SurfaceFlinger: previousFrameFence needs to check the current time

previousFrameFence returns the fence associated with the previous vsync.
In case that SF wake up was delayed due to a previous frame, up to the
point that the wake up occurs after the vsync, SF needs to check fence
N-1 and not N-2 since frame N-1 was targeting the vsync prior to SF
wake up.

Bug: 187886038
Test: run bouncy ball and simulate the first frame missed
Change-Id: I176c6641dd42132fcfd75a79898a5d2bd8da9c90

1 files changed, 5 insertions, 5 deletions

diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index ecee97783c..267dd6d9d0 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -1709,11 +1709,11 @@ void SurfaceFlinger::setVsyncEnabled(bool enabled) {
 }
 
 SurfaceFlinger::FenceWithFenceTime SurfaceFlinger::previousFrameFence() {
-    // We are storing the last 2 present fences. If sf's phase offset is to be
-    // woken up before the actual vsync but targeting the next vsync, we need to check
-    // fence N-2
-    return mVsyncModulator->getVsyncConfig().sfOffset > 0 ? mPreviousPresentFences[0]
-                                                          : mPreviousPresentFences[1];
+    const auto now = systemTime();
+    const auto vsyncPeriod = mScheduler->getDisplayStatInfo(now).vsyncPeriod;
+    const bool expectedPresentTimeIsTheNextVsync = mExpectedPresentTime - now <= vsyncPeriod;
+    return expectedPresentTimeIsTheNextVsync ? mPreviousPresentFences[0]
+                                             : mPreviousPresentFences[1];
 }
 
 bool SurfaceFlinger::previousFramePending(int graceTimeMs) {