I have mid 2011 Apple Imac A1311 (Full specs) with Radeon HD 6750M
$ lspci | grep VGA 01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Whistler [Radeon HD 6630M/6650M/6750M/7670M/7690M]e
I have acpi_video0 (and nothing else) but it ignores all my changes in /sys/class/backlight/acpi_video0/brightness
I tried all of those grub options but still nothing
acpi_backlight=video acpi_backlight=vendor acpi_backlight=native
I tried solutions from https://archived.forum.manjaro.org/t/solved-imac-27-mid-2011-radeon-hd-6770m-brightness-control/88672/5 too, didn't work for me (because of different GPU, I guess).
any ideas ?