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

[Helge Deller <deller@gmx.de>]

On 2/26/25 12:46, Arnd Bergmann wrote:
> 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).

Yes, it's rare. But on parisc, if no supported fbdev or drm
graphic card is found, it needs to stays on sticon (which always works).
Otherwise - if a card was found - the kernel switches dynamically to fbcon.

> 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.

As mentioned above, that should be optional then.

> 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.

Helge