Enable systemd-backlight@intel_backlight instead of systemd-backlight@acpi_video0

Question

I have 2 files in /sys/class/backlight/ - acpi_video0 and intel_backlight. intel_backlight is the one which controls my screen brightness (echo/cat), however acpi_video0 is the default.

By default, the service systemd-backlight@acpi_video0.service is enabled which should save/restore brightness at shutdown/startup, but it doesn't work since it points to the wrong file.

When trying to enable systemd-backlight@intel_backlight.service instead, I get this message:

The unit files have no [Install] section. They are not meant to be enabled
using systemctl.
Possible reasons for having this kind of units are:
1) A unit may be statically enabled by being symlinked from another unit's
   .wants/ or .requires/ directory.
2) A unit's purpose may be to act as a helper for some other unit which has
   a requirement dependency on it.
3) A unit may be started when needed via activation (socket, path, timer,
   D-Bus, udev, scripted systemctl call, ...).

How do I fix this?

Solution

It may be that you're just missing the backlight: part of the command:

# systemctl enable systemd-backlight@backlight:intel_backlight.service

You may also need to escape the :, i.e.:

# systemctl enable systemd-backlight@backlight\:intel_backlight.service

Since you have two folders in /sys/class/backlight/, you probably have two graphics cards. The Arch wiki suggests adding one of the following kernel parameters in your bootloader:

acpi_backlight=video
acpi_backlight=vendor
acpi_backlight=native
acpi_backlight=none

Comments

[Admin]

Side question: Is it normal to have two files in /sys/class/backlight/?

[Admin]

Don't know if it's normal but I have 2 dirs there on my laptop on default install.