compositor: add repaint delay timer

This timer delays the output_repaint towards the end of the refresh
period, reducing the time from repaint to present.

The length of the repaint window can be set in weston.ini.

The call to weston_output_schedule_repaint_reset() is delayed by one
more period.  If we exit the continuous repaint loop (set
output->repaint_scheduled to false) in finish_frame, we may call
start_repaint_loop() unnecessarily.  The problem case was actually
observed with two outputs on the DRM backend at 60 Hz, and 7 ms
repaint-window. During a window move, one output was constantly falling
off the continuous repaint loop and introducing additional one frame
latency, leading to jerky window motion. This code now avoids the
problem.

Changes in v2:

- Rename repaint_delay_timer to repaint_timer and
output_repaint_delay_handler to output_repaint_timer_handler.

- When computing the delay, take the current time into account. The timer
uses a relative timeout, so we have to subtract any time already gone.

Note, that 'gone' may also be negative. DRM has a habit of predicting
the page flip timestamp so it may be still in the future when we get the
completion event.

- Do also a sanity check 'msec > 1000'. In the unlikely case that
something fails to provide a good timestamp, never delay for more than
one second.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-By: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>

man/weston.ini.man

@@ -137,6 +137,16 @@ directory are:
 .fi
 .RE
 .TP 7
+.BI "repaint-window=" N
+Set the approximate length of the repaint window in milliseconds. The repaint
+window is used to control and reduce the output latency for clients. If the
+window is longer than the output refresh period, the repaint will be done
+immediately when the previous repaint finishes, not processing client requests
+in between. If the repaint window is too short, the compositor may miss the
+target vertical blank, increasing output latency. The default value is 7
+milliseconds. The allowed range is from -10 to 1000 milliseconds. Using a
+negative value will force the compositor to always miss the target vblank.
+.TP 7
 .BI "gbm-format="format
 sets the GBM format used for the framebuffer for the GBM backend. Can be
 .B xrgb8888,

src/compositor.c

@@ -62,6 +62,28 @@
 #include "git-version.h"
 #include "version.h"
 
+#define DEFAULT_REPAINT_WINDOW 7 /* milliseconds */
+
+#define NSEC_PER_SEC 1000000000
+
+static void
+timespec_sub(struct timespec *r,
+	     const struct timespec *a, const struct timespec *b)
+{
+	r->tv_sec = a->tv_sec - b->tv_sec;
+	r->tv_nsec = a->tv_nsec - b->tv_nsec;
+	if (r->tv_nsec < 0) {
+		r->tv_sec--;
+		r->tv_nsec += NSEC_PER_SEC;
+	}
+}
+
+static int64_t
+timespec_to_nsec(const struct timespec *a)
+{
+	return (int64_t)a->tv_sec * NSEC_PER_SEC + a->tv_nsec;
+}
+
 static struct wl_list child_process_list;
 static struct weston_compositor *segv_compositor;
 
@@ -2346,19 +2368,38 @@ weston_output_schedule_repaint_reset(struct weston_output *output)
 				     weston_compositor_read_input, compositor);
 }
 
+static int
+output_repaint_timer_handler(void *data)
+{
+	struct weston_output *output = data;
+	struct weston_compositor *compositor = output->compositor;
+
+	if (output->repaint_needed &&
+	    compositor->state != WESTON_COMPOSITOR_SLEEPING &&
+	    compositor->state != WESTON_COMPOSITOR_OFFSCREEN &&
+	    weston_output_repaint(output) == 0)
+		return 0;
+
+	weston_output_schedule_repaint_reset(output);
+
+	return 0;
+}
+
 WL_EXPORT void
 weston_output_finish_frame(struct weston_output *output,
 			   const struct timespec *stamp,
 			   uint32_t presented_flags)
 {
 	struct weston_compositor *compositor = output->compositor;
-	int r;
-	uint32_t refresh_nsec;
+	int32_t refresh_nsec;
+	struct timespec now;
+	struct timespec gone;
+	int msec;
 
 	TL_POINT("core_repaint_finished", TLP_OUTPUT(output),
 		 TLP_VBLANK(stamp), TLP_END);
 
-	refresh_nsec = 1000000000000UL / output->current_mode->refresh;
+	refresh_nsec = 1000000000000LL / output->current_mode->refresh;
 	weston_presentation_feedback_present_list(&output->feedback_list,
 						  output, refresh_nsec, stamp,
 						  output->msc,
@@ -2366,15 +2407,16 @@ weston_output_finish_frame(struct weston_output *output,
 
 	output->frame_time = stamp->tv_sec * 1000 + stamp->tv_nsec / 1000000;
 
-	if (output->repaint_needed &&
-	    compositor->state != WESTON_COMPOSITOR_SLEEPING &&
-	    compositor->state != WESTON_COMPOSITOR_OFFSCREEN) {
-		r = weston_output_repaint(output);
-		if (!r)
-			return;
-	}
+	weston_compositor_read_presentation_clock(compositor, &now);
+	timespec_sub(&gone, &now, stamp);
+	msec = (refresh_nsec - timespec_to_nsec(&gone)) / 1000000; /* floor */
+	msec -= compositor->repaint_msec;
 
-	weston_output_schedule_repaint_reset(output);
+	/* Also sanity check. */
+	if (msec < 1 || msec > 1000)
+		output_repaint_timer_handler(output);
+	else
+		wl_event_source_timer_update(output->repaint_timer, msec);
 }
 
 static void
@@ -3873,6 +3915,8 @@ weston_output_destroy(struct weston_output *output)
 
 	output->destroying = 1;
 
+	wl_event_source_remove(output->repaint_timer);
+
 	weston_presentation_feedback_discard_list(&output->feedback_list);
 
 	weston_compositor_remove_output(output->compositor, output);
@@ -4033,6 +4077,8 @@ weston_output_init(struct weston_output *output, struct weston_compositor *c,
 		   int x, int y, int mm_width, int mm_height, uint32_t transform,
 		   int32_t scale)
 {
+	struct wl_event_loop *loop;
+
 	output->compositor = c;
 	output->x = x;
 	output->y = y;
@@ -4053,6 +4099,10 @@ weston_output_init(struct weston_output *output, struct weston_compositor *c,
 	wl_list_init(&output->resource_list);
 	wl_list_init(&output->feedback_list);
 
+	loop = wl_display_get_event_loop(c->wl_display);
+	output->repaint_timer = wl_event_loop_add_timer(loop,
+					output_repaint_timer_handler, output);
+
 	output->id = ffs(~output->compositor->output_id_pool) - 1;
 	output->compositor->output_id_pool |= 1 << output->id;
 
@@ -4517,6 +4567,17 @@ weston_compositor_init(struct weston_compositor *ec,
 	weston_compositor_add_debug_binding(ec, KEY_T,
 					    timeline_key_binding_handler, ec);
 
+	s = weston_config_get_section(ec->config, "core", NULL, NULL);
+	weston_config_section_get_int(s, "repaint-window", &ec->repaint_msec,
+				      DEFAULT_REPAINT_WINDOW);
+	if (ec->repaint_msec < -10 || ec->repaint_msec > 1000) {
+		weston_log("Invalid repaint_window value in config: %d\n",
+			   ec->repaint_msec);
+		ec->repaint_msec = DEFAULT_REPAINT_WINDOW;
+	}
+	weston_log("Output repaint window is %d ms maximum.\n",
+		   ec->repaint_msec);
+
 	weston_compositor_schedule_repaint(ec);
 
 	return 0;

src/compositor.h

@@ -196,6 +196,7 @@ struct weston_output {
 	pixman_region32_t previous_damage;
 	int repaint_needed;
 	int repaint_scheduled;
+	struct wl_event_source *repaint_timer;
 	struct weston_output_zoom zoom;
 	int dirty;
 	struct wl_signal frame_signal;
@@ -683,6 +684,7 @@ struct weston_compositor {
 	int32_t kb_repeat_delay;
 
 	clockid_t presentation_clock;
+	int32_t repaint_msec;
 
 	int exit_code;
 };