Adding power related targets to systemd

systemd has a bunch of nice "power" related targets that come shipped by default. For example, sleep.target is extremely useful if you want to lock your screen on sleep.

One thing which systemd doesn't have, at least yet, is targets which relate to going on to and coming back off of AC and battery power. This is pretty easy to do, though, and provides the convenience of sleep.target for events like this. I want to use these to do some power tweaks when coming on to and off of battery.

First off, create the two targets that you want to use for AC/battery:

cat > /etc/systemd/system/ac.target << 'EOF'
[Unit]
Description=On AC power
DefaultDependencies=no
StopWhenUnneeded=yes
EOF

cat > /etc/systemd/system/battery.target << 'EOF'
[Unit]
Description=On battery power
DefaultDependencies=no
StopWhenUnneeded=yes
EOF

A systemd "target" is essentially a systemd unit that is used to perform actions when a certain state is reached in the system. We'll be using these as the binding points for services that we want to run when we switch to and from AC/battery.

Now, we need to tell udev to start ac.target when it sees us coming on to AC, and start battery.target when it sees us coming off of AC. To find the events we need to attach to, we can use udevadm monitor --environment:

# udevadm monitor --environment
UDEV  [7041.262327] change   /devices/.../power_supply/AC (power_supply)
[...]
POWER_SUPPLY_ONLINE=0

You can also use udevadm info -a -p /sys/class/power_supply/AC to tie this information to the relevant keys in udev:

% udevadm info -a -p /sys/class/power_supply/AC

looking at device '/devices/LNXSYSTM:00/LNXSYBUS:00/PNP0A08:00/device:19/PNP0C09:00/ACPI0003:00/power_supply/AC':
   KERNEL=="AC"
   SUBSYSTEM=="power_supply"
   DRIVER==""
   ATTR{SUBSYSTEM}=="power_supply"
   ATTR{POWER_SUPPLY_NAME}=="AC"
   ATTR{POWER_SUPPLY_ONLINE}=="1"

Based on this, we can see that we need to attach our target startup to the power_supply object, on the online attribute. All we have to do now is to tell udev to do this by creating a rules file:

cat > /etc/udev/rules.d/99-powertargets.rules << 'EOF'
SUBSYSTEM=="power_supply", KERNEL=="AC", ATTR{online}=="0", RUN+="/usr/sbin/systemctl start battery.target"
SUBSYSTEM=="power_supply", KERNEL=="AC", ATTR{online}=="1", RUN+="/usr/sbin/systemctl start ac.target"
EOF

We can now reload and apply udev's new config:

# udevadm control --reload-rules

We can now test it out by unplugging the power and seeing what happens:

% sudo systemctl status battery.target
● battery.target - On battery power
   Loaded: loaded (/etc/systemd/system/battery.target; static; vendor preset: disabled)
   Active: inactive (dead)

Oct 29 12:24:33 roujiamo systemd[1]: Reached target On battery power.
Oct 29 12:24:33 roujiamo systemd[1]: battery.target: Unit not needed anymore. Stopping.
Oct 29 12:24:33 roujiamo systemd[1]: Stopped target On battery power.

Sweet, it works! Now that everything is setup, we can add the services that we want to start based on these targets. Take this as an example, and note the [Install] section. You need to enable the service, as well, or the symlinks to battery.target.wants won't be created.

% systemctl cat powerdown.service
# /etc/systemd/system/powerdown.service
[Unit]
Description=Laptop battery savings

[Service]
Type=oneshot
ExecStart=/usr/local/bin/powerdown

[Install]
WantedBy=battery.target

Try unplugging again after enabling the service, and you should see it working, too:

% sudo systemctl status powerdown
● powerdown.service - Laptop battery savings
   Loaded: loaded (/etc/systemd/system/powerdown.service; enabled; vendor preset: disabled)
   Active: inactive (dead) since Sun 2017-10-29 02:24:33 GMT; 22min ago
  Process: 15977 ExecStart=/usr/local/bin/powerdown (code=exited, status=0/SUCCESS)
 Main PID: 15977 (code=exited, status=0/SUCCESS)

Oct 29 12:24:33 roujiamo systemd[1]: Starting Laptop battery savings...
Oct 29 12:24:33 roujiamo systemd[1]: Started Laptop battery savings.

Plug it back in, and you should also see the same for whatever you have attached to ac.target.