linux-kernel - Re: [PATCH v8 2/4] ttynull: Add an option to allow ttynull to be used as a console device

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Message-ID: <Z9GqI2HvOJaAOr-r@pathway.suse.cz>
Date: Wed, 12 Mar 2025 16:37:07 +0100
From: Petr Mladek <pmladek@...e.com>
To: adamsimonelli@...il.com
Cc: linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jiri Slaby <jirislaby@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andy Shevchenko <andy.shevchenko@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	John Ogness <john.ogness@...utronix.de>,
	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH v8 2/4] ttynull: Add an option to allow ttynull to be
 used as a console device

On Mon 2025-03-10 23:31:31, adamsimonelli@...il.com wrote:
> From: Adam Simonelli <adamsimonelli@...il.com>
> 
> The new config option, CONFIG_NULL_TTY_DEFAULT_CONSOLE will allow
> ttynull to be initialized by console_initcall() and selected as a
> possible console device.

It would be great to mention here the motivation. It is nicely
explained in the 3rd patch.

> --- a/drivers/tty/Kconfig
> +++ b/drivers/tty/Kconfig
> @@ -383,7 +383,20 @@ config NULL_TTY
>  	  available or desired.
>  
>  	  In order to use this driver, you should redirect the console to this
> -	  TTY, or boot the kernel with console=ttynull.
> +	  TTY, boot the kernel with console=ttynull, or enable
> +	  NULL_TTY_DEFAULT_CONSOLE.
> +
> +	  If unsure, say N.
> +
> +config NULL_TTY_DEFAULT_CONSOLE
> +	bool "Support for console on ttynull"
> +	depends on NULL_TTY=y && !VT_CONSOLE
> +	help
> +	  Say Y here if you want the NULL TTY to be used as a /dev/console
> +	  device.

I would provide few more details, something like:

<proposal>
	  Say Y here if you want the NULL TTY to be used as a /dev/console
	  device by default.

	  For example, it might be useful to prevent a VT-less kernel from
	  writing the system log to a random device connected to
	  the serial port.

	  Another console driver still might get preferred via the command line,
	  SPCR, or the device tree.
</proposal>

> +	  This is similar to VT_CONSOLE, but without the dependency on VT.
> +	  It uses the ttynull driver as the system console.

Honestly, I do not find this paragraph much useful. I would omit it,

>  	  If unsure, say N.
>  
> diff --git a/drivers/tty/ttynull.c b/drivers/tty/ttynull.c
> index d8b51edde628..67aad3e6f538 100644
> --- a/drivers/tty/ttynull.c
> +++ b/drivers/tty/ttynull.c
> @@ -96,6 +96,19 @@ static int __init ttynull_init(void)
>  	return 0;
>  }
>  
> +#ifdef CONFIG_NULL_TTY_DEFAULT_CONSOLE
> +static int __init ttynull_register(void)
> +{
> +	if (console_set_on_cmdline)
> +		return 0;
> +
> +	add_preferred_console("ttynull", 0, NULL);
> +
> +	return 0;
> +}
> +console_initcall(ttynull_register);
> +#endif

I have realized that this does not work without shuffling the linking
order (3rd patch).

I would prefer to avoid the linking order hack and rather call this
in console_init() directly, see
https://lore.kernel.org/r/Z9GPVVTnngGbmbuv@pathway.suse.cz
for more details.

Best Regards,
Petr