Re: FreeBSD driver for the OCP TAP Time Card

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John Hay writes:

> The other is that the conversion from the above value to bintime and later
> back to what is used elsewhere, seems to lose a little precision. The
> comments in sys/kern/kern_tc.c also note that.

Yes once the /resolution/ of the hardware gets into the nanosecond
domain, some of that resolution is lost, because a 64x64=>128
multiplication would have been both surplus to requirements and
beyond the pale.

Getting rid of 32bit platforms moves the needle, but it may still
not be warranted.

I will also note that most people who think they are approaching
nanosecond /precision/ are wrong about that:  Only a few of the
institutions listed in Circular T manage it.

The important thing is to make sure the added noise is bias free.

You should capture some millions of the the difference between the
HW counter and the hw->timecounter->bintime->timespec result, and
run that through the usual battery (Histogram, MVAR, FFT etc.)

> In the pps code, I wished that one could provide a timestamp with pps_capture
> (). It uses the time at which it handled the interrupt. The card latch the
> counter values when the pps happens, so it knows the correct time. Currently I
> hack around it by twiddling sc->sc_pps_state.capcount directly.

That is not hacking around, that is how it is done :-)

See for instance i386/i386/elan-mmcr.c

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