android: Update comment about invalid timestamps from SurfaceFlinger

build/android/pylib/surface_stats_collector.py

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import Queue
 import datetime
 import logging
 import re
 import threading
 
@@ skipping 201 matching lines @@
     # composited into a SurfaceView.
     results = self._adb.RunShellCommand(
         'dumpsys SurfaceFlinger --latency SurfaceView', log_result=True)
     if not len(results):
       return (None, None)
 
     timestamps = []
     nanoseconds_per_second = 1e9
     refresh_period = long(results[0]) / nanoseconds_per_second
 
-    # SurfaceFlinger sometimes gives an invalid timestamp for the very latest
-    # frame if it is queried while the frame is still being presented. We ignore
-    # these timestamps.
-    bad_timestamp = (1 << 63) - 1
+    # If a fence associated with a frame is still pending when we query the
+    # latency data, SurfaceFlinger gives the frame a timestamp of INT64_MAX.
+    # Since we only care about completed frames, we will ignore any timestamps
+    # with this value.
+    pending_fence_timestamp = (1 << 63) - 1
 
     for line in results[1:]:
       fields = line.split()
       if len(fields) != 3:
         continue
       timestamp = long(fields[1])
-      if timestamp == bad_timestamp:
+      if timestamp == pending_fence_timestamp:
         continue
       timestamp /= nanoseconds_per_second
       timestamps.append(timestamp)
 
     return (refresh_period, timestamps)
 
   def _GetSurfaceStatsLegacy(self):
     """Legacy method (before JellyBean), returns the current Surface index
        and timestamp.
 
@@ skipping 11 matching lines @@
       try:
         cur_surface = int(match.group(1), 16)
       except Exception:
         logging.error('Failed to parse current surface from ' + match.group(1))
     else:
       logging.warning('Failed to call SurfaceFlinger surface ' + results[0])
     return {
         'page_flip_count': cur_surface,
         'timestamp': datetime.datetime.now(),
     }