[sf] only update the last latch time if the layer has a buffer

If we apply a transaction that does not latch a buffer, we
incorrectly updated the last latch time. In the next vsync,
if we presented a buffer late, we would misclassify the
jank as buffer stuffing.

Fix this by only updating the latch time if the layer has
a buffer.

Test: verify in perfetto traces
Fixes: 270041584
Change-Id: I423a28803a2660ada80ef554469f3977c389fbfa

1 files changed, 8 insertions, 2 deletions

diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index eecfeb661a..f6063f4a97 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -3954,8 +3954,14 @@ bool SurfaceFlinger::latchBuffers() {
             mLayersWithQueuedFrames.emplace(sp<Layer>::fromExisting(layer));
         } else {
             layer->useEmptyDamage();
-            // If the layer has frames we will update the latch time when latching the buffer.
-            layer->updateLastLatchTime(latchTime);
+            if (!layer->hasBuffer()) {
+                // The last latch time is used to classify a missed frame as buffer stuffing
+                // instead of a missed frame. This is used to identify scenarios where we
+                // could not latch a buffer or apply a transaction due to backpressure.
+                // We only update the latch time for buffer less layers here, the latch time
+                // is updated for buffer layers when the buffer is latched.
+                layer->updateLastLatchTime(latchTime);
+            }
         }
     });
     mForceTransactionDisplayChange = false;