On 04/02/2015 01:37 PM, Pekka Paalanen wrote:
On Thu, 2 Apr 2015 07:10:50 +0200
Mario Kleiner <[email protected]> wrote:
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 <[email protected]>
---
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;
Hi,
are you sure this cannot overflow? I think tv_sec could be a int64_t.
That no-one gets the idea of initializing the clock in the kernel to
some huge value just to fish out this kind of overflows?
I almost literally copied it from compositor.c, your repaint delay
handling. I think it can't overflow the way it is used here to only
remap from kernel timestamps. The kernel delivers its timestamps in
"struct timeval", so tv_sec (== long int) from kernel could be a 64-bit
int on 64-Bit kernels, but the values put into it are always derived
from ktime_t which is "nanoseconds stored in 64 bit signed integers". So
at least with the current kernel implementation it can't overflow. And
32-Bit kernels would hit such overflows before user space as their
struct timeval seems to have 32 bit int for tv_sec. So i think with the
current kernel ioctl interfaces it should be safe.
+}
+
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)) {
No need to check tval_usec?
I don't think it is necessary for this quick crude early-out check and
would complicate the check to be almost as expensive as the check it is
trying to skip.
On kernels <= Linux 3.16 a zero time value tval_sec=tval_usec=0 meant
"invalid timestamp, please retry again later". This would happen on kms
drivers which don't support high precision instant vblank timestamping
if vblank interrupts were disabled (currently all but intel, radeon and
nouveau, so essentially all SoC's), because the only way to get a valid
timestamp there is to wait for a vblank irq and collect the timestamp
there, and we didn't want the "non-blocking" bits of drmWaitVblank ioctl
to block for up to a whole video refresh cycle.
The tval_sec > 0 check detects these invalid timestamps and immediately
skips to the pageflip fallback below. It would also skip to fallback if
weston would somehow manage to run within the first second of kernel
boot, but that's very unlikely and all it would cause is 1 extra frame
of lag during that second.
The reason we need the check for stale timestamps at all is because we
accidentally removed that "timestamp 0 == invalid ts" signalling during
some improvements to the vblank handling in kernel 3.17 which were
needed for the atomic modesetting stuff. At the moment timestamps will
be always correct and instantaneous on intel/radeon/nouveau-kms, but the
kernel would deliver stale timestamps for <= 1 video refresh duration
after vblank irqs were turned off and on again on the various SoC
display drivers.
I'll try to prepare a kernel drm patch to fix that signalling, but now
we have to deal with at least some broken kernels anyway.
I think it would be a good idea to find ways to implement the
instantaneous high precision vblank timestamping also on more kms
drivers for SoC's, as it would be not only good for timing precision,
but also for lag reduction and power saving. For any SoC that allows to
query the current crtc scanout position via some register this could be
easily done using the same shared drm helper routines we already use for
intel/radeon/nouveau. Or if they had some hardware vblank timestamp
register that could be mapped to CLOCK_MONOTONIC time.
+ 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) <
I'd write this difference in terms of timespec first and convert to
nsec afterwards, but I can do that also as a follow-up when I happen to
visit this part the next time. I have quite some timespec helpers lying
around that I should collect into one place at some point.
If you wanted to introduce some proper timespec helpers anyway then i'd
leave it to you when you get around doing that. I went for the easy
route here because i didn't want to duplicate too much helper code from
other places like compositor.c or open-code this stuff just for one
single check.
+ (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,
At least
Acked-by: Pekka Paalanen <[email protected]>
for now.
thanks,
-mario
Thanks,
pq
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