Re: [gpsd-dev] Port to QNX: PPS without SHM?

[Gary E. Miller]

Yo Hal!

On Tue, 05 Jan 2016 23:17:27 -0800
Hal Murray <address@hidden> wrote:

> You work mostly on gpsd.  It's your hammer.  I work mostly on ntpd.
> It's my hammer.

I submitted a lot of patches to ntpd, nothing ever got applied...

> One of the reasons we (both gpsd and ntpd) put the JSON stuff
> together is to handle cases where shared memory didn't work.  If it
> doesn't work got this case we should fix it.

First we have to agree on how it should work, that was an issue between
Pearly and I.

>   Maybe the
> documentation needs work.

No maybe about it.  But we need agreement first.

> > Yes, gpsd needs to be the ne getting the PPS signal.  Do not pass it
> > directly to ntpd.   
> 
> Why not?  I often run gpsd without a PPS.  It does what I expect.

If gpsd does not get the PPS then it can not pass the PPS on to its
other clients.

> I don't see any reason why ntpd wouldn't work correctly if it gets
> the NMEA timing info via gpsd and the PPS directly.  That would let
> gpsd see the position info and pass it on to other clients.

ntpd has no way to associate the gpsd data with the PPS data.  So it
has no way to do many of the validations that gpsd does between the NMEA
and PPS data.  In short ntpd can be using bad PPS data.  ntpd also can
not do the autodetection on the PPS that gpsd does.

So yes, when it works, it works, but when things go sideways ntpd can
be misled.

> > That would be a bad thing.  You have now bypassed all that gpsd
> > does to clean up the PPS signal for you.   
> 
> ntpd knows how to process PPS signals.

Sort of, in a primitive fashion.

> I'm not trying to say "don't feed the PPS to gpsd", but rather if
> there was a problem doing that, it should be possible to feed it
> directly to ntpd.

After 3.16 ships I'll look at having gpsd do PPS without needing NTPSHM.
ntpd can not be trusted with PPS aand gpsd at the same time.

>  That might be a useful way to learn more and/or
> avoid needing to write some code.

Given what I hear on the QNX porting there is no way to avoid some code.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

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