关于Layer组件的同步以及减少GPU带宽的问题

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问题：

1) Layer 之间能否进行单独更新，比如其中video 层上面弹出的状态栏场景如何优化；

初步构想每次仅更新video layer或则将video layer 绕过BQ 进行处理。

2）FW 以及GPU对video layer 的常见处理；

3）几个变量的理解；

（1）mCurrentTexture ，nextTextureImage；

（2）BufferItem，BufferQueue,mslots；slot，mqueuedframe，mqueuedItem；

（3）syncForReleaseLocked，updateAndReleaseLocked，releaseBufferLocked，releaseBuffer

（4）

/ acquireBuffer attempts to acquire ownership of the next pending buffer in the BufferQueue.
      // If no buffer is pending then it returns NO_BUFFER_AVAILABLE. If a buffer is successfully
      // acquired, the information about the buffer is returned in BufferItem.
      //
      // If the buffer returned had previously been acquired then the BufferItem::mGraphicBuffer field
      // of buffer is set to NULL and it is assumed that the consumer still holds a reference to the
      // buffer.
      //
      // If presentWhen is non-zero, it indicates the time when the buffer will be displayed on
      // screen. If the buffer's timestamp is farther in the future, the buffer won't be acquired, and
      // PRESENT_LATER will be returned. The presentation time is in nanoseconds, and the time base
      // is CLOCK_MONOTONIC.
      //
      // If maxFrameNumber is non-zero, it indicates that acquireBuffer should only return a buffer
      // with a frame number less than or equal to maxFrameNumber. If no such frame is available
      // (such as when a buffer has been replaced but the consumer has not received the
      // onFrameReplaced callback), then PRESENT_LATER will be returned.
      //
      // Return of NO_ERROR means the operation completed as normal.
      //
      // Return of a positive value means the operation could not be completed at this time, but the
      // user should try again later:
      // * NO_BUFFER_AVAILABLE - no buffer is pending (nothing queued by producer)
      // * PRESENT_LATER - the buffer's timestamp is farther in the future
      //
      // Return of a negative value means an error has occurred:
      // * INVALID_OPERATION - too many buffers have been acquired

  // Returned by releaseBuffer, after which the consumer must free any references to the
            // just-released buffer that it might have.
            STALE_BUFFER_SLOT = 1,
          // Returned by dequeueBuffer if there are no pending buffers available.
          NO_BUFFER_AVAILABLE,
          // Returned by dequeueBuffer if it's too early for the buffer to be acquired.
          PRESENT_LATER,

（5）mslots 与mframe 以及mframenumber 的区别

（6）onframavailable 和latchbuffer ，reject，updateteximage，的逻辑。

其中updateteximage；

(7)bufferitem 与mslots的区别

(8)bufferqueueconsumer 与bufferitemconsumer区别；

（9）acquirebuffer的fence 逻辑，fencefd是哪来的；

fence 相关接口定义参见ui/Fence.h

 status_t Fence::waitForever(const char* logname) {64      ATRACE_CALL();
      if (mFenceFd == -1) {
          return NO_ERROR;
      }
      int warningTimeout = 3000;
      int err = sync_wait(mFenceFd, warningTimeout);
      if (err < 0 && errno == ETIME) {
          ALOGE("%s: fence %d didn't signal in %u ms", logname, mFenceFd,
                  warningTimeout);
          err = sync_wait(mFenceFd, TIMEOUT_NEVER);
      }
      return err < 0 ? -errno : status_t(NO_ERROR);
  }

  struct EglSlot {
           EglSlot() : mEglFence(EGL_NO_SYNC_KHR) {}
            // mEglImage is the EGLImage created from mGraphicBuffer.
            sp<EglImage> mEglImage;
           // mFence 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 (optionally, based
           // on a compile-time option) set to a new sync object in updateTexImage.
            EGLSyncKHR mEglFence;
      };

（10）

updateAndReleaseLocked(item, &mPendingRelease)，

updateAndReleaseLocked(item)，

bindTextureImageLocked()

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