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

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Message-ID: <fnhpycf6yf4jmoaaryjjhrjp3mnb442t3zpyqsfuze3x3scy7w@hpgz432aswar>
Date: Wed, 31 Jan 2024 11:57:12 +0100
From: "mripard@...nel.org" <mripard@...nel.org>
To: Jason-JH Lin (林睿祥) <Jason-JH.Lin@...iatek.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
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Subject: Re: Re: Re: [PATCH] drm/atomic-helpers: remove legacy_cursor_update
 hacks

Hi,

On Wed, Jan 31, 2024 at 05:27:14AM +0000, Jason-JH Lin (林睿祥) wrote:
> 
> On Sun, 2024-01-28 at 10:24 +0100, Maxime Ripard wrote:
> > On Thu, Jan 25, 2024 at 07:17:21PM +0100, Daniel Vetter wrote:
> > > 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.
> > > 
> > > 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 there was also a regression with msm no one really figured
> > out?
> 
> OK...
> But I am only available on MediaTek platform.

I think most of us are in that situation, and which is part of the
reason it kind of stalled :)

> Does it also causes a regression with msn if I re-send a patch for
> drm_atomic_helper.c only?

Yes, that's my recollection at least.

Fortunately, Dmitry might be able to clear that up.

Maxime

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