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

[Maxime Ripard]

Hi,

On Mon, Sep 30, 2024 at 03:45:13PM -0400, Lyude Paul wrote:
> On Mon, 2024-09-30 at 09:06 +0200, Thomas Zimmermann wrote:
> > 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 at redhat.com>
> > > > Fixes: c2fcd274bce5 ("drm: Add atomic/plane helpers")
> > > > Cc: dri-devel at lists.freedesktop.org
> > > > Cc: <stable at vger.kernel.org> # 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.
> 
> Gotcha, FWIW the reason I went with this patch:
>  * atomic_update is actually documented as being optional in the kernel docs,
>    so we'd want to remove that if we make it mandatory

Sure, that makes total sense :)

>  * rvkms currently doesn't have an atomic_update. We will likely have one
>    whenever I get a chance to actually add CRC and/or writeback connector
>    supports - but for the time being all we do is register a KMS device with
>    vblank support.

WIP drivers can provide an empty implementation. And even if actually
didn't need it for $REASONS, I'd argue that an empty implementation (and
a comment) makes that explicit instead of making the reader guess why
it's not needed.

Maxime
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