rdp: Calculate frame times from mode refresh rate

Instead of hard coding a 16ms refresh interval, use the refresh rate
from the current mode to determine when the next frame should be.

Currently, we still hard code the refresh rate, so this will end up
with roughly the same value we've been using, but in the future
we'll allow setting it via command line.

Co-authored-by: Steve Pronovost <spronovo@microsoft.com>
Co-authored-by: Brenton DeGeer <brdegeer@microsoft.com>
Signed-off-by: Hideyuki Nagase <hideyukn@microsoft.com>
Signed-off-by: Steve Pronovost <spronovo@microsoft.com>
Signed-off-by: Brenton DeGeer <brdegeer@microsoft.com>

libweston/backend-rdp/rdp.c

@@ -243,6 +243,24 @@ rdp_output_repaint(struct weston_output *output_base, pixman_region32_t *damage)
 	struct rdp_output *output = container_of(output_base, struct rdp_output, base);
 	struct weston_compositor *ec = output->base.compositor;
 	struct rdp_peers_item *outputPeer;
+	struct timespec now, target;
+	int refresh_nsec = millihz_to_nsec(output_base->current_mode->refresh);
+	int refresh_msec = refresh_nsec / 1000000;
+	int next_frame_delta;
+
+	/* Calculate the time we should complete this frame such that frames
+	   are spaced out by the specified monitor refresh. Note that our timer
+	   mechanism only has msec precision, so we won't exactly hit our
+	   target refresh rate.
+	 */
+	weston_compositor_read_presentation_clock(ec, &now);
+
+	timespec_add_nsec(&target, &output_base->frame_time, refresh_nsec);
+
+	next_frame_delta = (int)timespec_sub_to_msec(&target, &now);
+	if (next_frame_delta < 1 || next_frame_delta > refresh_msec) {
+		next_frame_delta = refresh_msec;
+	}
 
 	pixman_renderer_output_set_buffer(output_base, output->shadow_surface);
 	ec->renderer->repaint_output(&output->base, damage);
@@ -260,7 +278,7 @@ rdp_output_repaint(struct weston_output *output_base, pixman_region32_t *damage)
 	pixman_region32_subtract(&ec->primary_plane.damage,
 				 &ec->primary_plane.damage, damage);
 
-	wl_event_source_timer_update(output->finish_frame_timer, 16);
+	wl_event_source_timer_update(output->finish_frame_timer, next_frame_delta);
 	return 0;
 }