Layer 之間同步以及如何減少GPU 帶寬

問題：

1) Layer 之間能否進行單獨更新，比如其中video 層上面彈出的狀態欄場景如何優化；

初步構想每次僅更新video layer或則將video layer 繞過BQ 進行處理。 

2）FW 以及GPU對video layer 的常見處理；

3）幾個變量的理解；

（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.
54      // If no buffer is pending then it returns NO_BUFFER_AVAILABLE. If a buffer is successfully
55      // acquired, the information about the buffer is returned in BufferItem.
56      //
57      // If the buffer returned had previously been acquired then the BufferItem::mGraphicBuffer field
58      // of buffer is set to NULL and it is assumed that the consumer still holds a reference to the
59      // buffer.
60      //
61      // If presentWhen is non-zero, it indicates the time when the buffer will be displayed on
62      // screen. If the buffer's timestamp is farther in the future, the buffer won't be acquired, and
63      // PRESENT_LATER will be returned. The presentation time is in nanoseconds, and the time base
64      // is CLOCK_MONOTONIC.
65      //
66      // If maxFrameNumber is non-zero, it indicates that acquireBuffer should only return a buffer
67      // with a frame number less than or equal to maxFrameNumber. If no such frame is available
68      // (such as when a buffer has been replaced but the consumer has not received the
69      // onFrameReplaced callback), then PRESENT_LATER will be returned.
70      //
71      // Return of NO_ERROR means the operation completed as normal.
72      //
73      // Return of a positive value means the operation could not be completed at this time, but the
74      // user should try again later:
75      // * NO_BUFFER_AVAILABLE - no buffer is pending (nothing queued by producer)
76      // * PRESENT_LATER - the buffer's timestamp is farther in the future
77      //
78      // Return of a negative value means an error has occurred:
79      // * INVALID_OPERATION - too many buffers have been acquired

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

（5）mslots 與mframe 以及mframenumber 的區別

（6）onframavailable 和latchbuffer ，reject，updateteximage，的邏輯。

其中updateteximage；

(7)bufferitem 與mslots的區別

(8)bufferqueueconsumer 與bufferitemconsumer區別；

（9）acquirebuffer的fence 邏輯，fencefd是哪來的；

fence 相關接口定義參見ui/Fence.h

 status_t Fence::waitForever(const char* logname) {
64      ATRACE_CALL();
65      if (mFenceFd == -1) {
66          return NO_ERROR;
67      }
68      int warningTimeout = 3000;
69      int err = sync_wait(mFenceFd, warningTimeout);
70      if (err < 0 && errno == ETIME) {
71          ALOGE("%s: fence %d didn't signal in %u ms", logname, mFenceFd,
72                  warningTimeout);
73          err = sync_wait(mFenceFd, TIMEOUT_NEVER);
74      }
75      return err < 0 ? -errno : status_t(NO_ERROR);
76  }

  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()