--- Comment #10 from Maurizio Galli firstname.lastname@example.org --- (In reply to Takashi Iwai from comment #9)
Ah, that's what I wrongly remembered, sorry.
But yes, testing with nouveau once would be interesting. This also allows testing with other older kernels more easily, too.
Switching to nouveau made no difference.
The last kernel allowing me to change display backlight is 5.7. So I guess the issue is introduced in kernel 5.8.
I'm attaching the dmesg and hwinfo next.