wayland: when possible, fill in actual and predicted presentation times

There is currently no Wayland protocol for providing presentation
timestamps or hints about when drawing will be presented onscreen.
However, by assuming the straightforward algorithm used by the
DRM backend to Weston, we can reverse engineer the right values.

https://bugzilla.gnome.org/show_bug.cgi?id=698864

gdk/wayland/gdkwindow-wayland.c

@@ -263,6 +263,50 @@ get_default_title (void)
   return title;
 }
 
+static void
+fill_presentation_time_from_frame_time (GdkFrameTimings *timings,
+                                        guint32          frame_time)
+{
+  /* The timestamp in a wayland frame is a msec time value that in some
+   * way reflects the time at which the server started drawing the frame.
+   * This is not useful from our perspective.
+   *
+   * However, for the DRM backend of Weston, on reasonably recent
+   * Linux, we know that the time is the
+   * clock_gettime(CLOCK_MONOTONIC) value at the vblank, and that
+   * backend starts drawing immediately after receiving the vblank
+   * notification. If we detect this, and make the assumption that the
+   * compositor will finish drawing before the next vblank, we can
+   * then determine the presentation time as the frame time we
+   * recieved plus one refresh interval.
+   *
+   * If a backend is using clock_gettime(CLOCK_MONOTONIC), but not
+   * picking values right at the vblank, then the presentation times
+   * we compute won't be accurate, but not really worse than then
+   * the alternative of not providing presentation times at all.
+   *
+   * The complexity here is dealing with the fact that we receive
+   * only the low 32 bits of the CLOCK_MONOTONIC value in milliseconds.
+   */
+  gint64 now_monotonic = g_get_monotonic_time ();
+  gint64 now_monotonic_msec = now_monotonic / 1000;
+  uint32_t now_monotonic_low = (uint32_t)now_monotonic_msec;
+
+  if (frame_time - now_monotonic_low < 1000 ||
+      frame_time - now_monotonic_low > (uint32_t)-1000)
+    {
+      /* Timestamp we received is within one second of the current time.
+       */
+      gint64 last_frame_time = now_monotonic + (gint64)1000 * (gint32)(frame_time - now_monotonic_low);
+      if ((gint32)now_monotonic_low < 0 && (gint32)frame_time > 0)
+        last_frame_time += (gint64)1000 * G_GINT64_CONSTANT(0x100000000);
+      else if ((gint32)now_monotonic_low > 0 && (gint32)frame_time < 0)
+        last_frame_time -= (gint64)1000 * G_GINT64_CONSTANT(0x100000000);
+
+      timings->presentation_time = last_frame_time + timings->refresh_interval;
+    }
+}
+
 static void
 frame_callback (void               *data,
                 struct wl_callback *callback,
@@ -294,6 +338,8 @@ frame_callback (void               *data,
         timings->refresh_interval = G_GINT64_CONSTANT(1000000000) / refresh_rate;
     }
 
+  fill_presentation_time_from_frame_time (timings, time);
+
   timings->complete = TRUE;
 
 #ifdef G_ENABLE_DEBUG
@@ -310,6 +356,30 @@ static void
 on_frame_clock_before_paint (GdkFrameClock *clock,
                              GdkWindow     *window)
 {
+  GdkFrameTimings *timings = gdk_frame_clock_get_current_timings (clock);
+  gint64 presentation_time;
+  gint64 refresh_interval;
+
+  gdk_frame_clock_get_refresh_info (clock,
+                                    timings->frame_time,
+                                    &refresh_interval, &presentation_time);
+
+  if (presentation_time != 0)
+    {
+      /* Assume the algorithm used by the DRM backend of Weston - it
+       * starts drawing at the next vblank after receiving the commit
+       * for this frame, and presentation occurs at the vblank
+       * after that.
+       */
+      timings->predicted_presentation_time = presentation_time + refresh_interval;
+    }
+  else
+    {
+      /* As above, but we don't actually know the phase of the vblank,
+       * so just assume that we're half way through a refresh cycle.
+       */
+      timings->predicted_presentation_time = timings->frame_time + refresh_interval / 2 + refresh_interval;
+    }
 }
 
 static void