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

[Jason-JH Lin (林睿祥)]

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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?

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