Screen brightness on iMac

My screen brightness doesn't work as many others have reported. So I dug around...

The following ls /sys/class/backlight generates two entries:

acpi_video0 radeon_bl0

This does nothing:
echo 20 > /sys/class/backlight/acpi_video0/brightness

This however does:
echo 20 > /sys/class/backlight/radeon_bl0/brightness

It makes me think the brightness control is targeting the wrong one. How can I fix this?

Ok so this solved it.


sudo nano /etc/default/grub

Add acpi_backlight=vendor to GRUB_CMDLINE_LINUX_DEFAULT and GRUB_CMDLINE_LINUX so it looks like this:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"

Save and and update grub:

sudo update-grub

Reboot and disco.

Fix this in installation. Or at the very least add it to the FAQ. This was way to hard to figure out.