viz: Eliminate delay in bumping up the refresh rate.

Be conservative when lowering down the refresh rate by waiting for a
consistent state for at least 60 frames but immediately switch if the
refresh rate should be increased.

R=dcastagna@chromium.org

Test: FrameRateDeciderTest.TogglesAfterMinNumOfFrames
Bug: 976583
Change-Id: I9483357807e81a28966254e1d89418dd12f03f4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079264
Commit-Queue: Khushal <khushalsagar@chromium.org>
Auto-Submit: Khushal <khushalsagar@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745726}

components/viz/service/display/frame_rate_decider.cc

@@ -139,11 +139,18 @@ void FrameRateDecider::UpdatePreferredFrameIntervalIfNeeded() {
   }
   last_computed_preferred_frame_interval_ = new_preferred_interval;
 
+  if (current_preferred_frame_interval_ == new_preferred_interval)
+    return;
+
   // The min num of frames heuristic is to ensure we see a constant pattern
-  // before toggling the global setting to avoid unnecessary switches.
-  if (num_of_frames_since_preferred_interval_changed_ >=
-          min_num_of_frames_to_toggle_interval_ &&
-      current_preferred_frame_interval_ != new_preferred_interval) {
+  // before toggling the global setting to avoid unnecessary switches when
+  // lowering the refresh rate. For increasing the refresh rate we toggle
+  // immediately to prioritize smoothness.
+  bool should_toggle =
+      current_preferred_frame_interval_ > new_preferred_interval ||
+      num_of_frames_since_preferred_interval_changed_ >=
+          min_num_of_frames_to_toggle_interval_;
+  if (should_toggle) {
     current_preferred_frame_interval_ = new_preferred_interval;
     client_->SetPreferredFrameInterval(new_preferred_interval);
   }

components/viz/service/display/frame_rate_decider_unittest.cc

@@ -298,6 +298,20 @@ TEST_F(FrameRateDeciderTest, TogglesAfterMinNumOfFrames) {
     frame_rate_decider_->OnSurfaceWillBeDrawn(surface);
   }
   EXPECT_EQ(display_interval_, preferred_interval);
+
+  // The frame rate is immediately toggled on drawing a surface with a lower
+  // interval.
+  frame_rate_decider_->set_min_num_of_frames_to_toggle_interval_for_testing(
+      50u);
+  FrameSinkId frame_sink_id2(1u, 1u);
+  preferred_intervals_[frame_sink_id2] = min_supported_interval;
+  auto* surface2 = CreateAndDrawSurface(frame_sink_id2);
+  {
+    FrameRateDecider::ScopedAggregate scope(frame_rate_decider_.get());
+    frame_rate_decider_->OnSurfaceWillBeDrawn(surface);
+    frame_rate_decider_->OnSurfaceWillBeDrawn(surface2);
+  }
+  EXPECT_EQ(display_interval_, min_supported_interval);
 }
 
 }  // namespace