linux-kernel - Re: [PATCH] drm/atomic_helper: Add missing NULL check for drm_plane_helper_funcs.atomic

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Message-ID: <bcf7e1e9-b876-4efc-83ef-b48403315d31@suse.de>
Date: Mon, 30 Sep 2024 09:06:34 +0200
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Maxime Ripard <mripard@...nel.org>, Lyude Paul <lyude@...hat.com>
Cc: dri-devel@...ts.freedesktop.org, stable@...r.kernel.org,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Sean Paul <seanpaul@...omium.org>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/atomic_helper: Add missing NULL check for
 drm_plane_helper_funcs.atomic_update

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@...hat.com>
>> Fixes: c2fcd274bce5 ("drm: Add atomic/plane helpers")
>> Cc: dri-devel@...ts.freedesktop.org
>> Cc: <stable@...r.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.


>
> Maxime

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
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