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

quoted

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:

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 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.

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

Regards,
Jason-JH.Lin

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 <redacted>

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

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