ublox refclock

[Trevor N.]

>> Is there any point to it without the matching kernel driver?
>
>Has anybody tried asking for the "echo" from user space?
>
>That is:
>  grab time
>  raise modem signal
>  grab time
>  read timestamp over serial port
>
>If you precede that with
>  grab time
>  lower modem signal
>I think that will warm up most of the cache.  Another interesting experiment 
>would be to see how much of a difference that makes.

The trimble driver already uses user-space for its time mark output
(the HW_poll function). It works quite well: on my test machine (50ns
precision, ovenized oscilator) a Praecis Cf has a jitter of less than
30ns, even with a 40k packets-per-second load.  I don't remember
pre-fetching a timestamp making much of a difference when I was
testing changes to the driver. HW_poll does not echo a PPS input, so
its alignment to the top of the second is arbitrary.

Attached is an ntpvis summary csv of a run I just finished on my test
machine while unloaded. Unit0 is a Praecis Cf with usermode timemark,
and unit1 is a Palisade using the timemark kernel module I am
developing. I am making another run now with the Palisade using the
usermode timemark for comparison.  Currently my kernel module has the
same arbitrary-alignment problem as ntpsec's HW_poll, so it's not
quite ready for general use.

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