[SOLVED] Udev rule to hibernate my laptop at certain battery level

Question

Hi!
I'm needing this kind of udev rule, to have my laptop automatically hibernate when battery level is about 3% (or 5, it doesn't matter).
I don't want to have a little script running in the background that every eg 5 seconds polls my battery level, because i find it a really rough system.
I guess i can use udev to reach what i want.
udevadm info --path=/sys/class/power_supply/BAT0
P: /devices/LNXSYSTM:00/device:00/PNP0A08:00/device:0f/PNP0C09:00/PNP0C0A:00/power_supply/BAT0
E: DEVPATH=/devices/LNXSYSTM:00/device:00/PNP0A08:00/device:0f/PNP0C09:00/PNP0C0A:00/power_supply/BAT0
E: POWER_SUPPLY_CHARGE_FULL=4800000
E: POWER_SUPPLY_CHARGE_FULL_DESIGN=4800000
E: POWER_SUPPLY_CHARGE_NOW=4800000
E: POWER_SUPPLY_CURRENT_NOW=0
E: POWER_SUPPLY_CYCLE_COUNT=0
E: POWER_SUPPLY_MANUFACTURER=SANYO
E: POWER_SUPPLY_MODEL_NAME=AS07B32
E: POWER_SUPPLY_NAME=BAT0
E: POWER_SUPPLY_PRESENT=1
E: POWER_SUPPLY_SERIAL_NUMBER= 7210
E: POWER_SUPPLY_STATUS=Full
E: POWER_SUPPLY_TECHNOLOGY=Li-ion
E: POWER_SUPPLY_VOLTAGE_MIN_DESIGN=14800000
E: POWER_SUPPLY_VOLTAGE_NOW=16142000
E: SUBSYSTEM=power_supply
Any help?
Thank you very much!
Last edited by nierro (2012-09-21 20:36:45)

Answer 1

It is indeed udev, displaying device info from its database in response to a query - that's what 'udevadm info' does. But you can't write a udev rule against that info - you need a uevent.
man udev wrote:The udev daemon, systemd-udevd.service(8), receives device uevents directly from the kernel whenever a device is added or removed from the system, or it changes its state. When udev receives a device event, it matches its configured set of rules against various device attributes to identify the device. Rules that match may provide additional device information to be stored in the udev database or to be used to create meaningful symlink names.
If you run cybertorture's 'udevadm monitor' command above, as your battery is running down, you will not see any uevents. If you connect or disconnect your AC adapter, you will see uevents - in fact, you're using those ones already, to trigger your powersave script. So although the general description I've quoted mentions kernel uevents generated when a device "changes its state", this does not cover all possible changes, and in this particular case, it does not cover changes in battery charge level.