linux-kernel - Re: [PATCH v4 0/2] Optionally allow ttynull to be selected as a default console

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Message-ID: <7410687.31r3eYUQgx@nerdopolis2>
Date: Sun, 23 Feb 2025 19:23:21 -0500
From: Adam Simonelli <adamsimonelli@...il.com>
To: Petr Mladek <pmladek@...e.com>, John Ogness <john.ogness@...utronix.de>,
 Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
 Steven Rostedt <rostedt@...dmis.org>,
 Andy Shevchenko <andy.shevchenko@...il.com>
Cc: linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
 Jiri Slaby <jirislaby@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject:
 Re: [PATCH v4 0/2] Optionally allow ttynull to be selected as a default
 console

On Sunday, February 23, 2025 4:10:10 PM EST Andy Shevchenko wrote:
> Sun, Feb 23, 2025 at 03:44:54PM -0500, adamsimonelli@...il.com kirjoitti:
> > From: Adam Simonelli <adamsimonelli@...il.com>
> > 
> > When switching to a CONFIG_VT=n world, at least on x86 systems,
> > /dev/console becomes /dev/ttyS0. This can cause some undesired effects.
> > /dev/console's behavior is now tied to the physical /dev/ttyS0, which when
> > disconnected can cause isatty() to fail when /dev/ttyS0 is disconnected,
> > and users who upgrade to a theoretical vt-less kernel from their
> > distribution who have a device such as a science instrument connected to
> > their /dev/ttyS0 port will suddenly see it receive kernel log messages.
> > 
> > When the new CONFIG_NULL_TTY_CONSOLE option is turned on, this will allow
> > the ttynull device to be leveraged as the default console. Distributions
> > that had CONFIG_VT turned on before will be able to leverage this option
> > to where /dev/console is still backed by a psuedo device, avoiding these
> > issues, without needing to enable the entire VT subsystem.
> 
> This rings a bell of the following
> 
> https://lore.kernel.org/all/20201111135450.11214-1-pmladek@suse.com/
> https://lore.kernel.org/all/20210107164400.17904-1-pmladek@suse.com/
> https://lore.kernel.org/all/20210108114847.23469-1-pmladek@suse.com/
> 
> I don't see any mention in the commit message about these, have you studied the
> cases? Will your change anyhow affect the described there?
> 
> 
I did see that sifting through commits, it looks kind of different though, as
that was to make ttynull more always on, and if I am understanding it correctly
it looks more of a last resort? I could be wrong, but I see it was attempted
and reverted because of conflicts on some hardware platforms?

The scope with this new patch set is much different, as it has to be manually
enabled on top of CONFIG_NULL_TTY, rather than assuming CONFIG_NULL_TTY is
enabled, and adding it to the list of preferred consoles. It is more for
Desktop configs that are looking to disable CONFIG_VT, but still want
/dev/console to not be a physical device by default.