How to Write to Tty from a Kernel Module

How can I write to TTY from a kernel module?

use tty = get_current_tty(); instead of tty = current->signal->tty;

that's it

you need to lock the tty before accessing it and get_current_tty does it internally

NOTE: get_current_tty is under EXPORT_SYMBOL_GPL, hence your module or code

How to write to a tty from kernel space with only major and minor device numbers available?

You don't do this by passing major/minor numbers to your syscall.

Instead, have the userspace side open the tty it wants then pass the resulting file descriptor to your syscall. You then use the sequence { fget(); kernel_write(); fput(); } to write to the supplied file descriptor.

How to write to `/dev/ttyUSB?` in kernel space?

SappyInsane on linuxquestions had the same problem and gave me his solution which worked
https://www.linuxquestions.org/questions/linux-kernel-70/reading-from-arduino-serial-in-a-kernel-module-4175704822/

How to write read() with poll() functionality in kernel for tty driver?

There is no read callback. The tty core buffers the data received by the tty drivers in a structure called struct tty_flip_buffer. Read Chapter 18 of LDD3

Custom char device linux module under stock major (like tty)

If you want your device to be exposed as a TTY, implement it as a serial driver, similar to other devices in drivers/tty/serial. The kernel will apply the TTY layer to it, and it will show up as a TTY.