/frameworks/native/libs/gui/include/gui/BufferSlot.h

struct BufferSlot {

BufferSlot()  
: mGraphicBuffer(nullptr),  
  mEglDisplay(EGL\_NO\_DISPLAY),  
  mBufferState(),  
  mRequestBufferCalled(false),  
  mFrameNumber(0),  
  mEglFence(EGL\_NO\_SYNC\_KHR),  
  mFence(Fence::NO\_FENCE),  
  mAcquireCalled(false),  
  mNeedsReallocation(false) {  
}

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

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

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

// 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;

// 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;

// 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;

// Indicates whether this buffer has been seen by a consumer yet  
bool mAcquireCalled;

// Indicates whether the buffer was re-allocated without notifying the  
// producer. If so, it needs to set the BUFFER\_NEEDS\_REALLOCATION flag when  
// dequeued to prevent the producer from using a stale cached buffer.  
bool mNeedsReallocation;  

};

• sp mGraphicBuffer;   // mGraphicBuffer指向这个槽位分配的缓冲区，如果没有分配缓冲区则为NULL
• EGLDisplay mEglDisplay;  // 用于创建EGLSyncKHR对象的EGLDisplay
• BufferState mBufferState; // 缓冲槽的当前状态
• bool mRequestBufferCalled; // mRequestBufferCalled用于验证生产者是否在被告知时调用了requestBuffer()。从技术上讲，这不是必需的，但对于调试和捕获生产者bug非常有用。
• uint64_t mFrameNumber; // mFrameNumber是此插槽的排队帧编号。这用于按LRU顺序将缓冲区出列（很有用，因为缓冲区可能在释放围栏发出信号之前被释放）。
• EGLSyncKHR mEglFence; // mEglFence是EGL sync对象，必须在与此缓冲槽关联的缓冲区退出队列之前发出信号。当创建缓冲区时，它被初始化为EGL_NO_SYNC_KHR，并且可以设置为releaseBuffer中的新同步对象(支持mFence时mEglFence被弃用）
• sp mFence;  // mFence是一个围栏，当缓冲区的前所有者启动的工作完成时，它将发出信号。

1. 当Buffer处在FREE时，Fence指示消费者何时已完成从缓冲区的读取，或者生产者何时已完成写入
2. 当Buffer处在QUEUED时，它指示生产者何时完成缓冲区填充。
3. 当Buffer处在DEQUEUED or ACQUIRED时，Fence连同缓冲区的所有权一起传递给消费者或生产者，并且mFence设置为NO_FENCE。

• bool mAcquireCalled; // 指示消费者consumer是否已看到此缓冲区

• bool mNeedsReallocation; //指示是否在未通知生产者的情况下重新分配了缓冲区。如果是这样，它需要在退出队列时设置BUFFER_NEEDS_REALLOCATION标志，以防止生产者使用过时的缓存缓冲区。

  // A buffer can be in one of five states, represented as below:
  //
  // | mShared | mDequeueCount | mQueueCount | mAcquireCount |
  // --------|---------|---------------|-------------|---------------|
  // FREE | false | 0 | 0 | 0 |
  // DEQUEUED| false | 1 | 0 | 0 |
  // QUEUED | false | 0 | 1 | 0 |
  // ACQUIRED| false | 0 | 0 | 1 |
  // SHARED | true | any | any | any |
  //
  // FREE indicates that the buffer is available to be dequeued by the
  // producer. The slot is "owned" by BufferQueue. It transitions to DEQUEUED
  // when dequeueBuffer is called.
  //
  // DEQUEUED indicates that the buffer has been dequeued by the producer, but
  // has not yet been queued or canceled. The producer may modify the
  // buffer's contents as soon as the associated release fence is signaled.
  // The slot is "owned" by the producer. It can transition to QUEUED (via
  // queueBuffer or attachBuffer) or back to FREE (via cancelBuffer or
  // detachBuffer).
  //
  // QUEUED indicates that the buffer has been filled by the producer and
  // queued for use by the consumer. The buffer contents may continue to be
  // modified for a finite time, so the contents must not be accessed until
  // the associated fence is signaled. The slot is "owned" by BufferQueue. It
  // can transition to ACQUIRED (via acquireBuffer) or to FREE (if another
  // buffer is queued in asynchronous mode).
  //
  // ACQUIRED indicates that the buffer has been acquired by the consumer. As
  // with QUEUED, the contents must not be accessed by the consumer until the
  // acquire fence is signaled. The slot is "owned" by the consumer. It
  // transitions to FREE when releaseBuffer (or detachBuffer) is called. A
  // detached buffer can also enter the ACQUIRED state via attachBuffer.
  //
  // SHARED indicates that this buffer is being used in shared buffer
  // mode. It can be in any combination of the other states at the same time,
  // except for FREE (since that excludes being in any other state). It can
  // also be dequeued, queued, or acquired multiple times.

• FREE状态，buffer及slot属于BufferQueue，producer可以通过调用dequeueBuffer获取该buffer,其状态转为DEQUEUED

• DEQUEUED状态，表示该buffer已经被producer出队列，但时还被queued或canceled。一旦与该buffer相关联的fence发出信号，producer就可以修改buffer的内容。这种状态下slot属于producer所有，当调用queueBuffer or attachBuffer后转为QUEUED，或调用cancelBuffer or detachBuffer转为FREE

• QUEUED状态，表示该buffer已经被producer填充数据，入队列让consumer使用。buffer内容可能会在有限的时间内继续修改，因此在相关fence发出信号之前，不得访问内容。此时slot归BufferQueue所有，buffer状态可以转为ACQUIRED(via acquireBuffer) 或FREE(另一个buffer异步模式下入队列)

• ACQUIRED状态，表示该buffer被consumer取得。fence信号发出后，消费者就可以访问其内容了。slot被consumer所拥有。 当调用releaseBuffer (or detachBuffer)可以转为FREE

• SHARED状态，表示此缓冲区正在共享缓冲区模式下使用