Display KVM switch on Linux with udev

  • Published 05 Sep 2020
  • Category software

Following up on Haim Gelfenbeyn’s excellent article on defining a software KVM display switch by using DCC/CI instructions, I made a Linux “version” with udev.

It essentially the same idea, listening for USB device events to see if the USB KVM switch is added or removed and issue a DCC/CI command to switch the monitor as well.

I made it for my specific setup: I have a main, always-on, Linux box and additional laptops, so all the logic is driving by the Linux machine, which simplifies installation a bit. I used this USB KVM switch.

DCC/CI commands with dccutils

I didn’t know this was a thing, but it is possible to control monitors from a computer by issuing DCC/CI commands!
dccutils is a tool to inspect the available capabilities and options of a monitor and emit commands such as “changing inputs”, changing brightness, etc…
I followed this post to query my monitor’s interface and tried first in a shell to change the input between DisplayPort (my Linux box) and HDMI-2 (my work MacBook), which in my case resulted in those commands:

# Set input to DisplayPort
sudo ddcutil setvcp 0x60 0x0f
# Set input to HDMI 2
sudo ddcutil setvcp 0x60 0x12

Need to be run as sudo since DCC/CI is implemented with the I2C protocol, which is fairly low-level and bad instructions could brick/crash the monitor.

Triggering on udev rules

Cool, we know how to set the monitor input from a machine! Now we just need to listen to USB event to see when the USB switch goes in or out and issue to corresponding ddcutils command. Enter udev !

udev is a Linux subsystem for managing device events, especially to respond to “hotplug” events, and run arbitrary code based on matching rules. The events come from the kernel but the resulting actions are executed in userspace. Useful for things like automatically backing up the content of a specific USB key to disk, etc…

Rules available in /etc/udev/rules.d on a Debian machine and look like this:

ACTION=="add", SUBSYSTEM=="USB", ATTR{idVendor}=="XXX", ATTR{idProduct}=="YYY", RUN+="/my/handler/script.sh

Which in this case instructs: if an USB event of type “add” is detected with the device’s vendor id of XXX and product of YYY then run the handler script. A good udev guide can be found here

Reload after adding/changing rules:

sudo udevadm control --reload
sudo udevadm trigger

Identifying the USB KVM device

Almost there! We just need to identify a few unique attributes for our rules. I used lsusb and turned on and off the device a few times to identify it:

Bus 007 Device 020: ID 05e3:0610 Genesys Logic, Inc. 4-port hub

Where vendorId is 05e3 and productId is 0610. We will use those attributes to match against “add” events and defined as ATTR{vendorId} and ATTR{productId} in the udev rule.
For the “remove” events, the story is a bit different: the remove event don’t have the same attributes/fields due to the device being removed and thus some data are not accessed/accessible anymore.

To find out, I ran udevadm monitor --environment add looked at the resulting “remove” event:


KERNEL[1635240.722923] remove   /devices/pci0000:00/0000:00:08.1/0000:2f:00.3/usb7/7-4/7-4:1.0 (usb)


I will match the remove event on the PRODUCT, which will be defined as ENV{PRODUCT} in the rule

Tidying up

We got all the moving parts for our switch !

The simplest implementation, for me, would be:


ACTION=="add", SUBSYSTEM=="usb", ATTR{idVendor}=="05e3", ATTR{idProduct}=="0610", RUN+="/usr/local/bin/kvm_added.sh"
ACTION=="remove", SUBSYSTEM=="usb", ENV{PRODUCT}=="5e3/610/9226", RUN+="/usr/local/bin/kvm_removed.sh"


#!/usr/bin/env bash

sudo ddcutil setvcp 0x60 0x0f


#!/usr/bin/env bash

sudo ddcutil setvcp 0x60 0x12


I’m very happy with my 23£ KVM and it only took a few hours to research and learn about udev and dcc/ci and have a working prototype. Will tidy up the implementation a bit as there is zero error handling and logging. Another great missed opportunity to write code, instead of making some bash cruft!