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

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Message-ID: <38dcc504-5fa7-49ff-a74c-9a877fd267d5@suse.de>
Date: Wed, 26 Nov 2025 08:44:07 +0100
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, simona@...ll.ch,
 airlied@...il.com, alexander.deucher@....com, christian.koenig@....com,
 lyude@...hat.com, dakr@...nel.org, deller@....de, mripard@...nel.org,
 jason.wessel@...driver.com, danielt@...nel.org, dianders@...omium.org
Cc: dri-devel@...ts.freedesktop.org, amd-gfx@...ts.freedesktop.org,
 nouveau@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/5] drm: Remove remaining support for kdb

Hi

Am 25.11.25 um 17:25 schrieb Maarten Lankhorst:
> Hey,
>
> I'm glad to see the old kdb handler gone.
>
> Could we perhaps extend the drm panic handler somehow for this to work?
> Restore could potentially be simply duplicating and committing the current state.

Yeah, I briefly outlined this in the cover letter. If we wanted to 
support kdb, we could get the scanout buffer and use the buffer's simple 
display update for showing the debugger. I think this still requires 
quite some work, but would avoid all the issues with the current approach.

Best regards
Thomas

>
> Kind regards,
> ~Maarten Lankhorst
>
> Den 2025-11-25 kl. 13:52, skrev Thomas Zimmermann:
>> 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
>>
>>   Documentation/process/debugging/kgdb.rst    |  28 -----
>>   drivers/gpu/drm/amd/amdgpu/dce_v10_0.c      |  35 ++-----
>>   drivers/gpu/drm/amd/amdgpu/dce_v6_0.c       |  35 ++-----
>>   drivers/gpu/drm/amd/amdgpu/dce_v8_0.c       |  35 ++-----
>>   drivers/gpu/drm/drm_fb_helper.c             | 108 --------------------
>>   drivers/gpu/drm/nouveau/dispnv04/crtc.c     |  24 +----
>>   drivers/gpu/drm/radeon/atombios_crtc.c      |  74 ++++----------
>>   drivers/gpu/drm/radeon/radeon_legacy_crtc.c |  23 ++---
>>   drivers/gpu/drm/radeon/radeon_mode.h        |  10 +-
>>   drivers/video/fbdev/core/fbcon.c            |  24 -----
>>   drivers/video/fbdev/core/fbcon.h            |   1 -
>>   include/drm/drm_fb_helper.h                 |  21 ----
>>   include/drm/drm_modeset_helper_vtables.h    |  23 -----
>>   include/linux/fb.h                          |   4 -
>>   14 files changed, 63 insertions(+), 382 deletions(-)
>>
>>
>> base-commit: 0a21e96e0b6840d2a4e0b45a957679eeddeb4362

-- 
--
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)