Tests: USB: Add a README for Linux users

[fkjagodzinski]

Description

Explain how to overcome xHCI limitations in kernels prior to v4.17 and
successfully run USB tests.

CC @c1728p9 @studavekar

Pull request type

[ ] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[x] Docs update
[ ] Test update
[ ] Breaking change

[studavekar]

Thanks @fkjagodzinski for updating docs, Is there some good instruction to use eHCI instead of xHCI the controller.

[cmonr]

🎉

This is awesome. Thank you for the research!

[c1728p9]

Excellent investigation and good find @fkjagodzinski !

[cmonr]

CI started

[cmonr]

I think 41 mins from Open to Merge is a new repo record 😄

[fkjagodzinski]

(...) Is there some good instruction to use eHCI instead of xHCI the controller.

@studavekar
I simply changed one of the BIOS settings on a laptop I used. If you'd rather avoid rebooting your machine you can try using setpci command. I've tried it, and it works; unfortunately I wasn't able to re-enable xHCI afterwards, so I had to reboot anyway.

The other Linux machine I used did not have that eHCI/xHCI setting, so I ended up using a newer mainline kernel.