linux-kernel - Re: [PATCH v6 2/3] ttynull: Add an option to allow ttynull to be used as a console device

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Message-ID: <CAHp75VeMm3rmPto_31vWCVKh0z76h2zFjpjLUaiZ-gS8REEG+Q@mail.gmail.com>
Date: Tue, 4 Mar 2025 08:07:12 +0200
From: Andy Shevchenko <andy.shevchenko@...il.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>, 
	Steven Rostedt <rostedt@...dmis.org>, John Ogness <john.ogness@...utronix.de>, 
	Sergey Senozhatsky <senozhatsky@...omium.org>, Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH v6 2/3] ttynull: Add an option to allow ttynull to be used
 as a console device

On Tue, Mar 4, 2025 at 5:55 AM <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.

...

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

I'm now at the non-monospace font mail client, but it looks like you
have a TAB/space mix in the above for the indentation. Please, double
check that all lines, except the first in the option starts with a
single TAB and help lines (after the 'help' keyword) additionally
indented by 2 (two) spaces.

>           If unsure, say N.

...

> +#ifdef CONFIG_NULL_TTY_DEFAULT_CONSOLE
> +static int __init ttynull_register(void)
> +{
> +       if (!console_set_on_cmdline)

Invert this. It might be that we want more in the future and this
conditional is not-scalable for that. Inverted does the trick. To be
precise, do like the fb driver for xen does:

  /* User defined console is present, avoid registering "ttynull" */
  if (..._set_on_cmdline)
    return 0;

> +               add_preferred_console("ttynull", 0, NULL);;
> +
> +       return 0;
> +}
> +console_initcall(ttynull_register);
> +#endif

-- 
With Best Regards,
Andy Shevchenko