Brightness control at startup with systemd

Question

I finally upgraded to systemd (I know... a bit later than I should have), and the transistion has been smooth, except for enabling brightness controll at boot.
I currently have a bit of a hack working to enable brightness control on my Dell XPS 13 ultrabook. I have two backlight interfaces (/sys/class/backlight/intel_backlight and /sys/class/backlight/acpi_video0), and I have found that the only way to alter my screen brightness is to set the intel_backlight brightness value to 0 and then alter the brightness file under the acpi_video0 interface. I must always write the 0 to the intel_backlight/brightness file, even if it already contains 0, to enable brightness control through acpi_video0.
I had had this code in my rc.local previously, and it worked exactly as I needed to enable brightness control:
#!/bin/bash
# set intel interface to 0, allowing acpi interface to take precedence
echo 0 > /sys/class/backlight/intel_backlight/brightness
# set the permissions of the acpi brightness file for video group (me) regular user modification
chgrp video /sys/class/backlight/acpi_video0/brightness
chmod 664 /sys/class/backlight/acpi_video0/brightness
# initialize brightness to a low level
echo 3 > /sys/class/backlight/acpi_video0/brightness
exit 0
Currently when I run this script normally it grants me the brightness control I desire through the modification of /sys/class/backlight/acpi_video0/brightness. I have made this script into a systemd service initiated at startup, with the service file below:
[Unit]
Description=Brightness Control Initialization
[Service]
Type=oneshot
ExecStart=/usr/lib/systemd/scripts/bright_init
RemainAfterExit=yes
[Install]
WantedBy=multi-user.target
My issue is that although the service is initiated at startup without error, my brightness control does not function like it does when I run the script manually myself. I have verified that systemd considers the service to have run successfully after startup, and I have also enabled both the multi-user.target and above service.
My questions are twofold:
1. Is there a better way to allow brightness control other than the somewhat hacky method of zeroing the /sys/class/backlight/intel_backlight/brightness file with the above script?
2. And does anyone have an idea as to why my service doesn't work, even though I can run the script after startup and it functions as I desire?
Last edited by tyler.heck (2012-11-16 01:45:46)

Answer 1

It is interesting to see that this is working for you. I had to add OWNER="root" to the rule. Otherwise when testing the rule with `udevadm test` I always recieved a permission denied error. But even with OWNER="root" Udev did not actually write the ATTR{brightness}="2" into the file.
Manually doing that by echo 2 > ... works well. I searched a bit on this topic and found a thread on the Gentoo forum where they say that it is not wise to use Udev for writing anything to /sys. Instead you should use Udev to run a script that does that:
[orschiro@thinkpad ~]$ cat /etc/udev/rules.d/50-powersave.rules
SUBSYSTEM=="power_supply", ENV{POWER_SUPPLY_ONLINE}=="0", RUN+="/home/orschiro/Scripts/powersaving battery"
SUBSYSTEM=="power_supply", ENV{POWER_SUPPLY_ONLINE}=="1", RUN+="/home/orschiro/Scripts/powersaving AC"
[orschiro@thinkpad ~]$ cat /usr/local/bin/powersaving
#!/bin/sh
case $1 in
battery)
echo "Running powersaving on AC in 3 seconds"
sleep 3
# screen power saving"
echo 200000 > /sys/class/backlight/intel_backlight/brightness
AC)
echo "Running powersaving on battery in 3 seconds"
sleep 3
# screen power saving
echo 4270725 > /sys/class/backlight/intel_backlight/brightness
esac
Last edited by orschiro (2013-03-01 15:29:02)