Proposal - drop the GPSD JSON driver

Gary E. Miller gem at
Mon Oct 17 23:08:18 UTC 2016

Yo Eric!

On Mon, 17 Oct 2016 18:28:22 -0400 (EDT)
esr at (Eric S. Raymond) wrote:

> I have come to think the best thing to do about the GPSD/JSON driver
> is just remove it.  This note explains why.  Please reply with
> agreement, disagreement, or comment.

Disagree.  The defaults are broken, and it has modes that can never
work, but it is fixable.

> Because it works so badly, it is highly unlikely that anyone will
> complain of its absence.

Probably true, but not a reason not to fix it.

> There might be an argument for fixing this driver rather than
> dropping it, but I don't see that it actually does anything useful.

Not yet, but the SHM driver also has problems.  SHM has security problems,
it is not extensible, it provides no synergy between the GPS and PPS
signals.  One nice extention to the JSON refclock might be to have it
autostart gpsd if required.

The GPSD JSON driver has documented 4 modes and undocumented 'features'.
That html documentation for those modes does not match what the code
does.  Bad doc is not a reason to remove the driver.  If ntpd forced the
driver into the one configuration that actually works (just remove the bad 
options), then fix the doc and it is good to go.

> For communication with GPSD, the SHM driver seems superior; it
> certainly has lower processing overhead and therefore introduces
> less noise into the delivery chain.

It introduces no extra timing noise and passes some data, like timing
source name, tdop, nSats, etc. that could be useful to ntpd.

> I understand there's a port problem with SHM on the Mac, but I'd
> rather fix that or write an equivalent driver using new POSIX
> shared memory than chase bugs in this one.

Seems to me any POSIX shared memory driver, which I would like to see,
would basically have to implement the GPSD JSON protocol to work.  We
know the GPSD JSON is a good protocol so no need to come up with a
new one.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at  Tel:+1 541 382 8588
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