frame clock: fix scheduling

Fix scheduling of the frame clock when we don't receive "frame drawn"
messages from the compositor.

If we received "frame drawn" events recently, then the "smooth frame
time" would be in sync with the vsync time. When we don't receive frame
drawn events, the "smooth frame time" is simply incremented by constant
multiples of the refresh interval. In both cases we can use this smooth
time as the basis for scheduling the next clock cycle.

By only using the "smooth frame time" as a basis we also benefit from
more consistent scheduling cadence. If, for example, we got "frame
drawn" events, then didn't receive them for a few frames, we would still
be in sync when we start receiving these events again.

1 files changed, 14 insertions, 20 deletions

diff --git a/gdk/gdkframeclockidle.c b/gdk/gdkframeclockidle.c
index 4884a81fb9..ec66950b70 100644
--- a/gdk/gdkframeclockidle.c
+++ b/gdk/gdkframeclockidle.c
@@ -140,6 +140,7 @@ gdk_frame_clock_idle_init (GdkFrameClockIdle *frame_clock_idle)
     gdk_frame_clock_idle_get_instance_private (frame_clock_idle);
 
   priv->freeze_count = 0;
+  priv->smoothed_frame_time_period = FRAME_INTERVAL;
 }
 
 static void
@@ -346,23 +347,6 @@ maybe_stop_idle (GdkFrameClockIdle *clock_idle)
     }
 }
 
-static gint64
-compute_min_next_frame_time (GdkFrameClockIdle *clock_idle,
-                             gint64             last_frame_time)
-{
-  gint64 presentation_time;
-  gint64 refresh_interval;
-
-  gdk_frame_clock_get_refresh_info (GDK_FRAME_CLOCK (clock_idle),
-                                    last_frame_time,
-                                    &refresh_interval, &presentation_time);
-
-  if (presentation_time == 0)
-    return last_frame_time + refresh_interval;
-  else
-    return presentation_time + refresh_interval / 2;
-}
-
 static gboolean
 gdk_frame_clock_flush_idle (void *data)
 {
@@ -656,9 +640,19 @@ gdk_frame_clock_paint_idle (void *data)
    */
   if (priv->freeze_count == 0)
     {
-      priv->min_next_frame_time = compute_min_next_frame_time (clock_idle,
-                                                               priv->smoothed_frame_time_base -
-                                                               priv->smoothed_frame_time_phase);
+      /*
+       * If we don't receive "frame drawn" events, smooth_cycle_start will simply be advanced in constant increments of
+       * the refresh interval. That way we get absolute target times for the next cycles, which should prevent skewing
+       * in the scheduling of the frame clock.
+       *
+       * Once we do receive "frame drawn" events, smooth_cycle_start will track the vsync, and do so in a more stable
+       * way compared to frame_time. If we then no longer receive "frame drawn" events, smooth_cycle_start will again be
+       * simply advanced in increments of the refresh interval, but this time we are in sync with the vsync. If we start
+       * receiving "frame drawn" events shortly after loosing them, then we should still be in sync.
+       */
+      gint64 smooth_cycle_start = priv->smoothed_frame_time_base - priv->smoothed_frame_time_phase;
+      priv->min_next_frame_time = smooth_cycle_start + priv->smoothed_frame_time_period;
+
       maybe_start_idle (clock_idle, FALSE);
     }