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

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Message-ID: <CAD=FV=X_-t2AF5osp7Hamoe7WYE_2YWJZCaPaOj=9seSbnwwVA@mail.gmail.com>
Date: Tue, 25 Nov 2025 07:26:33 -0800
From: Doug Anderson <dianders@...omium.org>
To: Thomas Zimmermann <tzimmermann@...e.de>
Cc: simona@...ll.ch, airlied@...il.com, alexander.deucher@....com, 
	christian.koenig@....com, lyude@...hat.com, dakr@...nel.org, deller@....de, 
	mripard@...nel.org, maarten.lankhorst@...ux.intel.com, 
	jason.wessel@...driver.com, danielt@...nel.org, 
	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, 
	Nir Lichtman <nir@...htman.org>
Subject: Re: [PATCH 0/5] drm: Remove remaining support for kdb

Hi,

On Tue, Nov 25, 2025 at 5:06 AM Thomas Zimmermann <tzimmermann@...e.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

Daniel may also have comments here?

-Doug