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

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Message-ID: <aScMprMh6Eh4JU5D@aspen.lan>
Date: Wed, 26 Nov 2025 14:20:22 +0000
From: Daniel Thompson <daniel@...cstar.com>
To: Thomas Zimmermann <tzimmermann@...e.de>
Cc: Daniel Thompson <danielt@...nel.org>,
	Doug Anderson <dianders@...omium.org>, 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,
	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

On Wed, Nov 26, 2025 at 02:32:32PM +0100, Thomas Zimmermann wrote:
> Hi
>
> Am 26.11.25 um 13:19 schrieb Daniel Thompson:
> > On Tue, Nov 25, 2025 at 07:26:33AM -0800, Doug Anderson wrote:
> > > On Tue, Nov 25, 2025 at 5:06 AM Thomas Zimmermann <tzimmermann@...e.de> wrote:
> > > > <snip>
> > > > 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?
> > TL;DR - I'm pretty relaxed about these changes... but I'd like
> >          to know how to test the changes.
> >
> > Like Doug I only really use kdb via serial but, since I'm maintain
> > the thing I do occasionally test kdb works on the qemu console. I don't
> > do it very often though because it's a manual test!
> >
> > I'd assume that will still work since it won't involve any of the
> > drivers above. I'm afraid I can't double check that since patch 4
> > doesn't apply cleanly in v6.18-rc7 (nor to linux-next... and neither
> > does the base-commit appear in linux-next).
>
> To test its effects, ignore this series and simply clear the two calbacks at
> [1]. This is where the debugger invokes fbcon. The series removes their
> implementation in the final patch.
>
> [1] https://elixir.bootlin.com/linux/v6.17.9/source/drivers/video/fbdev/core/fbcon.c#L3202

Thanks. Explanation in original cover letter was great and there's
certainly been no harm to QEMU.

Acked-by: Daniel Thompson (RISCstar) <danielt@...nel.org>


Daniel.