linux-kernel - Re: VT-less kernels, and /dev/console on x86

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Message-ID: <3669532.hdfAi7Kttb@nerdopolis2>
Date: Sun, 18 Aug 2024 08:33:25 -0400
From: nerdopolis <bluescreen_avenger@...izon.net>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: pmladek@...e.com, rostedt@...dmis.org, john.ogness@...utronix.de,
 senozhatsky@...omium.org, tglx@...utronix.de, tony@...mide.com,
 linux-kernel@...r.kernel.org
Subject: Re: VT-less kernels, and /dev/console on x86

On Sunday, August 18, 2024 1:12:14 AM EDT Greg KH wrote:
> On Sat, Aug 17, 2024 at 08:09:20PM -0400, nerdopolis wrote:
> > Hi
> > 
> > I originally brought this up on linux-serial, but I think it makes more sense
> > that it's part of how printk console device selection works. Without VTs, while
> > most software is able to handle the situation, some userspace programs expect
> > /dev/console to still be responsive. Namely systemd. It calls isatty() against
> > /dev/console, and since /dev/console on VT-less systems currently defaults to
> > /dev/ttyS0, and when /dev/ttyS0 is disconnected, the ioctl's fail, and it
> > refuses to write log messages to it.
> > 
> > There doesn't seem to be a mailing list for printk, so I had to use
> > get_maintainer.pl. Hopefully this is correct
> > 
> > 
> > After some grepping and guessing and testing, and playing around Something like
> > diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
> > index a45d423ad10f..f94a4632aab0 100644
> > --- a/drivers/tty/Kconfig
> > +++ b/drivers/tty/Kconfig
> > @@ -384,9 +384,12 @@ config NULL_TTY
> >  
> >           In order to use this driver, you should redirect the console to this
> >           TTY, or boot the kernel with console=ttynull.
> > -
> >           If unsure, say N.
> >  
> > +config NULL_TTY_CONSOLE
> > +        bool "Supports /dev/ttynull as a console automatically"
> > +        depends on NULL_TTY && !VT_CONSOLE
> > +
> >  config VCC
> >         tristate "Sun Virtual Console Concentrator"
> >         depends on SUN_LDOMS
> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index dddb15f48d59..c1554a789de8 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -3712,6 +3712,11 @@ void __init console_init(void)
> >         initcall_t call;
> >         initcall_entry_t *ce;
> >  
> > +#ifdef CONFIG_NULL_TTY_CONSOLE
> > +       if (!strstr(boot_command_line, "console="))
> > +               add_preferred_console("ttynull", 0, NULL);
> > +#endif
> > +
> >         /* Setup the default TTY line discipline. */
> >         n_tty_init();
> >  
> > 
> > 
> > 
> > seems to work, it conflicts with CONFIG_VT_CONSOLE since it is effectively
> > redundant, it is optional, so that it doesn't cause any changes to
> > configurations, that historically had CONFIG_VT_CONSOLE turned off in the past,
> > and for bootloader configs, it won't change any behavior if the kernel command
> > line has a console device specified
> 
> What is wrong with just setting the kernel command line for this
> instead?
> 
When they eventually start shipping kernels without VTs, they will then have to
include a script in their upgrade process that runs

sed -i "s/GRUB_CMDLINE_LINUX_DEFAULT=\"/GRUB_CMDLINE_LINUX_DEFAULT=\"nomodeset /g" /etc/default/grub

(Assuming they are using Grub)
> thanks,
> 
> greg k-h
>