https://bugzilla.kernel.org/show_bug.cgi?id=208661
--- Comment #4 from Shannon Gaines (iknstudio@protonmail.com) --- As suggested in IRC, I tried nouveau.modeset=0. It didn't help with the backlight issue, without or without non-free firmware. So I believe the issue isn't related to modesetting.
To be clear, I tested: - nouveau + modeset=0 + no firmware - nouveau + no firmware - nouveau + modeset=0 + firmware - nouveau + firmware
And nothing helped.