AW: PPS stopped once => stopped forever?

[Wielant, Ulrich]

Hi Gary,

thanks for your comments.

Attached you find the output of the gpsdebuginfo script at a point of time when 
everything was running well.

I configured the baud rate with -s 38400 to avoid the hunting. Attached you 
find the log of a case when a bad checksum was received.
gpsd was now able to reconnect correctly. But PPS was removed. Writing the time 
info to shm stopped although gpsd received NMEA data again.
I would expect that SHM 0 is still updated because as far as I understand the 
SHM 0 time info is taken from he NMEA data.

Result:
I am fine with using the -s parameter to configure a constant baud rate.
I vote for continuing to update the time information to the shm, even if the 
PPS signal fails.

I will try to find out if the hardware is buggy or if the cabling leads to 
faults.

Cheers
Ulrich


-----Ursprüngliche Nachricht-----
Von: gpsd-users-bounces+u.wielant=leonardogermany.com@nongnu.org 
<gpsd-users-bounces+u.wielant=leonardogermany.com@nongnu.org> Im Auftrag von 
Gary E. Miller
Gesendet: Dienstag, 1. Oktober 2024 05:01
An: gpsd-users@nongnu.org
Betreff: Re: PPS stopped once => stopped forever?



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Yo Ulrich!

On Mon, 30 Sep 2024 08:07:43 +0000
"Wielant, Ulrich" <U.Wielant@leonardogermany.com> wrote:

> I am running gpsd version 3.25. My GPS is a GARMIN device connected
> via serial port.

Good on the 3.25.  Sorry to hear about the Garmin.

> I have observed that gpsd sometimes stops feeding time via SHM to ntp
> or chrony. It seems to happen after gpsd got an NMEA sentence with bad
> checksum.

This woud be your root problem.  WHy are you getting bad checksums?
This should never happen.

> After that gpsd tries to find the correct speed (38400) which often
> does not succeed unless gpsd is restarted.

If you are always using 38,400, then you should use the option "-s 38400" to 
gpsd so it will not needlessly hunt.

> This may be a
> 2nd issue which I will post later.

Try git head, it has improvements in the hunt process.  Or, use -s and avoid 
the hunt altogether.  After you find you hardware problem.

> I wonder what happens here and why. Why does the kernel removes the
> pps0 device?

gpsd stopped seeing PPS, so it shut down the PPS monitor thread.
It has been that way a long time, but some newer GNSS receivers stop PPS 
intentionally, so that may need to be rethought.

> Why stops gpsd feeding the time information via SHM (even when gpsd is
> able to reconnect to the GPS device)?

We need some more information.  How about sending us the output of 
gpsdebuginfo.  Run it as root, not as a user or sudo.  Use the latest version 
here:

    https://gpsd.io/gpsdebuginfo

> I found a gpsd #277 ticket where Gary stated:
> "Nothing can be done about restoring PPS after outages. gpsd drops
> root at the start, so it can't reopen the root owned SHM()'s" What
> does that mean exactly?

It meansa after intialization gpsd no longer has root priviledges.
So can't do things, like open SHM(0), as user nobody.

> Once the connection to a GPS has been lost PPS will never work again
> unless gpsd is restarted?

Yup.  BUt this should never happen once you fix your hardware problem.

> Here are some logs around the problem. Please tell me if you need
> other logs or more logs.

Logs are only useful they are 100% complete.  Snippets lack critical context.

> 2024-09-29T23:08:31.495350+00:00 localhost gpsd[36346]: gpsd:WARN:
> bad checksum in NMEA packet; got 4F expected 59.

And there it is, bad hardware.

RGDS
GARY
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gpsdebuginfo.log
Description: gpsdebuginfo.log

gps_stop_pps.log
Description: gps_stop_pps.log