commit bf711ebeb6931f5b2094ac4ab4fe710b9dbebbe3
author Drew Davenport <ddavenport@google.com> Wed Feb 19 09:41:20 2025 -0700
committer Drew Davenport <ddavenport@google.com> Fri Feb 21 10:51:42 2025 -0700
tree 1ac7ca1ca7d552ac19b39aa926b35291bd5234e7
parent 35a395720a4d567e1aaf1749963088d9829ef8b8

drm_hwcomposer: Avoid resetting vsync timestamp

last_timestamp_ tracks the most recent timestamp from a vblank event. Do
not reset it when toggling vsync tracking on/off. This allows for
GetPhasedVSync to calculate a more precise approximation of the next
vsync relative to the current time, even if a vsync hasn't happened for
a while.

Reset last_timestamp_ when the vsync period changes, since the phased
vsync calculation is based on the current vsync.

Also change last_timestamp_ to std::optional type.

Change-Id: I101b36f87d6ba8197c321c83ac0bc230fba9cf7e
Signed-off-by: Drew Davenport <ddavenport@google.com>

drm/VSyncWorker.cpp

diff --git a/drm/VSyncWorker.cpp b/drm/VSyncWorker.cpp
index 5234f6b..5b3071d 100644
--- a/drm/VSyncWorker.cpp
+++ b/drm/VSyncWorker.cpp
@@ -55,7 +55,6 @@
   {
     const std::lock_guard<std::mutex> lock(mutex_);
     enabled_ = ShouldEnable();
-    last_timestamp_ = -1;
   }
 
   cv_.notify_all();
@@ -64,6 +63,7 @@
 void VSyncWorker::SetVsyncPeriodNs(uint32_t vsync_period_ns) {
   const std::lock_guard<std::mutex> lock(mutex_);
   vsync_period_ns_ = vsync_period_ns;
+  last_timestamp_ = std::nullopt;
 }
 
 void VSyncWorker::SetVsyncTimestampTracking(bool enabled) {
@@ -121,11 +121,11 @@
  *  timestamp.
  */
 int64_t VSyncWorker::GetPhasedVSync(int64_t frame_ns, int64_t current) const {
-  if (last_timestamp_ < 0)
+  if (!last_timestamp_.has_value())
     return current + frame_ns;
 
-  return (frame_ns * ((current - last_timestamp_) / frame_ns + 1)) +
-         last_timestamp_;
+  return (frame_ns * ((current - *last_timestamp_) / frame_ns + 1)) +
+         *last_timestamp_;
 }
 
 static const int64_t kOneSecondNs = 1LL * 1000 * 1000 * 1000;