How is PPS Processing related to the Kernel Discipline?
PPS processing can be done in application programs (see also Q: 6.2.4.5.1.), but it makes much more sense when done in the operating system kernel. When polling a time source every 20 minutes, an offset of 5ms is rather small, but when polling a signal every second, an offset of 5ms is very high. Therefore a high accuracy is required for PPS processing. Application programs usually can’t fulfil these demands. The kernel clock model described before also includes algorithms to discipline the clock through an external pulse, the PPS. The additional requirements consist of two mechanisms: Capturing an external event with high accuracy, and applying that event to the clock model. The first is nowadays solved by using the PPS API (Q: 6.2.4.5.1.), while the second is implemented mostly in a routine named hardpps(). The latter routine is called every time when an external PPS event has been detected.
