On Thu, Feb 18, 2021 at 06:03:05PM +0200, Ville Syrjala wrote: > From: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> > > drm_vblank_restore() exists because certain power saving states > can clobber the hardware frame counter. The way it does this is > by guesstimating how many frames were missed purely based on > the difference between the last stored timestamp vs. a newly > sampled timestamp. > > If we should call this function before a full frame has > elapsed since we sampled the last timestamp we would end up > with a possibly slightly different timestamp value for the > same frame. Currently we will happily overwrite the already > stored timestamp for the frame with the new value. This > could cause userspace to observe two different timestamps > for the same frame (and the timestamp could even go > backwards depending on how much error we introduce when > correcting the timestamp based on the scanout position). > > To avoid that let's not update the stored timestamp at all, > and instead we just fix up the last recorded hw vblank counter > value such that the already stored timestamp/seq number will > match. Thus the next time a vblank irq happens it will calculate > the correct diff between the current and stored hw vblank counter > values. > > Sidenote: Another possible idea that came to mind would be to > do this correction only if the power really was removed since > the last time we sampled the hw frame counter. But to do that > we would need a robust way to detect when it has occurred. Some > possibilities could involve some kind of hardare power well > transition counter, or potentially we could store a magic value > in a scratch register that lives in the same power well. But > I'm not sure either of those exist, so would need an actual > investigation to find out. All of that is very hardware specific > of course, so would have to be done in the driver code. Forgot to mention that I wasn't able to test this with PSR since HSW+PSR1 is bork, but I did test it a bit w/o PSR by artificially adding arbitrary offsets to the reported hw frame counter value. The behaviour seemed sane enough at least. > > Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@xxxxxxxxx> > Cc: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx> > Cc: Daniel Vetter <daniel.vetter@xxxxxxxx> > Signed-off-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> > --- > drivers/gpu/drm/drm_vblank.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c > index 2bd989688eae..3417e1ac7918 100644 > --- a/drivers/gpu/drm/drm_vblank.c > +++ b/drivers/gpu/drm/drm_vblank.c > @@ -1478,6 +1478,7 @@ static void drm_vblank_restore(struct drm_device *dev, unsigned int pipe) > u64 diff_ns; > u32 cur_vblank, diff = 1; > int count = DRM_TIMESTAMP_MAXRETRIES; > + u32 max_vblank_count = drm_max_vblank_count(dev, pipe); > > if (drm_WARN_ON(dev, pipe >= dev->num_crtcs)) > return; > @@ -1504,7 +1505,7 @@ static void drm_vblank_restore(struct drm_device *dev, unsigned int pipe) > drm_dbg_vbl(dev, > "missed %d vblanks in %lld ns, frame duration=%d ns, hw_diff=%d\n", > diff, diff_ns, framedur_ns, cur_vblank - vblank->last); > - store_vblank(dev, pipe, diff, t_vblank, cur_vblank); > + vblank->last = (cur_vblank - diff) & max_vblank_count; > } > > /** > -- > 2.26.2 -- Ville Syrjälä Intel _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
