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Linux HDMI hotplug

by Alejandro Liu

Updated: 2023-10-01

in 2021

tagged configuration, device, linux, windows

The point of this article is to document I workaround that I came up with to handle a HDMI KVM switch.

What happens is that if my Linux PC is turned on while the KVM switch is selecting the other PC, it fails to initialize the display, so when you switch back to the Linux PC, no display is shown.

The trick for this to work is to the use of udev and xrandr.

We use udev to detect the monitor being plugged in, and we use xrandr to tell X windows to update the display.

Figuring out udev

First in the agenda is to figure out what kind of event we should be looking at. For that, we use the command:

udevadm monitor

or

udevadm monitor --property

With that we can determine what kind of udev events to look for (if any).

Next we need to figure out what keys we need to match. Unfortunately there is some guess work required as you need to figure out the /dev device path, whereas udevadm monitor shows a /devices/ path.

However, you manage, you need to use the following command:

udevadm info --query=all --name=/dev/dri/card0 --attribute-walk

This will show possible attributes in the udev rules key format.

Once we know the keys to use, we can know create the rules files.

Rules are located in two locations:

  • /usr/lib/udev/rules.d/ : for system default rules
  • /etc/udev/rules.d/ : for local specific rules

Essentially, we are waiting for the monitor configuration to change and when that happens we will run a script. This is accomplish with the following rules file (99-xwin-hotplug.rules):

{! xwin-hotplug/99-xwin-hotplug.rules !}

See: hot-plug rules

Running xrandr

The script that is kicked off by udev does the following:

  1. Check if Xorg is running.
  2. Assumes DISPLAY is :0.0 (only one local display!) and tries to determine a suitable XAUTHORITY file.
  3. Run xrandr to try to determine what is the connected display.
  4. Calls xrandr --output "$monitor" --auto to re-configure the display
  5. Run xrefresh for good measure.

See script:

{! xwin-hotplug/xwin-hotplug !}

See: hot-plug script

See Also