struct BufferSlot {
    // mGraphicBuffer points to the buffer allocated for this slot or is NULL
    // if no buffer has been allocated.
    sp<GraphicBuffer> mGraphicBuffer;

• mGraphicBuffer 指向已分配的 buffer 或 NULL。GraphicBuffer 继承 Flattenable，所以可跨进程传递。

// mEglDisplay is the EGLDisplay used to create EGLSyncKHR objects.
EGLDisplay mEglDisplay;

// mBufferState is the current state of this buffer slot.
BufferState mBufferState;

• EGLDisplay 还不知道是什么。
• BufferState 表示当前 buffer slot 的状态，一共有 FREE、DEQUEUED、QUEUED、ACQUIRED、SHARED。注意，它不是 buffer 的状态。

// mRequestBufferCalled is used for validating that the producer did
// call requestBuffer() when told to do so. Technically this is not
// needed but useful for debugging and catching producer bugs.
bool mRequestBufferCalled;

• mRequestBufferCalled 表示生产者是否调用了 BufferQueueProducer::requestBuffer()，其实它是为了检查生产者持有的 buffer 是否是正确分配的。在 BufferQueueProducer::queueBuffer() 中如果 mRequestBufferCalled 是 false，则返回 BAD_VALUE。
• 在 BufferQueueProducer::dequeueBuffer() 中如果需要再分配，把 mRequestBufferCalled 设置 false，在 BufferQueueProducer::requestBuffer() 设置 true。
• 在 BufferQueueProducer::attachBuffer() 也设置为 true，因为该 buffer 是生产者连接的，所以假定 buffer 已经按照生产者要求分配的，这样之后的 BufferQueueProducer::queueBuffer() 才能成功。

// mFrameNumber is the number of the queued frame for this slot.  This
// is used to dequeue buffers in LRU order (useful because buffers
// may be released before their release fence is signaled).
uint64_t mFrameNumber;

mFrameNumber 表示该 slot 分配 buffer 之后 queue 过的帧数，如果再分配，则重新计数。它已经不用在 dequeue 时的 LRU 排序。目前它只用在 BufferQueueConsumer::releaseBuffer() 中判断该 slot 中的 buffer 是否再分配过。

BufferQueueProducer::queueBuffer() 中，先更新 BufferQueueCore::mFrameCount 后再设置给 mFrameNumber。

// mEglFence is the EGL sync object that must signal before the buffer
// associated with this buffer slot may be dequeued. It is initialized
// to EGL_NO_SYNC_KHR when the buffer is created and may be set to a
// new sync object in releaseBuffer.  (This is deprecated in favor of
// mFence, below.)
EGLSyncKHR mEglFence;

// mFence is a fence which will signal when work initiated by the
// previous owner of the buffer is finished. When the buffer is FREE,
// the fence indicates when the consumer has finished reading
// from the buffer, or when the producer has finished writing if it
// called cancelBuffer after queueing some writes. When the buffer is
// QUEUED, it indicates when the producer has finished filling the
// buffer. When the buffer is DEQUEUED or ACQUIRED, the fence has been
// passed to the consumer or producer along with ownership of the
// buffer, and mFence is set to NO_FENCE.
sp<Fence> mFence;

Fence 相关的以后单独写。