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 (F1, 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 acpi_video0 directory.

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


After searching around on how to fix this, I found an answer explaining the meaning of theacpi_backlight kernel parameter.

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 ACPI video.ko.

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 acpi_backlight=vendor:

GRUB_CMDLINE_LINUX_DEFAULT="quiet acpi_backlight=vendor"

3. Updating grub

Finally, I had to update grub to produce the actual configuration file being loaded at boot.

sudo update-grub

4. Reboot

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 radeon_bl0.


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:

Cat terminator

published by Lazarus Lazaridis on Jun 24, 2018 in category how-to.