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_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.

Adam Simonelli (2):
  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.

 drivers/tty/Kconfig   | 15 ++++++++++++++-
 drivers/tty/Makefile  | 10 ++++++++++
 drivers/tty/ttynull.c | 20 +++++++++++++++++++-
 3 files changed, 43 insertions(+), 2 deletions(-)

-- 
2.45.2