[PATCH] drm/imx: correct order of crtc disable
Robert Beckett
bob.beckett at collabora.com
Fri Jun 21 14:34:05 UTC 2019
On Thu, 2019-06-20 at 17:53 +0100, Robert Beckett wrote:
> On Thu, 2019-06-20 at 18:29 +0200, Daniel Vetter wrote:
> > On Thu, Jun 20, 2019 at 3:30 PM Robert Beckett
> > <bob.beckett at collabora.com> wrote:
> > > On Thu, 2019-06-20 at 14:32 +0200, Daniel Vetter wrote:
> > > > On Thu, Jun 20, 2019 at 12:12:13PM +0100, Robert Beckett wrote:
> > > > > On Thu, 2019-06-20 at 10:50 +0200, Daniel Vetter wrote:
> > > > > > On Wed, Jun 19, 2019 at 11:40 AM Philipp Zabel <
> > > > > > p.zabel at pengutronix.de> wrote:
> > > > > > >
> > > > > > > Hi Robert,
> > > > > > >
> > > > > > > thank you for the patch.
> > > > > > >
> > > > > > > On Tue, 2019-06-18 at 16:50 +0100, Robert Beckett wrote:
> > > > > > > > Notify drm core before sending pending events during
> > > > > > > > crtc
> > > > > > > > disable.
> > > > > > > > This fixes the first event after disable having an old
> > > > > > > > stale
> > > > > > > > timestamp
> > > > > > > > by having drm_crtc_vblank_off update the timestamp to
> > > > > > > > now.
> > > > > > > >
> > > > > > > > This was seen while debugging weston log message:
> > > > > > > > Warning: computed repaint delay is insane: -8212 msec
> > > > > > > >
> > > > > > >
> > > > > > > Would you say this
> > > > > > > Fixes: a474478642d5 ("drm/imx: fix crtc vblank state
> > > > > > > regression")
> > > > > > > ?
> > > > > > >
> > > > > > > > Signed-off-by: Robert Beckett <
> > > > > > > > bob.beckett at collabora.com>
> > > > > > > > ---
> > > > > > > > drivers/gpu/drm/imx/ipuv3-crtc.c | 6 +++---
> > > > > > > > 1 file changed, 3 insertions(+), 3 deletions(-)
> > > > > > > >
> > > > > > > > diff --git a/drivers/gpu/drm/imx/ipuv3-crtc.c
> > > > > > > > b/drivers/gpu/drm/imx/ipuv3-crtc.c
> > > > > > > > index 9cc1d678674f..c436a28d50e4 100644
> > > > > > > > --- a/drivers/gpu/drm/imx/ipuv3-crtc.c
> > > > > > > > +++ b/drivers/gpu/drm/imx/ipuv3-crtc.c
> > > > > > > > @@ -91,14 +91,14 @@ static void
> > > > > > > > ipu_crtc_atomic_disable(struct
> > > > > > > > drm_crtc *crtc,
> > > > > > > > ipu_dc_disable(ipu);
> > > > > > > > ipu_prg_disable(ipu);
> > > > > > > >
> > > > > > > > + drm_crtc_vblank_off(crtc);
> > > > > > > > +
> > > > > > >
> > > > > > > This is explained in the commit message and aligns with
> > > > > > > the
> > > > > > > drm_crtc_state @event documentation.
> > > > > >
> > > > > > This part here looks fishy. The drm_vblank.c code is
> > > > > > supposed
> > > > > > to
> > > > > > do
> > > > > > the right thing, no matter where or when you ask it to
> > > > > > generate
> > > > > > an
> > > > > > event. It definitely shouldn't generate a timestamp that's
> > > > > > a
> > > > > > few
> > > > > > seconds too early. Bunch of options:
> > > > > > - there's a bug in drm_vblank.c and it's mixing up
> > > > > > something
> > > > > > and
> > > > > > generating a totally bogus value.
> > > > > > - there's a lie in your imx vblank code, which trips the
> > > > > > drm_vblank.c
> > > > > > counter interpolation and results in a totally bogus value.
> > > > > >
> > > > > > drm_vblank.c assumes that if you do claim to have a hw
> > > > > > counter
> > > > > > and
> > > > > > generate timestamps, that those are perfectly accurate. It
> > > > > > only
> > > > > > falls
> > > > > > back to guestimating using the system timer if that's not
> > > > > > present.
> > > > > >
> > > > > > Either way, this very much smells like papering over a bug
> > > > > > if
> > > > > > this
> > > > > > change indeed fixes your wrong vblank timestamps.
> > > > > >
> > > > >
> > > > > A quick explaination of where the dodgy timestamp came from:
> > > > > 1. driver starts up
> > > > > 2. fbcon comes along and restores fbdev, enabling vblank
> > > > > 3. vblank_disable_fn fires via timer disabling vblank,
> > > > > keeping
> > > > > vblank
> > > > > seq number and time set at current value
> > > > > (some time later)
> > > > > 4. weston starts and does a modeset
> > > > > 5. atomic commit disables crtc while it does the modeset
> > > > > 6. ipu_crtc_atomic_disable sends vblank with old seq number
> > > > > and
> > > > > time
> > > > >
> > > > > It turns out the actual fix for the old vblank is the next
> > > > > change,
> > > > > which stops it being sent at all during the crtc disable as
> > > > > it
> > > > > is
> > > > > is
> > > > > still active, it would then go through drm_crtc_vblank_off,
> > > > > reseting
> > > > > the timestamp, and get delivered during the vblank enable as
> > > > > part
> > > > > of
> > > > > the atomic commit.
> > > >
> > > > This shouldn't fix your vblank timestamp troubles either. It
> > > > might
> > > > mean
> > > > that the timestamp is slightly too early (because you take it
> > > > while
> > > > shutting down the crtc, not while re-enabling), but not by
> > > > seconds.
> > > >
> > > > Quick experiment: Disable vblank disabling with
> > > > drm.vblankoffdelay =
> > > > 0. If
> > > > that also fixes the timestamps, then I'm pretty sure you have a
> > > > driver bug
> > > > somewhere and lie to the vblank core code about something.
> > > > -Daniel
> > > >
> > >
> > > Experiment done. The timestamp is fine, it is the timestamp of
> > > the
> > > previous vblank update. But, this would fix it because the vblank
> > > interrupt was never disabled.
> > >
> > > The original issue was that the event got sent after vblank was
> > > disabled and before it got re-enabled during the modeset, so
> > > nothing
> > > had happened to update the timestamp and seq number.
> > >
> > > What are you expecting to update the timestamp and seq number
> > > while
> > > the
> > > interrupt is disabled after vblank_disable_fn?
> >
> > Hm maybe this indeed needs to be shuffled around a bit, since
> > currently it's indeed not not allowed to call
> > drm_crtc_send_vblank_event if:
> > - your driver has vblank support (i.e. dev->num_crtcs > 0)
> > - the vblank irq is off (i.e. no one called drm_crtc_vblank_get)
> > - from the vblank code's pov the pipe is still running (i.e. not
> > in-between a drm_crtc_vblank_off()/on() pair)
> >
> > If all of these conditions hold then drm_crtc_send_vblank_event is
> > going to give you a garbage timestamp and and sequence number. If
> > you
> > call drm_crtc_vblank_get/put around it, or after vblank_off, then
> > either of those will have rolled things forward for you.
> >
> > The other option I was chasing is a get_vblank_counter driver
> > callback
> > which lies, but I think that's only relevant for immediate
> > disabling
> > support.
> >
> > Anyway, thanks for sticking with me on this. Can I volunteer you to
> > write a patch to improve the documentation of
> > drm_crtc_send_vblank_event, plus add a WARN_ON in there that fires
> > if
> > the above conditions are all true? I expect imx isn't the only one
> > that gets this wrong, would be good to close this trap for real.
It turns out it is not as simple as calling drm_crtc_vblank_off() to
update the timestamp and seq number before sending the event.
drm_crtc_vblank_off() calls drm_vblank_disable_and_save() to update the
info, but that doesnt actually do the update if !vblank->enabled for
the legit reason that the driver may have runtime disabled the
timestamp register HW.
In the situation where interrupts were disabled long ago, and the
modeset is disabling the crtc, we would want an up to date timestamp
and seq number for signalling the disable of a crtc, but cant get the
updated values if interrupts were previously disabled.
Im not sure how to handle this case generically.
We could just set the timestamp to now, but what to do about the seq
number? Increment it? keep it the same?
Any thoughts on this?
For the similar case where the crtc is being enabled during the modeset
with vblank enabled, the later fix of checking for !crtc->state->active
before sending the event still works as it will be delivered during
next vblank with an updated stamp.
>
> Sure, Ill add it as part of v3 once Ive figured out which combination
> of state I can use for the WARN_ON, along with the split up of this
> patch in to 2 separate patches. (Tomorrow)
>
> >
> > > Im struggling to see what this experiment was meant to
> > > test/prove.
> >
> > Worked all fine in confirming what I wanted to confirm.
> >
> > Thanks, Daniel
>
> No problem. Thanks for the due diligence.
>
> >
> > >
> > > > >
> > > > > So, in theory, we could just have the following change to fix
> > > > > the
> > > > > specific issue of a stale timestamp.
> > > > >
> > > > > However, given the documentation for the event in
> > > > > include/drm/drm_crtc.h:
> > > > >
> > > > > * - The event is for a CRTC which is being disabled
> > > > > through
> > > > > this
> > > > > * atomic commit. In that case the event can be
> > > > > send
> > > > > out
> > > > > any
> > > > > time
> > > > > * after the hardware has stopped scanning out the
> > > > > current
> > > > > * framebuffers. It should contain the timestamp
> > > > > and
> > > > > counter
> > > > > for the
> > > > > * last vblank before the display pipeline was
> > > > > shut
> > > > > off.
> > > > > The
> > > > > simplest
> > > > > * way to achieve that is calling
> > > > > drm_crtc_send_vblank_event()
> > > > > * somewhen after drm_crtc_vblank_off() has been
> > > > > called.
> > > > >
> > > > > This still seems like a sensible change for when the crtc is
> > > > > being
> > > > > disabled.
> > > > >
> > > > >
> > > > >
> > > > > > > > spin_lock_irq(&crtc->dev->event_lock);
> > > > > > > > - if (crtc->state->event)
> > > > > > > > {
> > > > > > > > + if (crtc->state->event && !crtc->state->active) {
> > > > > > >
> > > > > > > This is not mentioned though.
> > > > > > >
> > > > > > > If the pending event is not sent here, I assume it will
> > > > > > > be
> > > > > > > picked
> > > > > > > up by
> > > > > > > .atomic_flush and will then be sent after the first EOF
> > > > > > > interrupt
> > > > > > > after
> > > > > > > the modeset is complete. Can you explain this in the
> > > > > > > commit
> > > > > > > message?
> > > > > >
> > > > > > Yeah looks correct (you only want to generate the event
> > > > > > here
> > > > > > when
> > > > > > the
> > > > > > crtc stays off), if it gets re-enabled the event should
> > > > > > only
> > > > > > be
> > > > > > generated later on once that's all finished. But separate
> > > > > > bugfix.
> > > > > > -Daniel
> > > > > >
> > > > >
> > > > > It looks like this is actually the fix needed to avoid the
> > > > > bogus
> > > > > timestamp.
> > > > >
> > > > > I can split this patch up in to 2 commits if desired?
> > > > >
> > > > > > >
> > > > > > > With that,
> > > > > > > Reviewed-by: Philipp Zabel <p.zabel at pengutronix.de>
> > > > > > >
> > > > > > > > drm_crtc_send_vblank_event(crtc, crtc-
> > > > > > > > > state-
> > > > > > > > > event);
> > > > > > > >
> > > > > > > > crtc->state->event = NULL;
> > > > > > > > }
> > > > > > > > spin_unlock_irq(&crtc->dev->event_lock);
> > > > > > > > -
> > > > > > > > - drm_crtc_vblank_off(crtc);
> > > > > > > > }
> > > > > > > >
> > > > > > > > static void imx_drm_crtc_reset(struct drm_crtc *crtc)
> > > > > > >
> > > > > > > regards
> > > > > > > Philipp
> > > > > > > _______________________________________________
> > > > > > > dri-devel mailing list
> > > > > > > dri-devel at lists.freedesktop.org
> > > > > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> > > > > >
> > > > > >
> > > > > >
> > > >
> > > >
> >
> >
>
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