My solution (ArchLinux ARM on RaspberryPi) requires that a non-privileged user has to have access to the
/dev/ttyAMA0 port. In final implementation that user will be automatically logged in and a start-up script would be launched, but that is off-topic.
The problem is that the
/dev/ttyAMA0 port (owned by root:tty) has 0620 permissions, and though the non-privileged user is put in the tty group, the file permissions do not give him read access, and that is not good enough.
In this thread I was told I should use
/etc/tmpfiles.d feature to fix the permissions. However, adding a /etc/tmpfiles.d/solution.conf file with one line
F /dev/ttyAMA0 0660 root tty
does not change a thing. Perhaps I am not using the tmpfiles.d feature correctly.