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

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Message-ID: <CAHp75VeBaetiQBykfLk_weBHdzZF1nWp=k8BJu+OKNp6iYRRTg@mail.gmail.com>
Date: Mon, 24 Feb 2025 09:19:29 +0200
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: Adam Simonelli <adamsimonelli@...il.com>
Cc: Petr Mladek <pmladek@...e.com>, John Ogness <john.ogness@...utronix.de>, 
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>, Steven Rostedt <rostedt@...dmis.org>, 
	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 Mon, Feb 24, 2025 at 2:23 AM Adam Simonelli <adamsimonelli@...il.com> wrote:
> 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.

Thank you for elaboration. Even if this may be a different case, I
think it would be nice to have the PRINTK maintainers (who are also
involved in the console code) blessing before going in.

In case of a new version, please summarize above in the commit message.

-- 
With Best Regards,
Andy Shevchenko