From: Daniel Vetter <daniel@ffwll.ch> To: Ville Syrjala <ville.syrjala@linux.intel.com> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>, intel-gfx@lists.freedesktop.org, Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>, dri-devel@lists.freedesktop.org, Rodrigo Vivi <rodrigo.vivi@intel.com> Subject: Re: [PATCH] drm/vblank: Avoid storing a timestamp for the same frame twice Date: Thu, 4 Feb 2021 16:32:16 +0100 [thread overview] Message-ID: <YBwTgHwZwMr8PwMr@phenom.ffwll.local> (raw) In-Reply-To: <20210204020400.29628-1-ville.syrjala@linux.intel.com> On Thu, Feb 04, 2021 at 04:04:00AM +0200, Ville Syrjala wrote: > From: Ville Syrjälä <ville.syrjala@linux.intel.com> > > 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 unless we're > also incrementing the sequence counter. We do still want to update > vblank->last with the freshly sampled hw frame counter value so > that subsequent vblank irqs/queries can actually use the hw frame > counter to determine how many frames have elapsed. Hm I'm not getting the reason for why we store the updated hw vblank counter? There's definitely a race when we grab the hw timestamp at a bad time (which can't happen for the irq handler, realistically), so maybe we should first adjust that to make sure we never store anything inconsistent in the vblank state? And when we have that we should be able to pull the inc == 0 check out into _restore(), including comment. Which I think should be cleaner. Or I'm totally off with why you want to store the hw vblank counter? > > Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com> > Cc: Rodrigo Vivi <rodrigo.vivi@intel.com> > Cc: Daniel Vetter <daniel.vetter@ffwll.ch> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com> > --- > drivers/gpu/drm/drm_vblank.c | 11 +++++++++++ > 1 file changed, 11 insertions(+) > > diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c > index 893165eeddf3..e127a7db2088 100644 > --- a/drivers/gpu/drm/drm_vblank.c > +++ b/drivers/gpu/drm/drm_vblank.c > @@ -176,6 +176,17 @@ static void store_vblank(struct drm_device *dev, unsigned int pipe, > > vblank->last = last; > > + /* > + * drm_vblank_restore() wants to always update > + * vblank->last since we can't trust the frame counter > + * across power saving states. But we don't want to alter > + * the stored timestamp for the same frame number since > + * that would cause userspace to potentially observe two > + * different timestamps for the same frame. > + */ > + if (vblank_count_inc == 0) > + return; > + > write_seqlock(&vblank->seqlock); > vblank->time = t_vblank; > atomic64_add(vblank_count_inc, &vblank->count); > -- > 2.26.2 > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel
WARNING: multiple messages have this Message-ID (diff)
From: Daniel Vetter <daniel@ffwll.ch> To: Ville Syrjala <ville.syrjala@linux.intel.com> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>, intel-gfx@lists.freedesktop.org, Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>, dri-devel@lists.freedesktop.org Subject: Re: [Intel-gfx] [PATCH] drm/vblank: Avoid storing a timestamp for the same frame twice Date: Thu, 4 Feb 2021 16:32:16 +0100 [thread overview] Message-ID: <YBwTgHwZwMr8PwMr@phenom.ffwll.local> (raw) In-Reply-To: <20210204020400.29628-1-ville.syrjala@linux.intel.com> On Thu, Feb 04, 2021 at 04:04:00AM +0200, Ville Syrjala wrote: > From: Ville Syrjälä <ville.syrjala@linux.intel.com> > > 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 unless we're > also incrementing the sequence counter. We do still want to update > vblank->last with the freshly sampled hw frame counter value so > that subsequent vblank irqs/queries can actually use the hw frame > counter to determine how many frames have elapsed. Hm I'm not getting the reason for why we store the updated hw vblank counter? There's definitely a race when we grab the hw timestamp at a bad time (which can't happen for the irq handler, realistically), so maybe we should first adjust that to make sure we never store anything inconsistent in the vblank state? And when we have that we should be able to pull the inc == 0 check out into _restore(), including comment. Which I think should be cleaner. Or I'm totally off with why you want to store the hw vblank counter? > > Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com> > Cc: Rodrigo Vivi <rodrigo.vivi@intel.com> > Cc: Daniel Vetter <daniel.vetter@ffwll.ch> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com> > --- > drivers/gpu/drm/drm_vblank.c | 11 +++++++++++ > 1 file changed, 11 insertions(+) > > diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c > index 893165eeddf3..e127a7db2088 100644 > --- a/drivers/gpu/drm/drm_vblank.c > +++ b/drivers/gpu/drm/drm_vblank.c > @@ -176,6 +176,17 @@ static void store_vblank(struct drm_device *dev, unsigned int pipe, > > vblank->last = last; > > + /* > + * drm_vblank_restore() wants to always update > + * vblank->last since we can't trust the frame counter > + * across power saving states. But we don't want to alter > + * the stored timestamp for the same frame number since > + * that would cause userspace to potentially observe two > + * different timestamps for the same frame. > + */ > + if (vblank_count_inc == 0) > + return; > + > write_seqlock(&vblank->seqlock); > vblank->time = t_vblank; > atomic64_add(vblank_count_inc, &vblank->count); > -- > 2.26.2 > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ Intel-gfx mailing list Intel-gfx@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/intel-gfx
next prev parent reply other threads:[~2021-02-04 15:32 UTC|newest] Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-02-04 2:04 [PATCH] drm/vblank: Avoid storing a timestamp for the same frame twice Ville Syrjala 2021-02-04 2:04 ` [Intel-gfx] " Ville Syrjala 2021-02-04 3:12 ` [Intel-gfx] ✓ Fi.CI.BAT: success for " Patchwork 2021-02-04 5:44 ` [Intel-gfx] ✗ Fi.CI.IGT: failure " Patchwork 2021-02-04 15:32 ` Daniel Vetter [this message] 2021-02-04 15:32 ` [Intel-gfx] [PATCH] " Daniel Vetter 2021-02-04 15:55 ` Ville Syrjälä 2021-02-04 15:55 ` [Intel-gfx] " Ville Syrjälä 2021-02-05 15:46 ` Daniel Vetter 2021-02-05 15:46 ` [Intel-gfx] " Daniel Vetter 2021-02-05 16:24 ` Ville Syrjälä 2021-02-05 16:24 ` [Intel-gfx] " Ville Syrjälä 2021-02-05 21:19 ` Ville Syrjälä 2021-02-05 21:19 ` [Intel-gfx] " Ville Syrjälä 2021-02-08 9:56 ` Daniel Vetter 2021-02-08 9:56 ` [Intel-gfx] " Daniel Vetter 2021-02-08 16:58 ` Ville Syrjälä 2021-02-08 16:58 ` [Intel-gfx] " Ville Syrjälä 2021-02-08 17:43 ` Daniel Vetter 2021-02-08 17:43 ` [Intel-gfx] " Daniel Vetter 2021-02-08 18:05 ` Ville Syrjälä 2021-02-08 18:05 ` [Intel-gfx] " Ville Syrjälä 2021-02-09 10:07 ` Daniel Vetter 2021-02-09 10:07 ` [Intel-gfx] " Daniel Vetter 2021-02-09 15:40 ` Ville Syrjälä 2021-02-09 15:40 ` [Intel-gfx] " Ville Syrjälä 2021-02-09 16:44 ` Daniel Vetter 2021-02-09 16:44 ` [Intel-gfx] " Daniel Vetter 2021-02-18 16:03 ` [PATCH v2] drm/vblank: Do not store a new vblank timestamp in drm_vblank_restore() Ville Syrjala 2021-02-18 16:03 ` [Intel-gfx] " Ville Syrjala 2021-02-18 16:10 ` Ville Syrjälä 2021-02-18 16:10 ` [Intel-gfx] " Ville Syrjälä 2021-02-19 15:08 ` Daniel Vetter 2021-02-19 15:08 ` [Intel-gfx] " Daniel Vetter 2021-02-19 15:47 ` Ville Syrjälä 2021-02-19 15:47 ` [Intel-gfx] " Ville Syrjälä 2021-02-18 19:08 ` [Intel-gfx] ✗ Fi.CI.BAT: failure for drm/vblank: Avoid storing a timestamp for the same frame twice (rev2) Patchwork 2021-02-18 19:22 ` Ville Syrjälä 2021-02-18 19:51 ` Vudum, Lakshminarayana 2021-02-18 19:29 ` [Intel-gfx] ✓ Fi.CI.BAT: success " Patchwork 2021-02-18 20:58 ` [Intel-gfx] ✗ Fi.CI.IGT: failure " Patchwork 2021-02-21 4:18 ` [Intel-gfx] ✓ Fi.CI.BAT: success for drm/vblank: Avoid storing a timestamp for the same frame twice (rev3) Patchwork 2021-02-21 5:41 ` [Intel-gfx] ✓ Fi.CI.IGT: " Patchwork
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