On Wed, Mar 5, 2025 at 6:30 AM <adamsimonelli@xxxxxxxxx> wrote: > > From: Adam Simonelli <adamsimonelli@xxxxxxxxx> > > If CONFIG_NULL_TTY_DEFAULT_CONSOLE is enabled, and CONFIG_VT is disabled, > ttynull will become the default primary console device, based on the link > order. > > Many distributions ship with CONFIG_VT enabled. On tested desktop hardware > if CONFIG_VT is disabled, the default console device falls back to > /dev/ttyS0 instead of /dev/tty. > > This could cause issues in user space, and hardware problems: > > 1. The user space issues include the case where /dev/ttyS0 is > disconnected, and the TCGETS ioctl, which some user space libraries use > as a probe to determine if a file is a tty, is called on /dev/console and > fails. Programs that call isatty() on /dev/console and get an incorrect > false value may skip expected logging to /dev/console. > > 2. The hardware issues include the case if a user has a science instrument > or other device connected to the /dev/ttyS0 port, and they were to upgrade > to a kernel that is disabling the CONFIG_VT option, kernel logs will then be > sent to the device connected to /dev/ttyS0 unless they edit their kernel > command line manually. > > The new CONFIG_NULL_TTY_CONSOLE option will give users and distribution > maintainers an option to avoid this. Disabling CONFIG_VT and enabling > CONFIG_NULL_TTY_CONSOLE will ensure the default kernel console behavior > is not dependant on hardware configuration by default, and avoid > unexpected new behavior on devices connected to the /dev/ttyS0 serial > port. ... Since it touches the link order only under drivers/tty the commit message should mention what the effect will be on the consoles drivers for which are located elsewhere. -- With Best Regards, Andy Shevchenko
