linux-kernel - Re: [PATCH 1/3] dummycon: only build module if there are users

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Message-Id: <98ed694b-0f2f-4bfb-9280-729842d9f95c@app.fastmail.com>
Date: Wed, 26 Feb 2025 14:18:46 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Javier Martinez Canillas" <javierm@...hat.com>,
 "Thomas Zimmermann" <tzimmermann@...e.de>, "Arnd Bergmann" <arnd@...nel.org>,
 "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
 "Helge Deller" <deller@....de>
Cc: linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] dummycon: only build module if there are users

On Wed, Feb 26, 2025, at 13:05, Javier Martinez Canillas wrote:
> "Arnd Bergmann" <arnd@...db.de> writes:
>> in drivers/video/console, with the simpler alternative just
>> calling into fbcon functions. I'm not sure if we can already drop
>> vgacon from normal x86-64 distro configs, i.e. if there are cases
>> that are not already covered by any of efi-earlycon, efifb,
>> vga16fb, vesafb/uvesafb or a PCI DRM driver.
>>
>
> I believe vgacon is still useful for x86 with legacy BIOS, because
> vesafb (and simpledrm) only works if the user defines a mode using
> the vga= kernel cmdline parameter.

Right, at the minimum, a configuration without vgacon would
have to pick a graphical mode in arch/x86/boot/video*.c if one
wasn't already set by grub.

Looking through the git history of that code, it seems that there
are lots of corner cases with weird 32-bit hardware.
Anything from the past 20 years is probably reasonably safe,
but there still needs to be a way to configure vgacon back in
to be safe.

    Arnd