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[v8,0/4] Optionally allow ttynull to be selected as a default console

Message ID 20250311033133.1859963-1-adamsimonelli@gmail.com
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Series Optionally allow ttynull to be selected as a default console | expand

Message

Adam Simonelli March 11, 2025, 3:31 a.m. UTC 
From: Adam Simonelli <adamsimonelli@gmail.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 psuedo 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

Adam Simonelli (4):
  ttynull: Always initialize console index to -1
  ttynull: Add an option to allow ttynull to be used as a console device
  tty: Change order of ttynull to be linked sooner if enabled as a
    console.
  Documentation/serial-console: Document
    CONFIG_NULL_TTY_DEFAULT_CONSOLE.

 Documentation/admin-guide/serial-console.rst |  4 +++-
 drivers/tty/Kconfig                          | 15 ++++++++++++++-
 drivers/tty/Makefile                         | 12 ++++++++++++
 drivers/tty/ttynull.c                        | 14 ++++++++++++++
 4 files changed, 43 insertions(+), 2 deletions(-)