How to unload a kernel module which is in use and has a recursive module dependency?

Question

I habe a driver for my ELM 327 an it uses the pl2303 module which depends on the usbserial and this again depends on the pl2303 module.

What I tried:

sudo modprobe -r usbserial pl2303
sudo modprobe -r pl2303 usbserial
sudo modprobe -rf usbserial
sudo modprobe -rf pl2303
sudo rmmod --force pl2303
sudo rmmod --force usbserial

result of rmmod:

rmmod: ERROR: ../libkmod/libkmod-module.c:799 kmod_module_remove_module() could not remove 'usbserial': Resource temporarily unavailable
rmmod: ERROR: could not remove module usbserial: Resource temporarily unavailable

result of modprobe:

modprobe: FATAL: Module usbserial is in use.

Also I tried it with the drivers loaded and unloaded.

OS: Ubuntu 20.04

Answer 1

It's possible to blacklist kernel modules.

Checkout this post about How to blacklist kernel modules?

Answer 2

When you look into dmesg do you see a line where the device was detected when it was plugged in? Ubuntu should have FTDI support out of the box. Normally you would just access it by typing screen /dev/ttyUSB# 38400.