Re: pulse-per-second API status

[Warner Losh <imp%bsdimp.com@localhost>]

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