How to make systemd units depend on specific hardware

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Sometimes you need a systemd service to wait for a specific hardware device to be available before starting. For instance, when starting an X11 session as a service, you want to ensure the GPU is ready.

By default, systemd only creates device units for block and network devices. Other devices, like /dev/dri/card0, aren’t exposed as units you can depend on.

The solution involves two steps: first, tell udev to tag the device for systemd, then use that device as a dependency in your service.

Creating a udev rule

Create a udev rule that tags the device with systemd:

/etc/udev/rules.d/99-dev-dri-card0.rules

ENV{DEVNAME}=="/dev/dri/card0", TAG+="systemd"

The TAG+="systemd" is what makes systemd dynamically create a device unit for this device. Without it, systemd won’t know about the device.

After creating the file, reload udev rules:

sudo udevadm control --reload-rules
sudo udevadm trigger

Using the device as a dependency

Now you can reference dev-dri-card0.device in your service units. The device path gets converted to a unit name by replacing / with - and removing the leading slash.

/etc/systemd/system/[email protected]

[Unit]
Description=X11 session for %i
Wants=dev-dri-card0.device
After=dev-dri-card0.device graphical.target systemd-user-sessions.service

[Service]
# ...

Using Wants= ensures the device is requested but won’t fail the service if unavailable. The After= directive ensures proper ordering—the service only starts after the device is ready.

Reload systemd to apply the changes:

sudo systemctl daemon-reload