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

[Daniel Vetter]

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.

Cheers, Sima
> 
> Regards,
> Jason-JH.Lin
> 
> On Tue, 2023-03-07 at 15:56 +0100, Maxime Ripard wrote:
> > Hi,
> > 
> > On Thu, Feb 16, 2023 at 12:12:13PM +0100, Daniel Vetter wrote:
> > > The stuff never really worked, and leads to lots of fun because it
> > > out-of-order frees atomic states. Which upsets KASAN, among other
> > > things.
> > > 
> > > For async updates we now have a more solid solution with the
> > > ->atomic_async_check and ->atomic_async_commit hooks. Support for
> > > that
> > > for msm and vc4 landed. nouveau and i915 have their own commit
> > > routines, doing something similar.
> > > 
> > > For everyone else it's probably better to remove the use-after-free
> > > bug, and encourage folks to use the async support instead. The
> > > affected drivers which register a legacy cursor plane and don't
> > > either
> > > use the new async stuff or their own commit routine are: amdgpu,
> > > atmel, mediatek, qxl, rockchip, sti, sun4i, tegra, virtio, and
> > > vmwgfx.
> > > 
> > > Inspired by an amdgpu bug report.
> > 
> > Thanks for submitting that patch. It's been in the downstream RPi
> > tree
> > for a while, so I'd really like it to be merged eventually :)
> > 
> > Acked-by: Maxime Ripard <maxime at cerno.tech>
> > 
> > Maxime

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