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

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Message-ID: <87mse8zzb0.fsf@minerva.mail-host-address-is-not-set>
Date: Wed, 26 Feb 2025 13:05:23 +0100
From: Javier Martinez Canillas <javierm@...hat.com>
To: Arnd Bergmann <arnd@...db.de>, 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

"Arnd Bergmann" <arnd@...db.de> writes:

Hello Arnd,

> On Wed, Feb 26, 2025, at 09:16, Thomas Zimmermann wrote:
>> Am 26.02.25 um 08:55 schrieb Arnd Bergmann:
>> Here's another general question. vgacon and fbcon only seem usable with 
>> CONFIG_VT=y. Wouldn't it make sense to have them depend on CONFIG_VT=y? 
>> dummycon could then be implemented as part of the vt code, maybe even 
>> become a vt-internal thing. The console code is complex, so I'm probably 
>> missing something here?
>
> I think in theory one may have a system use fbcon purely to get the
> boot logo, but not actually support VT.  I had also assumed there might
> be a way to use fbcon as the console (i.e. printk) but not register
> the tty, but it looks like the console code still requires vt.
>
> After I looked at the vt and conswitchp code some more, I wonder
> if we could go the other way and instead of integrating it more
> make the conswitchp logic optional: most of the complexity here
> deals with switching between text console and fbcon dynamically,
> but having any text console support is getting very rare (vga
> on alpha/mips/x86-32, newport on mips-ip22, sti on parisc).
>
> If we do this, the conswitchp code could be merged with dummycon

This sounds like a much better approach indeed.

> 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.

>     Arnd
>

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat