On Nov 2, 2013, at 1:33 AM, Alan Barrett wrote: > On Fri, 01 Nov 2013, Greg Troxel wrote: >>> But if NetBSD enables PPS on ucom, there's going to be an expectation that >>> it is good enough for stratum-1 timekeeping, like PPS on real serial ports. >> >> I don't think there's any such expectation created. >> [...] >> People who expect the same as serial PPS are confused, and we are not >> responsible for that. > > I think that PPS on a device with very high "interrupt" latency is > sufficiently similar to PPS on a device with low interrupt latency that it > deserves to have the same API. I don't think it even needs a sysctl to > enable it. > > I think that it just needs careful documentation, in ucom(4) and wherever we > document the PPS API. Maybe the documentation for applications like ntpd > should also warn against using PPS on USB interfaces.
It isn't the latency that's the problem with the interrupt even. A 2ms latency that has a variance of 10ns is much much better for time keeping than a 10us latency with a 1us variance. Variance of the interrupt latency is the killer, since the on-time point can be calibrated and systemic delays can be compensated for rather easily. Warner