PPS (Pulse Per Second) is a special pulse provided by some GPS
antennae. Userland can use it to get a high-precision time
reference.

Some antennae's PPS signals are connected with the CD (Carrier
Detect) pin of the serial line they use to communicate with the
host. In this case use the SERIAL_LINE client support.

Some antennae's PPS signals are connected with some special host
inputs so you have to enable the corresponding client support.

To compile this driver as a module, choose M here: the module
will be called pps_core.ko.

Say Y here if you want the PPS support to produce a bunch of debug
messages to the system log.  Select this if you are having a
problem with PPS support and want to see more of what is going on.

This option adds support for direct in-kernel time
synchronization using an external PPS signal.

It doesn't work on tickless systems at the moment.