[chrony-users] Newbie requesting help

[Stuart Maclean <stuart@xxxxxxxxxxxxxxxxxx>]

I have a reference clock onboard an embedded system running a legacy
Linux 2.6.10.  That kernel has no 'Linux PPS' support, nor have I yet
looked into patching it in.

My clock, call it C, outputs a 'seconds since epoch' counter over   a
serial line to me, and also outputs a PPS, which is wired to an GPIO
pin.  At
start of deployment, I can set the counter from a GPS source.  GPS then
goes away.

There is no Internet access on this system.  I want to use clock C to
maintain good Linux system time.  I am hoping chronyd might help.  I
have basically asked this same question on the ntpd mailing list, to no
avail.

I have written a Linux device driver which exposes two char devices:

/dev/clock

/dev/clockpps

A userspace read of /dev/clock blocks until C next increments its
counter, which is does when its PPS fires.  The counter value is then
the result of the read.

A userspace read of /dev/clockpps blocks until C next outputs its PPS
signal.  In the device driver, I grab Linux system time right in the
interrupt handler for the PPS/GPIO signal.  I am installing the ISR
using a 'fast interrupt' handler too, so am doing all I can to minimize
latency between the pps occuring and me grabbing Linux time.

The linux system clock time at time of PPS is then fed back as the
result of the read of /dev/clockpps.

So, as I see it, I have two 'votes' for the correct time at moment of
PPS.  The first is C's vote for the time, its simple 'seconds counter'.
 The second is Linux's vote for the time, i.e. the software clock.  In
my system, the software clock is driven by a 32KHz clock, using a jiffy
of 10ms. The resolution of that clock is then about 30us.

I was then under the impression that I would supply these two 'votes'
for the correct time to ntpd/chrony??  But when I looked into how gpsd
feeds its time parameters (ie a PPS event and a GPRMC string?) over SHM
to ntpd, I see that not one but TWO clock sources are used.  Am I
supposed to split my data into these two sources?  Given that the
shmTime  struct has a 'clockTimestamp' AND a 'receiveTimestamp', do I
need FOUR and not TWO timestamps to complete the shm implementation?

Any help appreciated, and I apologize if I am not describing the problem
too well.

Stuart



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