Re: [gpsd-dev] RFC: Patch tp prevent call to ntpd_link_* for driver_nmea2000

[Gary E. Miller]

Yo Reinhard!

On Wed, 30 May 2012 09:13:54 +0200
"address@hidden" <address@hidden> wrote:

>       -- the usage of ntp with gpsd on the boat has no top priority
> for me, the support of more then one GPS device, the AIS transmitter,
> and other instruments come first. 

Just because it does not interest does not give you permission to break
things for others.

>       -- i see no way to test it, i have only one linux machine on
> the boat, perhaps you can tell me how to compare the ntp results of a
> gpsd using a conventional usb gps (u-blox 5) reciever, and a ntpd on
> the gpsd using the information from the nmea2000 bus. 

Easy, just send time from both of them and look at "ntpq -p".

>       -- as nmea2000 is a CAN bus (network), you need support in
> the CAN driver and perhaps even in the CAN interface hardware, to
> get a good timestamps. There are own time protocols for CAN, but not
> part ofnmea2000 as far as i know. 

Not the point, the timestamps in the nmea2000 GPS messages are what
we are looking for.  If those are not supported then nmea2000 is
useless.

>       -- i have spend some time trying to understand the
> interaction of the pps daemon and the other ntp stuff, but my
> understanding of the subsystem is not good enough to make a change
> here. My understanding is, the the ntp_link_activate() and
> ntpd_link_deactivate() only allocate/free the the shared memory
> segment for ntp, and then starts the pps subsystem, that is unusable
> for nmea2000 as it is today. 

Yes, and no.  ntp_link_activate() also enables the basic timestamp
support which should be working for you fine.  You need to look deeper
to disable pps support.  But why do you care to disable PPS support, is
it breaking something for you?  Turning it off buys you nothing.
> 
>       Question: Does the ntp subsystem (without pps support) work,
> if there is only the shared memory segment present?

Yes.

> then it is a nice
> option, to move my patches into ntp_link_activate() and
> ntpd_link_deactivate() after allocating/freeing the ntp shared memory
> segment. If this works, it will be for shure a far better solution! 

Possibly, send patches and I'll look at it.  

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
        address@hidden  Tel:+1(541)382-8588

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