linux-kernel - Re: [PATCH] drm/gud: fix NULL crtc dereference on display disable

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Message-ID: <cc51e712-a337-46b0-91cb-6c3af76a84c3@shenghaoyang.info>
Date: Tue, 20 Jan 2026 00:41:33 +0800
From: Shenghao Yang <me@...nghaoyang.info>
To: Thomas Zimmermann <tzimmermann@...e.de>, Ruben Wauters
 <rubenru09@....com>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org, kernel test robot <lkp@...el.com>,
 Dan Carpenter <dan.carpenter@...aro.org>
Subject: Re: [PATCH] drm/gud: fix NULL crtc dereference on display disable

Hi,

On 19/1/26 16:17, Thomas Zimmermann wrote:
> Hi,
> 
> thanks for the patch.

Thanks for taking a look. I did forget about this oops and the
smatch failure was a good reminder!

> 
> Am 18.01.26 um 13:50 schrieb Shenghao Yang:
>> Commit dc2d5ddb193e ("drm/gud: fix NULL fb and crtc dereferences
>> on USB disconnect") [1] only fixed the initial NULL crtc dereference
>> in gud_plane_atomic_update().
>>
>> However, planes can also be disabled in non-hotplug paths (e.g.
>> display disables via the DE). The drm_dev_enter() call would not
> 
> 'DE' ?

Ah - the desktop environment. I was scratching my head for why the
box kept oops-ing on boot even after the hotplug fix. It turned out
kscreen was applying the saved "disable display" setting.

> It seems to me that all these calls to GUD_REQ_SET_CONTROLLER_ENABLE(^1) and GUD_REQ_SET_DISPLAY_ENABLEshould rather go to the CRTC's atomic_enable/atomic_disable functions. Those currently seem missing from [1]. The atomic_update helper would then be reduced to damage handling. Best regards Thomas [1] https://elixir.bootlin.com/linux/v6.18.6/source/drivers/gpu/drm/gud/gud_drv.c#L341
> ^1: SET_CONTROLLER_ENABLE sounds like it could even be part of device probing and runtime PM management, but that is a more invasive change.

That feels like it'd be much cleaner. I'll respin with that in v2.

Shenghao