linux-kernel - [PATCH v9 0/1] Optionally allow ttynull to be selected as a default console

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Message-ID: <20250313210347.2987535-1-adamsimonelli@gmail.com>
Date: Thu, 13 Mar 2025 17:03:46 -0400
From: adamsimonelli@...il.com
To: linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jiri Slaby <jirislaby@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andy Shevchenko <andy.shevchenko@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	John Ogness <john.ogness@...utronix.de>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Petr Mladek <pmladek@...e.com>
Cc: Adam Simonelli <adamsimonelli@...il.com>
Subject: [PATCH v9 0/1] Optionally allow ttynull to be selected as a default console

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_DEFAULT_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 pseudo device, avoiding these
issues, without needing to enable the entire VT subsystem.

v2:
    rebase

v3:
    Clarify commit messages.

    Guard the all the register_console()s in ttynull to prevent it from being
    registered twice.

    Only change the link order if CONFIG_NULL_TTY_CONSOLE is enabled, otherwise
    use the existing order for ttynull if only CONFIG_NULL_TTY is enabled.

    Document why the link order changes in the drivers/tty/Makefile file.

    Replace #ifdefs

v4:
    Remember to actually include the changes to v3 in the cover letter.

v5:
    Correct code formatting in Makefile comment.

v6:
    Change to CONFIG_NULL_TTY_DEFAULT_CONSOLE

    Set the index to -1, and don't set the flags

    Use add_preferred_console() instead of register_console() in ttynull's
    console_initcall when CONFIG_NULL_TTY_DEFAULT_CONSOLE is enabled

v7
    Add a commit message to the first commit, and the Suggested-by

    Correct Kconfig tabs/spaces formatting

    Invert the console_set_on_cmdline check

v8
    Update Documentation.

    Remove the "CONFIG_" in the help text when mentioning other configuration
    options.

    Update commit message

v9
    Spell pseudo correctly in the cover letter.

    Use more clear, and suggested help text for NULL_TTY_DEFAULT_CONSOLE.

    Do not set ttynull's index to -1 anymore.

    Do not rely on link order, call add_preferred_console() in console_init().

    Merge the Documentation/admin-guide/serial-console.rst change into one commit.

Adam Simonelli (1):
  printk: Add an option to allow ttynull to be a default console device

 Documentation/admin-guide/serial-console.rst |  4 +++-
 drivers/tty/Kconfig                          | 19 ++++++++++++++++++-
 kernel/printk/printk.c                       |  5 +++++
 3 files changed, 26 insertions(+), 2 deletions(-)

-- 
2.45.2