linux-kernel - Re: [PATCH 1/2] drm/atomic: Allow vblank-enabled + self-refresh "disable"

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Date:   Fri, 6 Jan 2023 10:08:53 -0800
From:   Brian Norris <briannorris@...omium.org>
To:     Heiko Stübner <heiko@...ech.de>,
        Sean Paul <seanpaul@...omium.org>,
        Michel Dänzer <michel.daenzer@...lbox.org>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Sandy Huang <hjc@...k-chips.com>,
        linux-rockchip@...ts.infradead.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/2] drm/atomic: Allow vblank-enabled + self-refresh
 "disable"

Hi Daniel,

On Fri, Jan 06, 2023 at 06:53:49PM +0100, Daniel Vetter wrote:
> On Thu, Jan 05, 2023 at 05:40:17PM -0800, Brian Norris wrote:
> > The self-refresh helper framework overloads "disable" to sometimes mean
> > "go into self-refresh mode," and this mode activates automatically
> > (e.g., after some period of unchanging display output). In such cases,
> > the display pipe is still considered "on", and user-space is not aware
> > that we went into self-refresh mode. Thus, users may expect that
> > vblank-related features (such as DRM_IOCTL_WAIT_VBLANK) still work
> > properly.
> > 
> > However, we trigger the WARN_ONCE() here if a CRTC driver tries to leave
> > vblank enabled here.
> > 
> > Add a new exception, such that we allow CRTCs to be "disabled" (with
> > self-refresh active) with vblank interrupts still enabled.
> > 
> > Cc: <stable@...r.kernel.org> # dependency for subsequent patch
> > Signed-off-by: Brian Norris <briannorris@...omium.org>
> > ---
> > 
> >  drivers/gpu/drm/drm_atomic_helper.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> > index d579fd8f7cb8..7b5eddadebd5 100644
> > --- a/drivers/gpu/drm/drm_atomic_helper.c
> > +++ b/drivers/gpu/drm/drm_atomic_helper.c
> > @@ -1207,6 +1207,12 @@ disable_outputs(struct drm_device *dev, struct drm_atomic_state *old_state)
> >  
> >  		if (!drm_dev_has_vblank(dev))
> >  			continue;
> > +		/*
> > +		 * Self-refresh is not a true "disable"; let vblank remain
> > +		 * enabled.
> > +		 */
> > +		if (new_crtc_state->self_refresh_active)
> > +			continue;
> 
> This very fishy, because we check in crtc_needs_disable whether this
> output should stay on due to self-refresh. Which means you should never
> end up in here.

That's not what crtc_needs_disable() does w.r.t. self-refresh. In fact,
it's the opposite; see, for example, the
|new_state->self_refresh_active| clause. That clause means that if we're
entering self-refresh, we *intend* to disable (i.e., we return 'true').
That's because like I mention above, the self-refresh helpers overload
what "disable" means.

I'll also add my caveat again that I'm a bit new to DRM, so feel free to
continue to correct me if I'm wrong :) Or perhaps Sean Paul could
provide second opinions, as I believe he wrote this stuff.

> And yes vblank better work in self refresh :-) If it doesn't, then you
> need to fake it with a timer, that's at least what i915 has done for
> transparent self-refresh.

OK! Then that sounds like it at least ACKs my general idea for this
series. (Michel and I poked at a few ideas in the thread at [1] and
landed on approx. this solution, or else a fake/timer like you suggest.)

> We might need a few more helpers. Also, probably more igt, or is this
> something igt testing has uncovered? If so, please cite the igt testcase
> which hits this.

The current patch only fixes a warning that comes when I try to do the
second patch. The second patch is a direct product of an IGT test
failure (a few of kms_vblank's subtests), and I linked [1] the KernelCI
report there.

Brian

[1] Link: https://lore.kernel.org/dri-devel/Y5itf0+yNIQa6fU4@sirena.org.uk/
    Reported-by: "kernelci.org bot" <bot@...nelci.org>