[chrony-dev] pps source is still marked as synchronised when no absolute clock sources are available

["Hattink, Tjalling (FINT)" <T.Hattink@xxxxxxxx>]

Hi,

 

This is my report of a second issue I found when experimenting with Chrony. See the mail titled "makestep command sometimes makes chrony stop reading its sources" for more background info.

 

I use two reference sources for synchronising the system clock, a SHM interface providing absolute time from a GPS card, and a PPS interface providing precise relative time from a GPS card.

 

The daemon providing the SHM interface only provides timestamps when the GPS card is locked and its time is accurate. The PPS interface is always providing sub-second timestamps.

 

When chrony is started it does not discipline the system clock until the SHM interface is marked synchronised. After the SHM source is correct it starts to look at the PPS clock and after a while the PPS clock is used as main source. This is correct behaviour. Now I disconnect the antenna from the GPS card and the SHM interface stops providing timestamps, as the GPS clock is unreliable. You can see in the sources list that the SHM clock is marked unreachable. But the PPS pulses are still provided and Chrony keeps using the PPS as synchronisation source. I think this is not correct behaviour. Chrony should check if any absolute time source is reachable and synchronised every time when a PPS signal comes in. If not it should also mark the PPS sources unreachable. This behaviour is also shown by ntpd.

 

If you need more information or you want me to test patches regarding this issue, don't hesitate to contact me!

 

Kind regards,

 

Tjalling Hattink