Re: [PATCH] drm/atomic-helpers: remove legacy_cursor_update hacks

[maxime@xxxxxxxxxx]

Hi Dmitry,

On Wed, Jan 31, 2024 at 12:28:59PM +0100, Daniel Vetter wrote:
> On Wed, Jan 31, 2024 at 12:26:45PM +0200, Dmitry Baryshkov wrote:
> > On Wed, 31 Jan 2024 at 11:11, Daniel Vetter <daniel@xxxxxxxx> 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.

Since it looks like you're in a mood to deal with kms reworks vs msm, we
still have this one to address too :)

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