linux-kernel - Re: [PATCH] drm/atomic-helpers: remove legacy_cursor

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Message-ID: <Zbou-y7eNhQTMpKo@phenom.ffwll.local>
Date: Wed, 31 Jan 2024 12:28:59 +0100
From: Daniel Vetter <daniel@...ll.ch>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc: Jason-JH Lin (林睿祥) <Jason-JH.Lin@...iatek.com>,
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Subject: Re: [PATCH] drm/atomic-helpers: remove legacy_cursor_update hacks

On Wed, Jan 31, 2024 at 12:26:45PM +0200, Dmitry Baryshkov wrote:
> On Wed, 31 Jan 2024 at 11:11, Daniel Vetter <daniel@...ll.ch> wrote:
> >
> > On Wed, Jan 31, 2024 at 05:17:08AM +0000, Jason-JH Lin (林睿祥) wrote:
> > > On Thu, 2024-01-25 at 19:17 +0100, Daniel Vetter wrote:
> > > >
> > > > External email : Please do not click links or open attachments until
> > > > you have verified the sender or the content.
> > > >  On Tue, Jan 23, 2024 at 06:09:05AM +0000, Jason-JH Lin (林睿祥) wrote:
> > > > > Hi Maxime, Daniel,
> > > > >
> > > > > We encountered similar issue with mediatek SoCs.
> > > > >
> > > > > We have found that in drm_atomic_helper_commit_rpm(), when
> > > > disabling
> > > > > the cursor plane, the old_state->legacy_cursor_update in
> > > > > drm_atomic_wait_for_vblank() is set to true.
> > > > > As the result, we are not actually waiting for a vlbank to wait for
> > > > our
> > > > > hardware to close the cursor plane. Subsequently, the execution
> > > > > proceeds to drm_atomic_helper_cleanup_planes() to  free the cursor
> > > > > buffer. This can lead to use-after-free issues with our hardware.
> > > > >
> > > > > Could you please apply this patch to fix our problem?
> > > > > Or are there any considerations for not applying this patch?
> > > >
> > > > Mostly it needs someone to collect a pile of acks/tested-by and then
> > > > land
> > > > it.
> > > >
> > >
> > > Got it. I would add tested-by tag for mediatek SoC.
> > >
> > > > I'd be _very_ happy if someone else can take care of that ...
> > > >
> > > > There's also the potential issue that it might slow down some of the
> > > > legacy X11 use-cases that really needed a non-blocking cursor, but I
> > > > think
> > > > all the drivers where this matters have switched over to the async
> > > > plane
> > > > update stuff meanwhile. So hopefully that's good.
> > > >
> > >
> > > I think all the drivers should have switched to async plane update.
> > >
> > > Can we add the checking condition to see if atomic_async_update/check
> > > function are implemented?
> >
> > Pretty sure not all have done that, so really it boils down to whether we
> > break a real user's use-case. Which pretty much can only be checked by
> > merging the patch (hence the requirement to get as many acks as possible
> > from display drivers) and then being willing to handle any fallout that's
> > reported as regressions for a specific driver.
> >
> > It's a pile of work, at least when it goes south, that's why I'm looking
> > for volunteers.
> 
> I can check this on all sensible msm generations, including mdp4, but
> it will be next week, after the FOSDEM.
> 
> BTW, for technical reasons one of the msm platforms still has the
> legacy cursor implementation might it be related?

Yeah, msm is one of the drivers I had to change with some hacks to avoid
really bad fallout. It should still work like before, but that's one that
definitely needs testing.
-Sima

> 
> >
> > Note that handling the fallout doesn't mean you have to fix that specific
> > driver, the only realistic option might be to reinstate the legacy cursor
> > behaviour, but as an explicit opt-in that only that specific driver
> > enables.
> >
> > So maybe for next round of that patch it might be good to have a 2nd patch
> > which implements this fallback plan in the shared atomic modeset code?
> >
> > Cheers, Sima
> 
> 
> -- 
> With best wishes
> Dmitry

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch