Re: [chrony-users] Automotive usage of chronyd

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On Sat, Apr 04, 2015 at 09:07:17AM +0200, Olivier Delbeke wrote:
> Two questions :
> 1. Can chronyd use non-precise references like a GPS sensor without PPS
> (+-200ms) or an RDS FM timestamp (+-100ms)?

Yes, but you may need to use the minsamples option to prevent chronyd
from dropping too many samples to improve the accuracy as these
sources tend to have large non-random offsets.

> 2. Can chronyd set the system time when it has not been initialized yet
> (Jan 1, 1970) ?

Yes, use the makestep directive in chrony.conf.

> I made a first test in a near worst-case scenario : system without RTC and
> without network, but with only a USB GPS sensor (without PPS). Time/date
> after boot is Jan 1, 1970, 00:00:00. I send the timestamps of the GPS
> sensor to chronyd (over a socket), but chronyd rejects them. Even if I use
> the first GPS timestamp to force the system time, the subsequent timestamps
> are still rejected by chronyd ("refclock sample not valid age=-0.41" from
> refclock.c - valid_sample_time())). What is the condition for a valid
> sample ? I can understand that samples are rejected, but the "< 0.0"
> conditions looks like the drift is only allowed in one direction. Is that
> right ?

It checks if the sample time is not from the future or too old.
Maybe you are setting the sample time to the refence time instead of
local time?

Miroslav Lichvar

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