Don't delay initial output paint

On startup, we cannot lock on to the repaint timer because it is unknown
to us. We deal with this by claiming that the moment of entry into the
repaint loop is the moment a frame returned, causing finish_frame to
delay our initial repaint to (refresh_time - repaint_delay), typically
around 9ms of utterly wasted time.

Add an explicit stamp == NULL, to determine that we are just beginning
our repaint loop, that the timings are in fact totally invalid, and that
it would be beneficial to repaint the output immediately. This will only
trigger when the display had previously been disabled or the previous
state is unknown, e.g. at startup, or coming back from DPMS off.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

libweston/compositor-drm.c

@@ -932,8 +932,7 @@ drm_output_start_repaint_loop(struct weston_output *output_base)
 
 finish_frame:
 	/* if we cannot page-flip, immediately finish frame */
-	weston_compositor_read_presentation_clock(output_base->compositor, &ts);
-	weston_output_finish_frame(output_base, &ts,
+	weston_output_finish_frame(output_base, NULL,
 				   WP_PRESENTATION_FEEDBACK_INVALID);
 }
 

libweston/compositor.c

@@ -2383,6 +2383,14 @@ weston_output_finish_frame(struct weston_output *output,
 	TL_POINT("core_repaint_finished", TLP_OUTPUT(output),
 		 TLP_VBLANK(stamp), TLP_END);
 
+	assert(stamp || (presented_flags & WP_PRESENTATION_FEEDBACK_INVALID));
+
+	/* If we haven't been supplied any timestamp at all, we don't have a
+	 * timebase to work against, so any delay just wastes time. Push a
+	 * repaint as soon as possible so we can get on with it. */
+	if (!stamp)
+		goto out;
+
 	refresh_nsec = millihz_to_nsec(output->current_mode->refresh);
 	weston_presentation_feedback_present_list(&output->feedback_list,
 						  output, refresh_nsec, stamp,
@@ -2415,6 +2423,7 @@ weston_output_finish_frame(struct weston_output *output,
 	if (presented_flags == WP_PRESENTATION_FEEDBACK_INVALID && msec_rel < 0)
 		msec_rel += refresh_nsec / 1000000;
 
+out:
 	if (msec_rel < 1)
 		output_repaint_timer_handler(output);
 	else