Re: [PATCH 0/5] drm: Remove remaining support for kdb

Thomas Zimmermann
2025-11-25T16:22:10Z

Hi

Am 25.11.25 um 16:26 schrieb Doug Anderson:
> Hi,
>
> On Tue, Nov 25, 2025 at 5:06 AM Thomas Zimmermann <tzimmermann@suse.de> wrote:
>> Remove the rest of the kbd support from DRM. Driver support has been
>> broken for years without anyone complaining.
>>
>> Kdb cannot use regular DRM mode setting, so DRM drivers have to
>> implement an additional hook to make it work (in theory). As outlined
>> by Sima in commit 9c79e0b1d096 ("drm/fb-helper: Give up on kgdb for
>> atomic drivers") from 2017, kdb is not compatible with DRM atomic mode
>> setting. Non-atomic mode setting meanwhile has become rare.
>>
>> Only 3 DRM drivers implement the hooks for kdb support. Amdgpu and
>> nouveau use non-atomic mode setting on older devices. But both drivers
>> have switched to generic fbdev emulation, which isn't compatible with
>> kdb. Radeon still runs kdb, but it doesn't work in practice. See the
>> commits in this series for details
>>
>> Therefore remove the remaining support for kdb from the DRM drivers
>> and from DRM fbdev emulation. Also remove the hooks from fbdev, as
>> there are no fbdev drivers with kdb support.
>>
>> If we ever want to address kdb support within DRM drivers, a place to
>> start would be the scanout buffers used by DRM's panic screen. These
>> use the current display mode. They can be written and flushed without
>> mode setting involved.
>>
>> Note: kdb over serial lines is not affected by this series and continues
>> to work as before.
>>
>> Thomas Zimmermann (5):
>> drm/amdgpu: Do not implement mode_set_base_atomic callback
>> drm/nouveau: Do not implement mode_set_base_atomic callback
>> drm/radeon: Do not implement mode_set_base_atomic callback
>> drm/fbdev-helper: Remove drm_fb_helper_debug_enter/_leave()
>> fbcon: Remove fb_debug_enter/_leave from struct fb_ops
> Personally, I've never worked with kdb over anything other than
> serial, so this won't bother any of my normal workflows. That being
> said, at least as of a year ago someone on the lists was talking about
> using kdb with a keyboard and (presumably) a display. You can see a
> thread here:
>
> http://lore.kernel.org/r/20241031192350.GA26688@lichtman.org

I wonder which driver or kernel that person was using. None of the
current drivers would be working.

Best regards
Thomas

>
> Daniel may also have comments here?
>
> -Doug
>

--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)