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.
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);
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
On Thu, Feb 04, 2021 at 04:32:16PM +0100, Daniel Vetter wrote:
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?
Because next time a vblank irq happens the code will do: diff = current_hw_counter - vblank->last
which won't work very well if vblank->last is garbage.
Updating vblank->last is pretty much why drm_vblank_restore() exists at all.
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?
Not sure what race you mean, or what inconsistent thing we store?
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
On Thu, Feb 04, 2021 at 05:55:28PM +0200, Ville Syrjälä wrote:
On Thu, Feb 04, 2021 at 04:32:16PM +0100, Daniel Vetter wrote:
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?
Because next time a vblank irq happens the code will do: diff = current_hw_counter - vblank->last
which won't work very well if vblank->last is garbage.
Updating vblank->last is pretty much why drm_vblank_restore() exists at all.
Oh sure, _restore has to update this, together with the timestamp.
But your code adds such an update where we update the hw vblank counter, but not the timestamp, and that feels buggy. Either we're still in the same frame, and then we should story nothing. Or we advanced, and then we probably want a new timestampt for that frame too.
Advancing the vblank counter and not advancing the timestamp sounds like a bug in our code.
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?
Not sure what race you mean, or what inconsistent thing we store?
For the drm_handle_vblank code we have some fudge so we don't compute something silly when the irq fires (like it often does) before top-of-frame. Ofc that fudge is inheritedly racy, if the irq is extremely delay (almost an entire frame) we'll get it wrong.
In practice it doesn't matter.
Now _restore can be called anytime, so we might end up in situations where the exact point where we jump to the next frame count, and the exact time where the hw counter jumps, don't lign up. And I think in that case funny things can happen, and I'm not sure your approach of "update hw counter but don't update timestamp" is the right way.
I think if we instead ignore any update if our fudge-corrected timestamp is roughly the same, then we handle that race correctly and there's no jumping around.
Cheers, Daniel
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
-- Ville Syrjälä Intel
On Fri, Feb 05, 2021 at 04:46:27PM +0100, Daniel Vetter wrote:
On Thu, Feb 04, 2021 at 05:55:28PM +0200, Ville Syrjälä wrote:
On Thu, Feb 04, 2021 at 04:32:16PM +0100, Daniel Vetter wrote:
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?
Because next time a vblank irq happens the code will do: diff = current_hw_counter - vblank->last
which won't work very well if vblank->last is garbage.
Updating vblank->last is pretty much why drm_vblank_restore() exists at all.
Oh sure, _restore has to update this, together with the timestamp.
But your code adds such an update where we update the hw vblank counter, but not the timestamp, and that feels buggy. Either we're still in the same frame, and then we should story nothing. Or we advanced, and then we probably want a new timestampt for that frame too.
Even if we're still in the same frame the hw frame counter may already have been reset due to the power well having been turned off. That is what I'm trying to fix here.
Now I suppose that's fairly unlikely, at least with PSR which probably does impose some extra delays before the power gets yanked. But at least theoretically possible.
Advancing the vblank counter and not advancing the timestamp sounds like a bug in our code.
We're not advancing the vblank counter. We're storing a new timestamp for a vblank counter value which already had a timestamp.
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?
Not sure what race you mean, or what inconsistent thing we store?
For the drm_handle_vblank code we have some fudge so we don't compute something silly when the irq fires (like it often does) before top-of-frame. Ofc that fudge is inheritedly racy, if the irq is extremely delay (almost an entire frame) we'll get it wrong.
Sorry, still no idea what fudge you mean.
In practice it doesn't matter.
Now _restore can be called anytime, so we might end up in situations where the exact point where we jump to the next frame count, and the exact time where the hw counter jumps, don't lign up. And I think in that case funny things can happen, and I'm not sure your approach of "update hw counter but don't update timestamp" is the right way.
I think if we instead ignore any update if our fudge-corrected timestamp is roughly the same, then we handle that race correctly and there's no jumping around.
We can't just not update vblank->last, assuming the theory holds that the power well may turn off even if the last vblank timestamp was sampled less than a full frame ago.
That will cause the next diff=current_hw_counter-vblank->last to generate total garbage and then the vblank seq number will jump to some random value. Which is exactly the main problem drm_vblank_restore() is trying to prevent.
On Fri, Feb 05, 2021 at 06:24:08PM +0200, Ville Syrjälä wrote:
On Fri, Feb 05, 2021 at 04:46:27PM +0100, Daniel Vetter wrote:
On Thu, Feb 04, 2021 at 05:55:28PM +0200, Ville Syrjälä wrote:
On Thu, Feb 04, 2021 at 04:32:16PM +0100, Daniel Vetter wrote:
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?
Because next time a vblank irq happens the code will do: diff = current_hw_counter - vblank->last
which won't work very well if vblank->last is garbage.
Updating vblank->last is pretty much why drm_vblank_restore() exists at all.
Oh sure, _restore has to update this, together with the timestamp.
But your code adds such an update where we update the hw vblank counter, but not the timestamp, and that feels buggy. Either we're still in the same frame, and then we should story nothing. Or we advanced, and then we probably want a new timestampt for that frame too.
Even if we're still in the same frame the hw frame counter may already have been reset due to the power well having been turned off. That is what I'm trying to fix here.
Now I suppose that's fairly unlikely, at least with PSR which probably does impose some extra delays before the power gets yanked. But at least theoretically possible.
Pondering about this a bit further. I think the fact that the current code takes the round-to-closest approach I used for the vblank handler is perhaps a bit bad. It could push the seq counter forward if we're past the halfway point of a frame. I think that rounding behaviour makes sense for the irq since those tick steadily and so allowing a bit of error either way seems correct to me. Perhaps round-down might be the better option for _restore(). Not quites sure, need more thinking probably.
Another idea that came to me now is that maybe we should actually just check if the current hw frame counter value looks sane, as in something like:
diff_hw_counter = current_hw_counter-stored_hw_counter diff_ts = (current_ts-stored_ts)/framedur
if (diff_hw_counter ~= diff_ts) diff = diff_hw_counter; else diff = diff_ts;
and if they seem to match then just keep trusting the hw counter. So only if there's a significant difference would we disregard the diff of the hw counter and instead use the diff based on the timestamps. Not sure what "significant" is though; One frame, two frames?
On Fri, Feb 05, 2021 at 11:19:19PM +0200, Ville Syrjälä wrote:
On Fri, Feb 05, 2021 at 06:24:08PM +0200, Ville Syrjälä wrote:
On Fri, Feb 05, 2021 at 04:46:27PM +0100, Daniel Vetter wrote:
On Thu, Feb 04, 2021 at 05:55:28PM +0200, Ville Syrjälä wrote:
On Thu, Feb 04, 2021 at 04:32:16PM +0100, Daniel Vetter wrote:
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?
Because next time a vblank irq happens the code will do: diff = current_hw_counter - vblank->last
which won't work very well if vblank->last is garbage.
Updating vblank->last is pretty much why drm_vblank_restore() exists at all.
Oh sure, _restore has to update this, together with the timestamp.
But your code adds such an update where we update the hw vblank counter, but not the timestamp, and that feels buggy. Either we're still in the same frame, and then we should story nothing. Or we advanced, and then we probably want a new timestampt for that frame too.
Even if we're still in the same frame the hw frame counter may already have been reset due to the power well having been turned off. That is what I'm trying to fix here.
Now I suppose that's fairly unlikely, at least with PSR which probably does impose some extra delays before the power gets yanked. But at least theoretically possible.
Pondering about this a bit further. I think the fact that the current code takes the round-to-closest approach I used for the vblank handler is perhaps a bit bad. It could push the seq counter forward if we're past the halfway point of a frame. I think that rounding behaviour makes sense for the irq since those tick steadily and so allowing a bit of error either way seems correct to me. Perhaps round-down might be the better option for _restore(). Not quites sure, need more thinking probably.
Yes this is the rounding I'm worried about.
But your point above that the hw might reset the counter again is also valid. I'm assuming what you're worried about is that we first do a _restore (and the hw vblank counter hasn't been trashed yet), and then in the same frame we do another restore, but now the hw frame counter has been trashe, and we need to update it?
Another idea that came to me now is that maybe we should actually just check if the current hw frame counter value looks sane, as in something like:
diff_hw_counter = current_hw_counter-stored_hw_counter diff_ts = (current_ts-stored_ts)/framedur
if (diff_hw_counter ~= diff_ts) diff = diff_hw_counter; else diff = diff_ts;
and if they seem to match then just keep trusting the hw counter. So only if there's a significant difference would we disregard the diff of the hw counter and instead use the diff based on the timestamps. Not sure what "significant" is though; One frame, two frames?
Hm, another idea: The only point where we can trust the entire hw counter + timestamp sampling is when the irq happens. Because then we know the driver will have properly corrected for any hw oddities (like hw counter flipping not at top-of-frame, like the core expects).
So what if _restore always goes back to the last such trusted hw counter for computing the frame counter diff and all that stuff? That way if we have a bunch of _restore with incosisten hw vblank counter, we will a) only take the last one (fixes the bug you're trying to fix) b) still use the same last trusted baseline for computations (addresses the race I'm seeing).
Or does this not work?
It does complicate the code a bit, because we'd need to store the count/timestamp information from _restore outside of the usual vblank ts array. But I think that addresses everything. -Daniel
On Mon, Feb 08, 2021 at 10:56:36AM +0100, Daniel Vetter wrote:
On Fri, Feb 05, 2021 at 11:19:19PM +0200, Ville Syrjälä wrote:
On Fri, Feb 05, 2021 at 06:24:08PM +0200, Ville Syrjälä wrote:
On Fri, Feb 05, 2021 at 04:46:27PM +0100, Daniel Vetter wrote:
On Thu, Feb 04, 2021 at 05:55:28PM +0200, Ville Syrjälä wrote:
On Thu, Feb 04, 2021 at 04:32:16PM +0100, Daniel Vetter wrote:
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?
Because next time a vblank irq happens the code will do: diff = current_hw_counter - vblank->last
which won't work very well if vblank->last is garbage.
Updating vblank->last is pretty much why drm_vblank_restore() exists at all.
Oh sure, _restore has to update this, together with the timestamp.
But your code adds such an update where we update the hw vblank counter, but not the timestamp, and that feels buggy. Either we're still in the same frame, and then we should story nothing. Or we advanced, and then we probably want a new timestampt for that frame too.
Even if we're still in the same frame the hw frame counter may already have been reset due to the power well having been turned off. That is what I'm trying to fix here.
Now I suppose that's fairly unlikely, at least with PSR which probably does impose some extra delays before the power gets yanked. But at least theoretically possible.
Pondering about this a bit further. I think the fact that the current code takes the round-to-closest approach I used for the vblank handler is perhaps a bit bad. It could push the seq counter forward if we're past the halfway point of a frame. I think that rounding behaviour makes sense for the irq since those tick steadily and so allowing a bit of error either way seems correct to me. Perhaps round-down might be the better option for _restore(). Not quites sure, need more thinking probably.
Yes this is the rounding I'm worried about.
Actually I don't think this is really an issue since we are working with the corrected timestamps here. Those always line up with frames, so unless the correction is really buggy or the hw somehow skips a partial frame it should work rather well. At least when operating with small timescales. For large gaps the error might creep up, but I don't think a small error in the predicted seq number over a long timespan is really a problem.
But your point above that the hw might reset the counter again is also valid. I'm assuming what you're worried about is that we first do a _restore (and the hw vblank counter hasn't been trashed yet), and then in the same frame we do another restore, but now the hw frame counter has been trashe, and we need to update it?
Yeah, although the pre-trashing _restore could also just be a vblank irq I think.
Another idea that came to me now is that maybe we should actually just check if the current hw frame counter value looks sane, as in something like:
diff_hw_counter = current_hw_counter-stored_hw_counter diff_ts = (current_ts-stored_ts)/framedur
if (diff_hw_counter ~= diff_ts) diff = diff_hw_counter; else diff = diff_ts;
and if they seem to match then just keep trusting the hw counter. So only if there's a significant difference would we disregard the diff of the hw counter and instead use the diff based on the timestamps. Not sure what "significant" is though; One frame, two frames?
Hm, another idea: The only point where we can trust the entire hw counter
- timestamp sampling is when the irq happens. Because then we know the
driver will have properly corrected for any hw oddities (like hw counter flipping not at top-of-frame, like the core expects).
i915 at least gives out correct data regardless of when you sample it. Well, except for the cases where the hw counter gets trashed, in which case the hw counter is garbage (when compared with .last) but the timestamp is still correct.
So what if _restore always goes back to the last such trusted hw counter for computing the frame counter diff and all that stuff? That way if we have a bunch of _restore with incosisten hw vblank counter, we will a) only take the last one (fixes the bug you're trying to fix) b) still use the same last trusted baseline for computations (addresses the race I'm seeing).
Or does this not work?
I don't think I really understand what you're suggesting here. _restore is already using the last trusted data (the stored timestamp + .last).
So the one thing _restore will have to update is .last. I think it can either do what it does now and set .last to the current hw counter value + update the timestamp to match, or it could perhaps adjust the stored .last such that the already stored timestamp and the updated .last match up. But I think both of those options have the same level or inaccuracy since both would still do the same ts_diff->hw_counter_diff prediction.
It does complicate the code a bit, because we'd need to store the count/timestamp information from _restore outside of the usual vblank ts array. But I think that addresses everything.
Hmm. So restore would store this extra information somewhere else, and not update the normal stuff at all? What exactly would we do with that extra data?
On Mon, Feb 8, 2021 at 5:58 PM Ville Syrjälä ville.syrjala@linux.intel.com wrote:
On Mon, Feb 08, 2021 at 10:56:36AM +0100, Daniel Vetter wrote:
On Fri, Feb 05, 2021 at 11:19:19PM +0200, Ville Syrjälä wrote:
On Fri, Feb 05, 2021 at 06:24:08PM +0200, Ville Syrjälä wrote:
On Fri, Feb 05, 2021 at 04:46:27PM +0100, Daniel Vetter wrote:
On Thu, Feb 04, 2021 at 05:55:28PM +0200, Ville Syrjälä wrote:
On Thu, Feb 04, 2021 at 04:32:16PM +0100, Daniel Vetter wrote: > 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?
Because next time a vblank irq happens the code will do: diff = current_hw_counter - vblank->last
which won't work very well if vblank->last is garbage.
Updating vblank->last is pretty much why drm_vblank_restore() exists at all.
Oh sure, _restore has to update this, together with the timestamp.
But your code adds such an update where we update the hw vblank counter, but not the timestamp, and that feels buggy. Either we're still in the same frame, and then we should story nothing. Or we advanced, and then we probably want a new timestampt for that frame too.
Even if we're still in the same frame the hw frame counter may already have been reset due to the power well having been turned off. That is what I'm trying to fix here.
Now I suppose that's fairly unlikely, at least with PSR which probably does impose some extra delays before the power gets yanked. But at least theoretically possible.
Pondering about this a bit further. I think the fact that the current code takes the round-to-closest approach I used for the vblank handler is perhaps a bit bad. It could push the seq counter forward if we're past the halfway point of a frame. I think that rounding behaviour makes sense for the irq since those tick steadily and so allowing a bit of error either way seems correct to me. Perhaps round-down might be the better option for _restore(). Not quites sure, need more thinking probably.
Yes this is the rounding I'm worried about.
Actually I don't think this is really an issue since we are working with the corrected timestamps here. Those always line up with frames, so unless the correction is really buggy or the hw somehow skips a partial frame it should work rather well. At least when operating with small timescales. For large gaps the error might creep up, but I don't think a small error in the predicted seq number over a long timespan is really a problem.
That corrected timestamp is what can go wrong I think: There's no guarantee that drm_crtc_vblank_helper_get_vblank_timestamp_internal() flips to top-of-frame at the exact same time than when the hw vblank counter flips. Or at least I'm not seeing where we correct them both together.
But your point above that the hw might reset the counter again is also valid. I'm assuming what you're worried about is that we first do a _restore (and the hw vblank counter hasn't been trashed yet), and then in the same frame we do another restore, but now the hw frame counter has been trashe, and we need to update it?
Yeah, although the pre-trashing _restore could also just be a vblank irq I think.
Another idea that came to me now is that maybe we should actually just check if the current hw frame counter value looks sane, as in something like:
diff_hw_counter = current_hw_counter-stored_hw_counter diff_ts = (current_ts-stored_ts)/framedur
if (diff_hw_counter ~= diff_ts) diff = diff_hw_counter; else diff = diff_ts;
and if they seem to match then just keep trusting the hw counter. So only if there's a significant difference would we disregard the diff of the hw counter and instead use the diff based on the timestamps. Not sure what "significant" is though; One frame, two frames?
Hm, another idea: The only point where we can trust the entire hw counter
- timestamp sampling is when the irq happens. Because then we know the
driver will have properly corrected for any hw oddities (like hw counter flipping not at top-of-frame, like the core expects).
i915 at least gives out correct data regardless of when you sample it. Well, except for the cases where the hw counter gets trashed, in which case the hw counter is garbage (when compared with .last) but the timestamp is still correct.
Hm where/how do we handle this? Maybe I'm just out of date with how it all works nowadays.
So what if _restore always goes back to the last such trusted hw counter for computing the frame counter diff and all that stuff? That way if we have a bunch of _restore with incosisten hw vblank counter, we will a) only take the last one (fixes the bug you're trying to fix) b) still use the same last trusted baseline for computations (addresses the race I'm seeing).
Or does this not work?
I don't think I really understand what you're suggesting here. _restore is already using the last trusted data (the stored timestamp + .last).
So the one thing _restore will have to update is .last. I think it can either do what it does now and set .last to the current hw counter value + update the timestamp to match, or it could perhaps adjust the stored .last such that the already stored timestamp and the updated .last match up. But I think both of those options have the same level or inaccuracy since both would still do the same ts_diff->hw_counter_diff prediction.
It does complicate the code a bit, because we'd need to store the count/timestamp information from _restore outside of the usual vblank ts array. But I think that addresses everything.
Hmm. So restore would store this extra information somewhere else, and not update the normal stuff at all? What exactly would we do with that extra data?
Hm I guess I didn't think this through. But the idea I had was: - _restore always recomputes back from the las drm_crtc_handl_vblank-stored timestamp. - the first drm_crtc_handle_vblank bakes in any corrections that _restore has prepared meanwhile - same applies to all the sampling functions we might look at lastes timestamps/counter values. -Daniel
On Mon, Feb 08, 2021 at 06:43:53PM +0100, Daniel Vetter wrote:
On Mon, Feb 8, 2021 at 5:58 PM Ville Syrjälä ville.syrjala@linux.intel.com wrote:
On Mon, Feb 08, 2021 at 10:56:36AM +0100, Daniel Vetter wrote:
On Fri, Feb 05, 2021 at 11:19:19PM +0200, Ville Syrjälä wrote:
On Fri, Feb 05, 2021 at 06:24:08PM +0200, Ville Syrjälä wrote:
On Fri, Feb 05, 2021 at 04:46:27PM +0100, Daniel Vetter wrote:
On Thu, Feb 04, 2021 at 05:55:28PM +0200, Ville Syrjälä wrote: > On Thu, Feb 04, 2021 at 04:32:16PM +0100, Daniel Vetter wrote: > > 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? > > Because next time a vblank irq happens the code will do: > diff = current_hw_counter - vblank->last > > which won't work very well if vblank->last is garbage. > > Updating vblank->last is pretty much why drm_vblank_restore() > exists at all.
Oh sure, _restore has to update this, together with the timestamp.
But your code adds such an update where we update the hw vblank counter, but not the timestamp, and that feels buggy. Either we're still in the same frame, and then we should story nothing. Or we advanced, and then we probably want a new timestampt for that frame too.
Even if we're still in the same frame the hw frame counter may already have been reset due to the power well having been turned off. That is what I'm trying to fix here.
Now I suppose that's fairly unlikely, at least with PSR which probably does impose some extra delays before the power gets yanked. But at least theoretically possible.
Pondering about this a bit further. I think the fact that the current code takes the round-to-closest approach I used for the vblank handler is perhaps a bit bad. It could push the seq counter forward if we're past the halfway point of a frame. I think that rounding behaviour makes sense for the irq since those tick steadily and so allowing a bit of error either way seems correct to me. Perhaps round-down might be the better option for _restore(). Not quites sure, need more thinking probably.
Yes this is the rounding I'm worried about.
Actually I don't think this is really an issue since we are working with the corrected timestamps here. Those always line up with frames, so unless the correction is really buggy or the hw somehow skips a partial frame it should work rather well. At least when operating with small timescales. For large gaps the error might creep up, but I don't think a small error in the predicted seq number over a long timespan is really a problem.
That corrected timestamp is what can go wrong I think: There's no guarantee that drm_crtc_vblank_helper_get_vblank_timestamp_internal() flips to top-of-frame at the exact same time than when the hw vblank counter flips. Or at least I'm not seeing where we correct them both together.
We do this seqlock type of thing: do { cur_vblank = __get_vblank_counter(dev, pipe); rc = drm_get_last_vbltimestamp(dev, pipe, &t_vblank, in_vblank_irq); } while (cur_vblank != __get_vblank_counter(dev, pipe) && --count > 0);
which guarantees the timestamp really is for the frame we think it is for.
But your point above that the hw might reset the counter again is also valid. I'm assuming what you're worried about is that we first do a _restore (and the hw vblank counter hasn't been trashed yet), and then in the same frame we do another restore, but now the hw frame counter has been trashe, and we need to update it?
Yeah, although the pre-trashing _restore could also just be a vblank irq I think.
Another idea that came to me now is that maybe we should actually just check if the current hw frame counter value looks sane, as in something like:
diff_hw_counter = current_hw_counter-stored_hw_counter diff_ts = (current_ts-stored_ts)/framedur
if (diff_hw_counter ~= diff_ts) diff = diff_hw_counter; else diff = diff_ts;
and if they seem to match then just keep trusting the hw counter. So only if there's a significant difference would we disregard the diff of the hw counter and instead use the diff based on the timestamps. Not sure what "significant" is though; One frame, two frames?
Hm, another idea: The only point where we can trust the entire hw counter
- timestamp sampling is when the irq happens. Because then we know the
driver will have properly corrected for any hw oddities (like hw counter flipping not at top-of-frame, like the core expects).
i915 at least gives out correct data regardless of when you sample it. Well, except for the cases where the hw counter gets trashed, in which case the hw counter is garbage (when compared with .last) but the timestamp is still correct.
Hm where/how do we handle this? Maybe I'm just out of date with how it all works nowadays.
There's not much to handle. We know when exactly the counters increment and thus can give out the correct answer to the question "which frame is this?".
So what if _restore always goes back to the last such trusted hw counter for computing the frame counter diff and all that stuff? That way if we have a bunch of _restore with incosisten hw vblank counter, we will a) only take the last one (fixes the bug you're trying to fix) b) still use the same last trusted baseline for computations (addresses the race I'm seeing).
Or does this not work?
I don't think I really understand what you're suggesting here. _restore is already using the last trusted data (the stored timestamp + .last).
So the one thing _restore will have to update is .last. I think it can either do what it does now and set .last to the current hw counter value + update the timestamp to match, or it could perhaps adjust the stored .last such that the already stored timestamp and the updated .last match up. But I think both of those options have the same level or inaccuracy since both would still do the same ts_diff->hw_counter_diff prediction.
It does complicate the code a bit, because we'd need to store the count/timestamp information from _restore outside of the usual vblank ts array. But I think that addresses everything.
Hmm. So restore would store this extra information somewhere else, and not update the normal stuff at all? What exactly would we do with that extra data?
Hm I guess I didn't think this through. But the idea I had was:
- _restore always recomputes back from the las
drm_crtc_handl_vblank-stored timestamp.
- the first drm_crtc_handle_vblank bakes in any corrections that
_restore has prepared meanwhile
- same applies to all the sampling functions we might look at lastes
timestamps/counter values.
So I guess instead of _restore adjusting .last we would instead mainatian a separate correction information and apply it when doing the diff between the current hw counter vs. .last. Not sure why that would be particularly better than just adjusting .last directly.
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.
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
Ok, top-posting because lol I got confused. I mixed up the guesstimation work we do for when we don't have a vblank counter with the precise vblank timestamp stuff.
I think it'd still be good to maybe lock down/document a bit better the requirements for drm_crtc_vblank_restore, but I convinced myself now that your patch looks correct.
Reviewed-by: Daniel Vetter daniel.vetter@ffwll.ch
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
On Tue, Feb 09, 2021 at 11:07:53AM +0100, Daniel Vetter wrote:
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.
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
Ok, top-posting because lol I got confused. I mixed up the guesstimation work we do for when we don't have a vblank counter with the precise vblank timestamp stuff.
I think it'd still be good to maybe lock down/document a bit better the requirements for drm_crtc_vblank_restore, but I convinced myself now that your patch looks correct.
Reviewed-by: Daniel Vetter daniel.vetter@ffwll.ch
Ta.
Though I wonder if we should just do something like this instead: - store_vblank(dev, pipe, diff, t_vblank, cur_vblank); + vblank->last = (cur_vblank - diff) & max_vblank_count;
to make it entirely obvious that this exists only to fix up the stored hw counter value?
Would also avoid the problem the original patch tries to fix because we'd simply never store a new timestamp here.
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
On Tue, Feb 9, 2021 at 4:41 PM Ville Syrjälä ville.syrjala@linux.intel.com wrote:
On Tue, Feb 09, 2021 at 11:07:53AM +0100, Daniel Vetter wrote:
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.
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
Ok, top-posting because lol I got confused. I mixed up the guesstimation work we do for when we don't have a vblank counter with the precise vblank timestamp stuff.
I think it'd still be good to maybe lock down/document a bit better the requirements for drm_crtc_vblank_restore, but I convinced myself now that your patch looks correct.
Reviewed-by: Daniel Vetter daniel.vetter@ffwll.ch
Ta.
Though I wonder if we should just do something like this instead:
store_vblank(dev, pipe, diff, t_vblank, cur_vblank);
vblank->last = (cur_vblank - diff) & max_vblank_count;to make it entirely obvious that this exists only to fix up the stored hw counter value?
Would also avoid the problem the original patch tries to fix because we'd simply never store a new timestamp here.
Hm yeah, I think that would nicely limit the impact. But need to check overflow/underflow math is all correct. And I think that would neatly implement the trick I proposed to address the bug that wasn't there :-)
The only thing that I've thought of as issue is that we might have more wrap-around of the hw vblank counter, but that shouldn't be worse than without this - anytime we have the vblank on for long enough we fix the entire thing, and I think our wrap handling is now consistent enough (there was some "let's just add a large bump" stuff for dri1 userspace iirc) that this shouldn't be any problem.
Plus the comment about _restore being very special would be in the restore function, so this would also be rather tidy. If you go with this maybe extend the kerneldoc for ->last to mention that drm_vblank_restore() adjusts it?
The more I ponder this, the more I like it ... which probably means I'm missing something, because this is drm_vblank.c?
Cheers, Daniel
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
-- Ville Syrjälä Intel
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 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.
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 | 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; }
/**
On Thu, Feb 18, 2021 at 06:03:05PM +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 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@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 | 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
On Thu, Feb 18, 2021 at 06:03:05PM +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 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.
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
Reviewed-by: Daniel Vetter daniel.vetter@ffwll.ch
For testing, there's nothing else than hsw psr that needs this, or that's just the box you have locally? -Daniel
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
dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel
On Fri, Feb 19, 2021 at 04:08:09PM +0100, Daniel Vetter wrote:
On Thu, Feb 18, 2021 at 06:03:05PM +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 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.
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
Reviewed-by: Daniel Vetter daniel.vetter@ffwll.ch
For testing, there's nothing else than hsw psr that needs this, or that's just the box you have locally?
Just the one I happen to have.
Any machine with PSR should be able to hit this. But now that I refresh my memory I guess HSW/BDW don't actually fully reset the hw frame counter since they don't have the DC5/6 stuff. But even on HSW/BDW the frame counter would certainly stop while in PSR, so maintaining sensible vblank seq numbers will still require drm_vblank_restore(). Just my further idea of checking some power well counter/scratch register would not help in cases where DC states are not used. Instead we'd need some kind of PSR residency counter/etc.
dri-devel@lists.freedesktop.org
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Ville Syrjälä