systemd and i2c

[bassplyr647]

In the past, I've had a Pi 3 running Jessie and created a service that would start after the i2c module loaded during boot, the service would run a small program to place a message, via i2c using wiringPI, to a character LCD. This has been working great, deployed on numerous Pi's etc..

I just received a Pi 3 B+. I've installed Stretch Lite and have the Pi starting from USB. I've created a systemd service that should start after i2c is active during boot (I've got i2c enabled and working), but I get the following error

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 systemctl status startmsg.service
startmsg.service - LCD Message
   Loaded: loaded (/lib/systemd/system/startmsg.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Tue 2018-08-28 11:08:51 CDT; 2h 16min ago
  Process: 88 ExecStart=/home/pi/startmsg (code=exited, status=1/FAILURE)
 Main PID: 88 (code=exited, status=1/FAILURE)

Aug 28 11:08:51 raspberrypi startmsg[88]: Unable to open I2C device: No such file or directory
Warning: Journal has been rotated since unit was started. Log output is incomplete or unavailable.

The program startmsg works fine after the Pi has booted up.

This is the service I created:

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[Unit]
Description=LCD Message
DefaultDependencies=false
Requires=systemd-modules-load.service
After=systemd-modules.load.service
Before=sysvinit.target
ConditionPathExists=/sys/class/i2c-adapter

[Service]
Type=oneshot
ExecStart=/home/pi/startmsg
Restart=no

[Install]
WantedBy=sysinit.target

Any suggestions would be greatly appreciated.

[DougieLawson]

Do you have

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i2c-dev

in /etc/modules?

[bassplyr647]

Yes.

[TimSmall]

You can make systemd track the availability of the i2c devices by creating a file e.g.

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/etc/udev/rules.d/99-i2c.rules

which looks like this:

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SUBSYSTEM=="i2c-dev", TAG+="systemd"

After rebooting you can then verify this has worked with:

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systemctl --all --full -t device | grep i2c

You can then add one the resulting i2c .device units as a dependency in your own unit file e.g.

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cat /etc/systemd/system/mything.service
[Unit]
...
Requires=dev-i2c\x2d1.device
After=dev-i2c\x2d1.device
...