[PATCH] drm/imx: correct order of crtc disable

[Robert Beckett]

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.

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
> > _______________________________________________
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> 
> 
>