[mutter] clutter/frame-clock: Evenly space updates when presentation times are zero

[Marge Bot <marge-bot src gnome org>]

commit 56fc09151d5da6ec2439c8844ec15499856f373e
Author: Daniel van Vugt <daniel van vugt canonical com>
Date:   Thu Feb 11 15:48:40 2021 +0800

    clutter/frame-clock: Evenly space updates when presentation times are zero
    
    This is for the Nvidia-X11 driver where `last_presentation_time_us` is
    always zero. Other drivers should be unaffected.
    
    The existing `calculate_next_update_time_us` algorithm only provides a
    guarantee of not scheduling faster than the refresh rate in the presence
    of a valid `last_presentation_time_us`. When `last_presentation_time_us`
    is zero there is no solid foundation to guarantee we're not occasionally
    scheduling too early. So introduce one now.
    
    By introducing a hard guarantee that updates are never scheduled faster
    than the refresh rate, we avoid keeping Nvidia's triple (or quad?) buffer
    queue full. So this avoids the high latency and random stalls experienced
    on Nvidia.
    
    Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/818,
            https://gitlab.gnome.org/GNOME/mutter/-/issues/1273,
            https://gitlab.gnome.org/GNOME/mutter/-/issues/1287,
            https://gitlab.gnome.org/GNOME/mutter/-/issues/1291,
            https://gitlab.gnome.org/GNOME/mutter/-/issues/1583
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1726>

 clutter/clutter/clutter-frame-clock.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)
---
diff --git a/clutter/clutter/clutter-frame-clock.c b/clutter/clutter/clutter-frame-clock.c
index bcc6bae6a0..05e79e6626 100644
--- a/clutter/clutter/clutter-frame-clock.c
+++ b/clutter/clutter/clutter-frame-clock.c
@@ -70,6 +70,7 @@ struct _ClutterFrameClock
   int64_t frame_count;
 
   ClutterFrameClockState state;
+  int64_t last_dispatch_time_us;
   int64_t last_presentation_time_us;
 
   gboolean is_next_presentation_time_valid;
@@ -240,6 +241,17 @@ calculate_next_update_time_us (ClutterFrameClock *frame_clock,
   refresh_rate = frame_clock->refresh_rate;
   refresh_interval_us = (int64_t) (0.5 + G_USEC_PER_SEC / refresh_rate);
 
+  if (frame_clock->last_presentation_time_us == 0)
+    {
+      *out_next_update_time_us =
+        frame_clock->last_dispatch_time_us ?
+        frame_clock->last_dispatch_time_us + refresh_interval_us :
+        now_us;
+
+      *out_next_presentation_time_us = 0;
+      return;
+    }
+
   min_render_time_allowed_us = refresh_interval_us / 2;
   max_render_time_allowed_us = refresh_interval_us - SYNC_DELAY_US;
 
@@ -444,7 +456,8 @@ clutter_frame_clock_schedule_update (ClutterFrameClock *frame_clock)
       calculate_next_update_time_us (frame_clock,
                                      &next_update_time_us,
                                      &frame_clock->next_presentation_time_us);
-      frame_clock->is_next_presentation_time_valid = TRUE;
+      frame_clock->is_next_presentation_time_valid =
+        (frame_clock->next_presentation_time_us != 0);
       break;
     case CLUTTER_FRAME_CLOCK_STATE_SCHEDULED:
       return;
@@ -469,6 +482,7 @@ clutter_frame_clock_dispatch (ClutterFrameClock *frame_clock,
 
   COGL_TRACE_BEGIN_SCOPED (ClutterFrameClockDispatch, "Frame Clock (dispatch)");
 
+  frame_clock->last_dispatch_time_us = time_us;
   g_source_set_ready_time (frame_clock->source, -1);
 
   frame_clock->state = CLUTTER_FRAME_CLOCK_STATE_DISPATCHING;