In this post I will describe the steps I had to follow to be able to change the brightness on my iMac using the related keyboard function keys.
After installing Debian 9.4 (stretch) on my iMac, I was no longer able to control the brightness from the keyboard (
F2 keys). When pressing these keys, although a notification was shown for increasing/decreasing it, the brightness was not changing.
Identifying the problem
In my case, I had two entries for the backlight class in my system.
$ ls /sys/class/backlight/ acpi_video0 radeon_bl0
The keyboard keys were affecting the brightness value stored in the file
brightness located in the
I tried to change the brightness level manually in the
brightness file located in the second directory, the one named
radeon_bl0 and the iMac’s brightness did actually change.
echo 190 | sudo tee /sys/class/backlight/radeon_bl0/brightness
So, all I had to do was to set my grub configuration to pass the
vendor value in the
acpi_backlight kernel parameter upon boot so that the vendor specific driver (in my case
Radeon) is preferred instead of the default
1. Editing the grub configuration file
I edited the grub configuration file with vim:
sudo vim /etc/default/grub
2. Setting the kernel parameter in the appropriate section
In the line starting with
GRUB_CMDLINE_LINUX_DEFAULT I appended the desired kernel parameter
3. Updating grub
Finally, I had to update grub to produce the actual configuration file being loaded at boot.
After rebooting, I was able to manage the brightness with the brightness function keys.
In addition, the
/sys/class/backlight directory now lists only one directory, the vendor’s one
The steps described in this post, have been tested on my system with:
- Product Name: iMac11,3
- VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Broadway PRO [Mobility Radeon HD 5850]
and I can’t know if they are valid for other cases. Follow them at your own risk :)
To find your iMac model, use:
sudo dmidecode | grep -A 9 "System Information"
and your VGA compatible controller:
lspci | grep "VGA compatible controller"
That’s all, cat photo: