Android GraphicBuffer-Fence

这里需要介绍一个伴随着GraphicBuffer的Fence，包括Fence的诞生，Fence的处理。

在前面介绍Surface的时候，提到过Surface会通过BufferQueueProducer申请GraphicBuffer，用作显存，所以我们这里看一下BufferQueueProducer/Consumer是怎么管理GraphicBuffer的。（这里有一个问题，是否可以脱离BufferQueueProducer/Consumer去使用GraphicBuffer？）

BufferQueueProducer(简称producer）和BufferQueueConsumer（简称consumer)公用BufferQueueCore（简称core）；core里面有一个mSlots = struct BufferSlot[64];同时mMaxAcquiredBufferCount=1，mMaxDequeuedBufferCount=1，同时getMaxBufferCountLocked 为3或者2：

int BufferQueueCore::getMaxBufferCountLocked() const {
    int maxBufferCount = mMaxAcquiredBufferCount + mMaxDequeuedBufferCount +
            ((mAsyncMode || mDequeueBufferCannotBlock) ? 1 : 0);

    // limit maxBufferCount by mMaxBufferCount always
    maxBufferCount = std::min(mMaxBufferCount, maxBufferCount);

    return maxBufferCount;
}
//BufferQueueCore创建的时候会初始化mFreeSlots(最多3个元素）， mUnusedSlots有64-3个元素

   int numStartingBuffers = getMaxBufferCountLocked();
   for (int s = 0; s < numStartingBuffers; s++) {
     mFreeSlots.insert(s);
   }
   for (int s = numStartingBuffers; s < BufferQueueDefs::NUM_BUFFER_SLOTS;  s++）{
     mUnusedSlots.push_front(s);
   }

在APP拿到BufferQueueProducer的时候，在绘制之前，会调用BufferQueueProducer.allocateBuffers，预先分配足够的GraphicBuffer，防止在draw的时候分配造成delay；

分配Buffer的数量和mFreeSlots元素数目相同，也就是3个：

void BufferQueueProducer::allocateBuffers(uint32_t width, uint32_t height,
        PixelFormat format, uint64_t usage) {
         newBufferCount = mCore->mFreeSlots.size();
         。。。。。。
　　　　   //创建3个GraphicBuffer
         for (size_t i = 0; i <  newBufferCount; ++i) {
            sp<GraphicBuffer> graphicBuffer = new GraphicBuffer(
                    allocWidth, allocHeight, allocFormat, BQ_LAYER_COUNT,
                    allocUsage, allocName);

            status_t result = graphicBuffer->initCheck();

            if (result != NO_ERROR) {
                BQ_LOGE("allocateBuffers: failed to allocate buffer (%u x %u, format"
                        " %u, usage %#" PRIx64 ")", width, height, format, usage);
                Mutex::Autolock lock(mCore->mMutex);
                mCore->mIsAllocating = false;
                mCore->mIsAllocatingCondition.broadcast();
                return;
            }
            buffers.push_back(graphicBuffer);
        }
       。。。。。
        //将新创建的GraphicBuffer添加到mSlots数组中。注意mSlots的index，这个index很重要，mSlots中的数据是和index绑定的，类似于ID；其中的graphicBuffer可能会发生变化。
        for (size_t i = 0; i < newBufferCount; ++i) {
                if (mCore->mFreeSlots.empty()) {
                    BQ_LOGV("allocateBuffers: a slot was occupied while "
                            "allocating. Dropping allocated buffer.");
                    continue;
                }
                auto slot = mCore->mFreeSlots.begin();
                mCore->clearBufferSlotLocked(*slot); // Clean up the slot first
                mSlots[*slot].mGraphicBuffer = buffers[i];
                mSlots[*slot].mFence = Fence::NO_FENCE;

                // freeBufferLocked puts this slot on the free slots list. Since
                // we then attached a buffer, move the slot to free buffer list.
                mCore->mFreeBuffers.push_front(*slot);

                BQ_LOGV("allocateBuffers: allocated a new buffer in slot %d",
                        *slot);

                // Make sure the erase is done after all uses of the slot
                // iterator since it will be invalid after this point.
                mCore->mFreeSlots.erase(slot);
            }
    
}        

下面看一下GraphicBuffer的创建流程：

GraphicBuffer经过GrallocService->GrallocHal，在libgralloc里面分配；其中比较常见的方式是使用ION驱动进行分配，分配完毕之后还要为buffer分配Fence。