Re: [PATCH] drm/atomic_helper: Add missing NULL check for drm_plane_helper_funcs.atomic_update

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Hi

Am 30.09.24 um 09:01 schrieb Maxime Ripard:
Hi,

On Fri, Sep 27, 2024 at 04:46:16PM GMT, Lyude Paul wrote:
Something I discovered while writing rvkms since some versions of the
driver didn't have a filled out atomic_update function - we mention that
this callback is "optional", but we don't actually check whether it's NULL
or not before calling it. As a result, we'll segfault if it's not filled
in.

   rvkms rvkms.0: [drm:drm_atomic_helper_commit_modeset_disables] modeset on [ENCODER:36:Virtual-36]
   BUG: kernel NULL pointer dereference, address: 0000000000000000
   PGD 0 P4D 0
   Oops: Oops: 0010 [#1] PREEMPT SMP NOPTI
   Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS edk2-20240813-1.fc40 08/13/2024
   RIP: 0010:0x0

So, let's fix that.

Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
Fixes: c2fcd274bce5 ("drm: Add atomic/plane helpers")
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: <stable@xxxxxxxxxxxxxxx> # v3.19+
So we had kind of a similar argument with drm_connector_init early this
year, but I do agree we shouldn't fault if we're missing a callback.

I do wonder how we can implement a plane without atomic_update though?
Do we have drivers in such a case?

That would likely be an output with an entirely static display. Hard to imaging, I think.


If not, a better solution would be to make it mandatory and check it
when registering.

Although I r-b'ed the patch already, I'd also prefer this solution.



Maxime

--
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Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)





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