[PATCH 2/6] compositor-drm: Allow instant start of repaint loop.

[Mario Kleiner]

drm_output_start_repaint_loop() incurred a delay of
one refresh cycle by using a no-op page-flip to get
an accurate vblank timestamp as reference. This causes
unwanted lag whenever Weston exited its repaint loop, e.g.,
whenever an application wants to repaint with less than
full video refresh rate but still minimum lag.

Try to use the drmWaitVblank ioctl to get a proper
timestamp instantaneously without lag. If that does
not work, fall back to the old method of idle page-flip.

This optimization should work on any drm/kms driver
which supports high precision vblank timestamping.
As of Linux 4.0 these would be intel, radeon and
nouveau on all supported gpu's.

Signed-off-by: Mario Kleiner <mario.kleiner.de at gmail.com>
---
 src/compositor-drm.c | 41 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

diff --git a/src/compositor-drm.c b/src/compositor-drm.c
index fe59811..4a7baa1 100644
--- a/src/compositor-drm.c
+++ b/src/compositor-drm.c
@@ -225,6 +225,9 @@ static const char default_seat[] = "seat0";
 static void
 drm_output_set_cursor(struct drm_output *output);
 
+static void
+drm_output_update_msc(struct drm_output *output, unsigned int seq);
+
 static int
 drm_sprite_crtc_supported(struct drm_output *output, uint32_t supported)
 {
@@ -704,6 +707,12 @@ err_pageflip:
 	return -1;
 }
 
+static int64_t
+timespec_to_nsec(const struct timespec *a)
+{
+	return (int64_t)a->tv_sec * 1000000000000LL + a->tv_nsec;
+}
+
 static void
 drm_output_start_repaint_loop(struct weston_output *output_base)
 {
@@ -711,7 +720,13 @@ drm_output_start_repaint_loop(struct weston_output *output_base)
 	struct drm_compositor *compositor = (struct drm_compositor *)
 		output_base->compositor;
 	uint32_t fb_id;
-	struct timespec ts;
+	struct timespec ts, tnow;
+	int ret;
+	drmVBlank vbl = {
+		.request.type = DRM_VBLANK_RELATIVE,
+		.request.sequence = 0,
+		.request.signal = 0,
+	};
 
 	if (output->destroy_pending)
 		return;
@@ -721,6 +736,30 @@ drm_output_start_repaint_loop(struct weston_output *output_base)
 		goto finish_frame;
 	}
 
+	/* Try to get current msc and timestamp via instant query */
+	vbl.request.type |= drm_waitvblank_pipe(output);
+	ret = drmWaitVBlank(compositor->drm.fd, &vbl);
+
+	/* Error return or zero timestamp means failure to get valid timestamp */
+	if ((ret == 0) && (vbl.reply.tval_sec > 0)) {
+		ts.tv_sec = vbl.reply.tval_sec;
+		ts.tv_nsec = vbl.reply.tval_usec * 1000;
+
+		/* Valid timestamp for most recent vblank - not stale? Stale ts could
+		 * happen on Linux 3.17+, so make sure it is not older than 1 refresh
+		 * duration since now.
+		 */
+		weston_compositor_read_presentation_clock(&compositor->base, &tnow);
+		if (timespec_to_nsec(&tnow) - timespec_to_nsec(&ts) <
+			(1000000000000LL / output_base->current_mode->refresh)) {
+			drm_output_update_msc(output, vbl.reply.sequence);
+			weston_output_finish_frame(output_base, &ts,
+									   PRESENTATION_FEEDBACK_INVALID);
+			return;
+		}
+	}
+
+	/* Immediate query didn't provide valid timestamp. Use pageflip fallback */
 	fb_id = output->current->fb_id;
 
 	if (drmModePageFlip(compositor->drm.fd, output->crtc_id, fb_id,
-- 
1.9.1