I found some problems when trying to set up the ntpd server connected to a GPS.
First, I had to solve how to create the devices required by ntpd: /dev/gps0 and /dev/gpspps0. This is easy: just create a udev rules file, for example /etc/udev/rules.d/90-gps.rules:
SUBSYSTEM=="pps", MODE="0660" GROUP="uucp" SYMLINK="gps%k"
the above creates the following links:
gps0 -> ttyS0
gpspps0 -> pps0
and the target devices are accessible to members of the uucp group. Therefore, the 'ntp' user must belong to the uucp group:
# usermod -G ntp,uucp ntp
Then ntpd experienced some serious jitter problems in the system clock, and I found out that the ntpd driver was not using the PPS signal, but only the NMEA output. Running ntpd in debug mode (-d flag) showed a 'permission denied' error when opening /dev/gpspps0. The file permissions are ok, but the problem was in the selinux layer. I inadvertently had the selinux enabled. Disabling it fixed the problem (set to 'disable' in /etc/selinux/config).
Also, the serial port had to be carefully set up before ntpd was started, otherwise, some interactions in the tty driver would be executed, and the DCD interrupt detection would be lost, as explained in the previous posting. The minimum settings of the serial port at start-up are:
# stty -F /dev/gps0 raw ispeed 4800 ospeed 4800 -hupcl
In particular, finding the selinux problem was quite confusing sometimes. If ntpd is started from the command-line as root, the "permission denied" error does not show up. However, if started using the 'service ntpd start' command, the error appears.