HWC2: Redefine retire fence as present fence

Redefines the retire fence returned by presentDisplay to be a present
fence with more strictly defined semantics: this fence should fire
at the vsync when the frame is picked up by hardware, either for
scanout (in the case of video-mode panels) or for transfer to panel
memory (in the case of command-mode panels).

Bug: 29771461
Change-Id: Idf5f86c92a8748426cf3368242c08876a48d81dd
diff --git a/include/hardware/hwcomposer2.h b/include/hardware/hwcomposer2.h
index a51efd7..f6a6eba 100644
--- a/include/hardware/hwcomposer2.h
+++ b/include/hardware/hwcomposer2.h
@@ -1328,7 +1328,7 @@
         hwc2_device_t* device, hwc2_display_t display, uint32_t* outNumElements,
         hwc2_layer_t* outLayers, int32_t* outFences);
 
-/* presentDisplay(..., outRetireFence)
+/* presentDisplay(..., outPresentFence)
  * Descriptor: HWC2_FUNCTION_PRESENT_DISPLAY
  * Must be provided by all HWC2 devices
  *
@@ -1342,15 +1342,16 @@
  * setLayerBuffer), then it is safe to call this function without first
  * validating the display.
  *
- * If this call succeeds, outRetireFence will be populated with a file
- * descriptor referring to a retire sync fence object. For physical displays,
- * this fence will be signaled when the result of composition of the prior frame
- * is no longer necessary (because it has been copied or replaced by this
- * frame). For virtual displays, this fence will be signaled when writes to the
- * output buffer have completed and it is safe to read from it.
+ * If this call succeeds, outPresentFence will be populated with a file
+ * descriptor referring to a present sync fence object. For physical displays,
+ * this fence will be signaled at the vsync when the result of composition of
+ * this frame starts to appear (for video-mode panels) or starts to transfer to
+ * panel memory (for command-mode panels). For virtual displays, this fence will
+ * be signaled when writes to the output buffer have completed and it is safe to
+ * read from it.
  *
  * Parameters:
- *   outRetireFence - a sync fence file descriptor as described above; pointer
+ *   outPresentFence - a sync fence file descriptor as described above; pointer
  *       will be non-NULL
  *
  * Returns HWC2_ERROR_NONE or one of the following errors:
@@ -1361,7 +1362,8 @@
  *       for this display
  */
 typedef int32_t /*hwc2_error_t*/ (*HWC2_PFN_PRESENT_DISPLAY)(
-        hwc2_device_t* device, hwc2_display_t display, int32_t* outRetireFence);
+        hwc2_device_t* device, hwc2_display_t display,
+        int32_t* outPresentFence);
 
 /* setActiveConfig(..., config)
  * Descriptor: HWC2_FUNCTION_SET_ACTIVE_CONFIG