[LinuxPPS] state of PPS

[Cirilo Bernardo]

While people are talking about the state of PPS, I can't help thinking that the PPS mechanism is inherently faulty. In the NTP implementation the serial message is parsed and then a call is made to obtain the timestamp.  There is no guarantee that the timestamp belongs to that ASCII message - there is a race here which may result in the timestamp being for the future ASCII message.  For this race condition to cause problems of course, the CPU has to be loaded (or the receiver has to be encumbered with output messages) - admittedly none of the systems I have worked with will meet that condition, but assuming that such a condition will not be met is unecessary and sloppy. One solution which doesn't really work is to check the timestamp associated with the incoming message and ensure that it was more recent than the PPS value - if it is earlier than the PPS value, then the message is obsolete - we cannot retrieve the actual timestamp for that message, so we have no option but to drop the message and hope that the next one comes through OK.

Another problem of course is that we are forever patching various routines to do things they were not originally meant for - everything is being shoehorned into the default kernel serial comms system which sits on top of the TTY layer, and the TTY layer really doesn't fit neatly into the scheme of things.

I have a WinSystems PCM-GPS board which uses a Trimble LassenSQ receiver and (fortuitously) uses a separate interrupt for PPS rather than any of the various serial line status signals.  Since I had to write a driver to make NMEA and TSIP messages as well as position etc available to a number of processes, I just implemented yet another pseudo-device which provides  the NMEA message with a PPS timestamp - I don't guarantee that the PPS value represets the NMEA message either, but with this specialized driver implementing such a guarantee should be trivial.  I think for attaining the ultimate precision that a particular NMEA message source can provide, we simply have to write specialized drivers and do away with the usual TTY and serial layer.  I really don't believe the PPS system as it exists is a system for the future - it is a technical kludge which may have worked fine for devices 10 years ago and was relatively easy to implement, but I believe that the system needs to be rethought and the old PPS ultimately dumped.  The great challenge of course is that any one person would only be familiar with a very small number of gadgets, but a sensible proposal will need input from a large number of people and (unfortunately) a very long discussion phase and also a very long implementation and testing phase.

Any comments/suggestions/abuse?

- Cirilo
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