linux-kernel - Re: [PATCH v7 3/3] tty: Change order of ttynull to be linked sooner if enabled as a console.

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Message-ID: <CAHp75Ve02Ma1EswjnsxiV5uyh3A9NU0B90QLFGS13ifr594QFw@mail.gmail.com>
Date: Wed, 5 Mar 2025 21:22:06 +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 v7 3/3] tty: Change order of ttynull to be linked sooner
 if enabled as a console.

On Wed, Mar 5, 2025 at 6:30 AM <adamsimonelli@...il.com> wrote:
>
> From: Adam Simonelli <adamsimonelli@...il.com>
>
> If CONFIG_NULL_TTY_DEFAULT_CONSOLE is enabled, and CONFIG_VT is disabled,
> ttynull will become the default primary console device, based on the link
> order.
>
> 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_CONSOLE option will give users and distribution
> maintainers an option to avoid this. Disabling CONFIG_VT and enabling
> CONFIG_NULL_TTY_CONSOLE will ensure the default kernel console behavior
> is not dependant on hardware configuration by default, and avoid
> unexpected new behavior on devices connected to the /dev/ttyS0 serial
> port.

...

Since it touches the link order only under drivers/tty the commit
message should mention what the effect will be on the consoles drivers
for which are located elsewhere.

-- 
With Best Regards,
Andy Shevchenko