SurfaceFlinger: tune number of buffers for low activity layers

Tune number of buffers indicating whether a layer is low activity
or not to 2 accommodate more cases of low activity layers.

Bug: 140948091
Test: 1) Set the static wallpaper
2) Go to Settings->About Phone -> scroll all the way down
3) Move device to Zone1 and launch settings page
4) Move device to Non Zone1 without touch and wait for 1-2 secs and observe

Change-Id: I6a06fa4945843a8d991463c30cb0e6db68f99365

2 files changed, 7 insertions, 4 deletions

diff --git a/services/surfaceflinger/Scheduler/LayerInfo.h b/services/surfaceflinger/Scheduler/LayerInfo.h
index a7337817e3..17afddac28 100644
--- a/services/surfaceflinger/Scheduler/LayerInfo.h
+++ b/services/surfaceflinger/Scheduler/LayerInfo.h
@@ -109,7 +109,7 @@ class LayerInfo {
         bool isLowActivityLayer() const {
             // We want to make sure that we received more than two frames from the layer
             // in order to check low activity.
-            if (mElements.size() < 2) {
+            if (mElements.size() < scheduler::LOW_ACTIVITY_BUFFERS + 1) {
                 return false;
             }
 
@@ -118,7 +118,8 @@ class LayerInfo {
             // Check the frame before last to determine whether there is low activity.
             // If that frame is older than LOW_ACTIVITY_EPSILON_NS, the layer is sending
             // infrequent updates.
-            if (mElements.at(mElements.size() - 2) < obsoleteEpsilon) {
+            if (mElements.at(mElements.size() - (scheduler::LOW_ACTIVITY_BUFFERS + 1)) <
+                obsoleteEpsilon) {
                 return true;
             }
 
diff --git a/services/surfaceflinger/Scheduler/SchedulerUtils.h b/services/surfaceflinger/Scheduler/SchedulerUtils.h
index ced1899109..ac10f83ad9 100644
--- a/services/surfaceflinger/Scheduler/SchedulerUtils.h
+++ b/services/surfaceflinger/Scheduler/SchedulerUtils.h
@@ -42,9 +42,11 @@ static constexpr uint32_t HWC2_SCREEN_OFF_CONFIG_ID = 0xffffffff;
 // or waiting idle in messaging app, when cursor is blinking.
 static constexpr std::chrono::nanoseconds OBSOLETE_TIME_EPSILON_NS = 1200ms;
 
-// Layer is considered low activity if the buffers come more than LOW_ACTIVITY_EPSILON_NS
-// apart. This is helping SF to vote for lower refresh rates when there is not activity
+// Layer is considered low activity if the LOW_ACTIVITY_BUFFERS buffers come more than
+// LOW_ACTIVITY_EPSILON_NS  apart.
+// This is helping SF to vote for lower refresh rates when there is not activity
 // in screen.
+static constexpr int LOW_ACTIVITY_BUFFERS = 2;
 static constexpr std::chrono::nanoseconds LOW_ACTIVITY_EPSILON_NS = 250ms;
 
 // Calculates the statistical mean (average) in the data structure (array, vector). The