> Note that phc2sys can (and usually does) work without that kernel PPS event. In fact, depending on ISR latency, the synchronization probably works better without the PPS.
That's interesting. Thanks for pointing this out. It goes counter to my intuition (but it makes sense). I see in the phc2sys code that when not using PPS, it uses ioctls to get time offsets (PTP_SYS_OFFSET_PRECISE etc.). I suppose you are saying that this a more accurate mechanism than the one based on PPS. How close can you get with the ioctls? What kind of accuracy can we expect? On Mon, May 20, 2019 at 9:32 AM Richard Cochran <richardcoch...@gmail.com> wrote: > On Mon, May 20, 2019 at 07:18:22AM -0400, Sanjay Bhandari wrote: > > Thanks for the response. Between what you wrote and some more > spelunking, I > > think I finally understand. I'll summarize it here in case it's useful to > > others. > > That is a good summary. > > > 1. The PPS from the GPS that you feed into the PHC has nothing to do with > > the kernel PPS system. It is simply an input on one of the pins of the > PHC. > > The PHC has the capability of timestamping such an input event (once > > programmed correctly). It timestamps the event with the PHC time. This > > timestamp then effectively gives you the offset between the GPS time and > > PHC time. Which can be used to train the PHC clock to match the GPS time. > > > > 2. The PPS that the PHC itself generates is fed into the kernel PPS > system, > > and that works as usual whereby the kernel timestamps the PPS event with > > the timestamp from the sysclock. This gives you the offset between the > PHC > > and sysclock, which can be used to train the sysclock to match the PHC > > (which is what phc2sys does). > > Note that phc2sys can (and usually does) work without that kernel PPS > event. In fact, depending on ISR latency, the synchronization > probably works better without the PPS. New Intel CPUs and MACs can > use the ART to synchronize the two in hardware, and phc2sys does > support this mode. > > Thanks, > Richard >
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