Arch Linux User Repository

Just an FYI: I had to add myself to the i2c group and copy over the ddcci udev rules to get this working on startup

Well, it depends on situation. There is no hang at boot now, I confirm, but ddcci_backlight still can't detect external (HDMI) monitor during sddm.service startup. So, in my use case (laptop with intel's video and external HDMI monitor) I still need systemd unit with Before=sddm.service.

fyi once the package is updated to 0.4 Andrej.Korshikov workaround will no longer be needed, ddcci_backlight can be loaded from /etc/modules-load.d/

Andrej.Korshikov you are right. So I am going to correct myself: you don't need this dkms if you want to control backlight only on one screen :-).

Hi jiri.one, thanks for the info. From that reddit thread,

PowerDevil cannot deal with more than one brightness source,

so

while both of the monitors change their brightness value on the same scale (from 0 to 100), the effective brightness is different because the two have different backlight illumination at the same value.

For example, my brightness values usually are 65 for laptop screen and 40 for external monitor. As far as I understand I can't independently manage brightness values for different monitors without this dkms. I would be glad if I'm wrong.

And, without this dkms there is no backlight device in /sys/class/backlight for my external monitor.

This dkms is no more needed, because now is powerdevil compiled with

-DHAVE_DDCUTIL=On

But still is needed to have i2c_dev loaded before sddm, so the systemd unit, which I am using is:

[Unit]
After=multi-user.target
Before=sddm.service

[Service]
Type=oneshot
ExecStart=/usr/bin/modprobe i2c_dev
ExecStop=/usr/bin/modprobe --remove i2c_dev
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

More info is here: https://www.reddit.com/r/kde/comments/epgs13/kde_my_journey_to_get_a_brighness_slider_for_my/

Andrej.Korshikov thanks a lot for your comment, it really helped me.

/etc/systemd/system/ddcci-backlight.service:

# Placing "ddcci_backlight" into /etc/modules-load.d
# leads to hang on boot with external (HDMI) monitor
# connected to the laptop, so we need to add the module later.

# And ddcci_backlight can't detect monitor during sddm.service startup.

[Unit]
After=multi-user.target
Before=sddm.service

[Service]
Type=oneshot
ExecStart=/usr/bin/modprobe ddcci_backlight
ExecStop=/usr/bin/modprobe --remove ddcci_backlight
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

@Chrissss What if you have a linux-lts kernel?

dkms already has an optional dependency on all the kernels in the repository.

I think the linux-headers should be a dependecy. Without them I get the following error, when installing the PKGBUILD:

[...]
(2/3) Install DKMS modules
==> Unable to install module ddcci/0.3.3 for kernel *: Missing kernel headers.
(3/3) Removing old cached pacakges...
[...]

After running pacman -S linux-headers the module gets built and installed successfully.