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

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Message-Id: <79d35e3b-9a0d-41a5-ab07-797bfa1e19cb@app.fastmail.com>
Date: Wed, 26 Feb 2025 12:46:56 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "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 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
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.

    Arnd