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Message-ID: <2024081824-leggings-omission-562a@gregkh>
Date: Sun, 18 Aug 2024 07:12:14 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: nerdopolis <bluescreen_avenger@...izon.net>
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 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?
thanks,
greg k-h
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