In order to automatically update my monitor setup and activate/deactivate my external monitor when plugging my ThinkPad into its dock, I found a way to hook into the ACPI events and run arbitrary scripts.

This was tested on a T420 with a ThinkPad Dock Series 3 as well as a T440p and a T460p with a ThinkPad Ultra Dock.

The only requirement is the ThinkPad kernel module. On most ThinkPads it's the tp_smapi module (which you can find in the tp-smapi-dkms package in Debian) but on newer hardware, that interface is gone and you can simply use the thinkpad_acpi module built into the kernel. That's what generates the ibm/hotkey events we will listen for.

Hooking into the events

Create the following ACPI event scripts as suggested in this guide.

Firstly, /etc/acpi/events/thinkpad-dock:

event=ibm/hotkey LEN0068:00 00000080 00006030
action=su francois -c "/home/francois/bin/external-monitor dock"

Secondly, /etc/acpi/events/thinkpad-undock:

event=ibm/hotkey LEN0068:00 00000080 00004011
action=su francois -c "/home/francois/bin/external-monitor undock"

then restart acpid:

sudo systemctl restart acpid.service

Note that I'm not using the real "docking" event (ibm/hotkey LEN0068:00 00000080 00004010) because it seems to be triggered too early and the new displays aren't ready.

Finding the right events

To make sure the events are the right ones, lift them off of:

sudo acpi_listen

and ensure that your script is actually running by adding:

logger "ACPI event: $*"

at the begininng of it and then looking in /var/log/syslog for lines like:

logger: external-monitor undock
logger: external-monitor dock

If that doesn't work for some reason, try using an ACPI event script like this:

action=logger %e

to see which event you should hook into.

Using xrandr inside an ACPI event script

Because the script will be running outside of your user session, the xrandr calls must explicitly set the display variable (-d). This is what I used:

logger "ACPI event: $*"
xrandr -d :0.0 --output DP2 --auto
xrandr -d :0.0 --output eDP1 --auto
xrandr -d :0.0 --output DP2 --left-of eDP1