On Tue, Aug 20, 2019 at 06:56:56PM +0200, Hubert Feurstein wrote: > Am Di., 20. Aug. 2019 um 17:40 Uhr schrieb Miroslav Lichvar > > I think a large jitter is ok in this case. We just need to timestamp > > something that we know for sure happened after the PHC timestamp. It > > should have no impact on the offset and its stability, just the > > reported delay. A test with phc2sys should be able to confirm that. > > phc2sys selects the measurement with the shortest delay, which has > > least uncertainty. I'd say that applies to both interrupt and polling. > > > > If it is difficult to specify the minimum interrupt delay, I'd still > > prefer an overly pessimistic interval assuming a zero delay. > > > Currently I do not see the benefit from this. The original intention was to > compensate for the remaining offset as good as possible.
That's ok, but IMHO the change should not break the assumptions of existing application and users. > The current code > of phc2sys uses the delay only for the filtering of the measurement record > with the shortest delay and for reporting and statistics. Why not simple shift > the timestamps with the offset to the point where we expect the PHC timestamp > to be captured, and we have a very good result compared to where we came > from. Because those reports/statistics are important in calculation of maximum error. If someone had a requirement for a clock to be accurate to 1.5 microseconds and the ioctl returned a delay indicating a sufficient accuracy when in reality it could be worse, that would be a problem. BTW, phc2sys is not the only user of the ioctl. -- Miroslav Lichvar