The printk changes submitted for the Linux 6.15 kernel introduce a new “NULL_TTY_DEFAULT_CONSOLE” Kconfig build-time option for allowing the null TTY to be the default for those building the Linux kernel without virtual terminal (VT) support.
There has long been the ability to build the Linux kernel without VT support via the “CONFIG_VT” switch but in that case the default output becomes the serial console. With the new CONFIG_NULL_TTY_DEFAULT_CONSOLE, the null TTY output can become the default in these VT-less kernel configurations.
The patch from Adam Simonelli elaborates:
“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.
ttynull will be the only console device usually with this option enabled. Some architectures do call add_preferred_console() which may add another console though.
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.”
This option is part of the printk changes for the Linux 6.15 kernel. Separately this pull also introduces a new “printk.debug_non_panic_cpus” option to store printk messages from non-panic CPUs during a kernel panic.
