linux-kernel - Re: [PATCH v11 1/1] printk: Add an option to allow ttynull to be a default console device

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Message-ID: <CAHp75VdaDyyNEKi=S4Y9wKR-Dqb0M30c+rHVs6FiWc3L0vc=Uw@mail.gmail.com>
Date: Fri, 14 Mar 2025 15:55:07 +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 v11 1/1] printk: Add an option to allow ttynull to be a
 default console device

On Fri, Mar 14, 2025 at 2:52 PM <adamsimonelli@...il.com> wrote:
>
> From: Adam Simonelli <adamsimonelli@...il.com>
>
> The new option is CONFIG_NULL_TTY_DEFAULT_CONSOLE.
>
> if enabled, and CONFIG_VT is disabled, ttynull will become the default
> primary console device.
>
> ttynull will be the only console device usually with this option enabled.
> Some architectures do call add_preferred_console() which may add another
> console though.
>
> Motivation:
>
> Many distributions ship with CONFIG_VT enabled. On tested desktop hardware
> if CONFIG_VT is disabled, the default console device falls back to
> /dev/ttyS0 instead of /dev/tty.
>
> This could cause issues in user space, and hardware problems:
>
> 1. The user space issues include the case where  /dev/ttyS0 is
> disconnected, and the TCGETS ioctl, which some user space libraries use
> as a probe to determine if a file is a tty, is called on /dev/console and
> fails. Programs that call isatty() on /dev/console and get an incorrect
> false value may skip expected logging to /dev/console.
>
> 2. The hardware issues include the case if a user has a science instrument
> or other device connected to the /dev/ttyS0 port, and they were to upgrade
> to a kernel that is disabling the CONFIG_VT option, kernel logs will then be
> sent to the device connected to /dev/ttyS0 unless they edit their kernel
> command line manually.
>
> The new CONFIG_NULL_TTY_DEFAULT_CONSOLE option will give users and
> distribution maintainers an option to avoid this. Disabling CONFIG_VT and
> enabling CONFIG_NULL_TTY_DEFAULT_CONSOLE will ensure the default kernel
> console behavior is not dependent on hardware configuration by default, and
> avoid unexpected new behavior on devices connected to the /dev/ttyS0 serial
> port.

> Signed-off-by: Adam Simonelli <adamsimonelli@...il.com>

You forgot to collect Petr's tags. Use `b4` (should be available in
your distro) to gather them from the mailing list (as of the previous
round of the review).

-- 
With Best Regards,
Andy Shevchenko