer == NULL) ||
- buffer->needsReallocation(width, height, format, usage))
- {
- mSlots[found].mAcquireCalled = false;
- mSlots[found].mGraphicBuffer = NULL;
- mSlots[found].mRequestBufferCalled = false;
- mSlots[found].mEglDisplay = EGL_NO_DISPLAY;
- mSlots[found].mEglFence = EGL_NO_SYNC_KHR;
- mSlots[found].mFence = Fence::NO_FENCE;
- mCore->mBufferAge = 0;
-
- returnFlags |= BUFFER_NEEDS_REALLOCATION;
- } else {
-
-
- mCore->mBufferAge =
- mCore->mFrameCounter + 1 - mSlots[found].mFrameNumber;
- }
-
- BQ_LOGV("dequeueBuffer: setting buffer age to %" PRIu64,
- mCore->mBufferAge);
-
- if (CC_UNLIKELY(mSlots[found].mFence == NULL)) {
- BQ_LOGE("dequeueBuffer: about to return a NULL fence - "
- "slot=%d w=%d h=%d format=%u",
- found, buffer->width, buffer->height, buffer->format);
- }
-
- eglDisplay = mSlots[found].mEglDisplay;
- eglFence = mSlots[found].mEglFence;
- *outFence = mSlots[found].mFence;
- mSlots[found].mEglFence = EGL_NO_SYNC_KHR;
- mSlots[found].mFence = Fence::NO_FENCE;
-
- mCore->validateConsistencyLocked();
- }
-
- if (returnFlags & BUFFER_NEEDS_REALLOCATION) {
- status_t error;
- BQ_LOGV("dequeueBuffer: allocating a new buffer for slot %d", *outSlot);
- sp<GraphicBuffer> graphicBuffer(mCore->mAllocator->createGraphicBuffer(
- width, height, format, usage, &error));
- if (graphicBuffer == NULL) {
- BQ_LOGE("dequeueBuffer: createGraphicBuffer failed");
- return error;
- }
-
- {
- Mutex::Autolock lock(mCore->mMutex);
-
- if (mCore->mIsAbandoned) {
- BQ_LOGE("dequeueBuffer: BufferQueue has been abandoned");
- return NO_INIT;
- }
-
- graphicBuffer->setGenerationNumber(mCore->mGenerationNumber);
- mSlots[*outSlot].mGraphicBuffer = graphicBuffer;
- }
- }
- ......
mAllocator的类型是IGraphicBufferAlloc,也是一个Binder对象,它是在BufferQueueCore的构造函数中得到的,这个时候allocator为空的,具体要从Layer的构造函数中调用BufferQueue::createBufferQueue函数是,那个时候allocator参数就为空。然后通过getComposerService来调用createGraphicBufferAlloc函数来创建这个mAllocator对象。
之前的博客分析过getComposerService返回的是和SurfaceFlinger进程的Binder对象,因此最后是到SurfaceFlinger的createGraphicBufferAlloc函数中去了(但是这里有点搞不明白明明是在一个进程中为什么要用Binder呢?)。
- ......
- if (allocator == NULL) {
- sp<ISurfaceComposer> composer(ComposerService::getComposerService());
- mAllocator = composer->createGraphicBufferAlloc();
- if (mAllocator == NULL) {
- BQ_LOGE("createGraphicBufferAlloc failed");
- }
- }
- ......
下面我们来看SurfaceFlinger的createGraphicBufferAlloc函数。
- sp<IGraphicBufferAlloc> SurfaceFlinger::createGraphicBufferAlloc()
- {
- sp<GraphicBufferAlloc> gba(new GraphicBufferAlloc());
- return gba;
- }
因此最后BufferQueueProducer中的dequeueBuffer函数中调用mCore->mAllocator的createGraphicBuffer函数就是调用了GraphicBufferAlloc的createGraphicBufferAlloc函数。
- sp<GraphicBuffer> GraphicBufferAlloc::createGraphicBuffer(uint32_t width,
- uint32_t height, PixelFormat format, uint32_t usage, status_t* error) {
- sp<GraphicBuffer> graphicBuffer(
- new GraphicBuffer(width, height, format, usage));
- status_t err = graphicBuffer->initCheck();
- *error = err;
- ......
- return graphicBuffer;
- }
我们来看下GraphicBuffer对象的构造函数中调用了initSize函数。
- GraphicBuffer::GraphicBuffer(uint32_t inWidth, uint32_t inHeight,
- PixelFormat inFormat, uint32_t inUsage)
- : BASE(), mOwner(ownData), mBufferMapper(GraphicBufferMapper::get()),
- mInitCheck(NO_ERROR), mId(getUniqueId())
- {
- ......
- mInitCheck = initSize(inWidth, inHeight, inFormat, inUsage);
- }
在initSize函数中调用GraphicBufferAllocator的alloc来分配内存。
- status_t GraphicBuffer::initSize(uint32_t inWidth, uint32_t inHeight,
- PixelFormat inFormat, uint32_t inUsage)
- {
- GraphicBufferAllocator& allocator = GraphicBufferAllocator::get();
- uint32_t outStride = 0;
- status_t err = allocator.alloc(inWidth, inHeight, inFormat, inUsage,
- &handle, &outStride);
- if (err == NO_ERROR) {
- width = static_cast<int>(inWidth);
- height = static_cast<int>(inHeight);
- format = inFormat;
- usage = static_cast<int>(inUsage);
- stride = static_cast<int>(outStride);
- }
- return err;
- }
alloc又调用了成员变量mAllocDev的alloc函数。- 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<int>(width),
- static_cast<int>(height), format, static_cast<int>(usage), handle,
- &outStride);
在GraphicBufferAllocator的构造函数中装载了Gralloc模块,因此mAllocDev指向了Gralloc模块。这个会在后面的博客中分析
- GraphicBufferAllocator::GraphicBufferAllocator()
- : mAllocDev(0)
- {
- hw_module_t const* module;
- int err = hw_get_module(GRALLOC_HARDWARE_MODULE_ID, &module);
- ALOGE_IF(err, "FATAL: can't find the %s module", GRALLOC_HARDWARE_MODULE_ID);
- if (err == 0) {
- gralloc_open(module, &mAllocDev);
- }
- }
这里调用alloc分配了一块共享的内存缓冲区,alloc函数将返回共享区的fd和缓冲区的指针。既然GraphicBuffer中的缓冲区是共享内存,我们知道使用共享内存需要传递共享内存的句柄fd。下面我们看看是如何传到客户进程的。
2.2 内存缓冲区的fd传递到客户进程
GraphicBuffer类从模板类Flattenable派生,这个派生类可以通过Parcel传递,通常派生类需要重载flatten和unflatten方法,用于对象的序列化和反序列化。
- class GraphicBuffer
- : public ANativeObjectBase< ANativeWindowBuffer, GraphicBuffer, RefBase >,
- public Flattenable<GraphicBuffer>
我们先来看下flatten函数,fds参数用来传递文件句柄,函数把handle中的句柄复制到fds中,因此这些句柄就能通过binder传递到目标进程中去。
- status_t GraphicBuffer::flatten(void*& buffer, size_t& size, int*& fds, size_t& count) const {
- size_t sizeNeeded = GraphicBuffer::getFlattenedSize();
- if (size < sizeNeeded) return NO_MEMORY;
-
- size_t fdCountNeeded = GraphicBuffer::getFdCount();
- if (count < fdCountNeeded) return NO_MEMORY;
-
- int32_t* buf = static_cast<int32_t*>(buffer);
- buf[0] = 'GBFR';
- buf[1] = width;
- buf[2] = height;
- buf[3] = stride;
- buf[4] = format;
- buf[5] = usage;
- buf[6] = static_cast<int32_t>(mId >> 32);
- buf[7] = static_cast<int32_t>(mId & 0xFFFFFFFFull);
- buf[8] = static_cast<int32_t>(mGenerationNumber);
- buf[9] = 0;
- buf[10] = 0;
-
- if (handle) {
- buf[9] = handle->numFds;
- buf[10] = handle->numInts;
- memcpy(fds, handle->data,
- static_cast<size_t>(handle->numFds) * sizeof(int));
- memcpy(&buf[11], handle->data + handle->numFds,
- static_cast<size_t>(handle->numInts) * sizeof(int));
- }
-
- buffer = static_cast<void*>(static_cast<uint8_t*>(buffer) + sizeNeeded);
- size -= sizeNeeded;
- if (handle) {
- fds += handle->numFds;
- count -= static_cast<size_t>(handle->numFds);
- }
-
- return NO_ERROR;
- }
再来看unflatten函数,调用这个函数时,共享区的文件句柄已经准备好了,但是内存还没有进行映射,调用了mBufferMapper.registerBuffer函数来进行内存映射。- 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);
- ......
- }
-
- buffer = static_cast<void const*>(static_cast<uint8_t const*>(buffer) + sizeNeeded);
- size -= sizeNeeded;
- fds += numFds;
- count -= numFds;
-
- return NO_ERROR;
- }
我们看下GraphicBufferMapper::registerBuffer函数,调用了mAllocMod的registerBuffer函数,mAllocMod同样是指向了Gralloc模块的指针,在GraphicBufferMapper的构造函数中创建,因此实际是调用了Gralloc模块的gralloc_register_buffer函数。这个函数就是调用了mmap来进行共享内存的映射。在后续的博客我们再详细分析。- status_t GraphicBufferMapper::registerBuffer(buffer_handle_t handle)
- {
- ATRACE_CALL();
- status_t err;
-
- err = mAllocMod->registerBuffer(mAllocMod, handle);
-
- ALOGW_IF(err, "registerBuffer(%p) failed %d (%s)",
- handle, err, strerror(-err));
- return err;
- }
在硬件设备支持Framebuffer缓冲区的情况下,Surface中绘制图形的缓冲区就是Framebuffer的缓冲区,绘制完成后,如果不需要进行图像合成,只需要flip操作就能完成图像的输出,中间完全不用复制的过程,很高效。