On Wednesday, March 5, 2025 2:22:06 PM EST Andy Shevchenko wrote: > 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. > > OK, I will think of what to say. Based on what I tested in throwing a preferred console in x86 though, it seems like ttynull was still at the top of the list of /proc/consoles still, I just have to come up with a good way to mention that in the commit message.
