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Re: [gpsd-dev] GPSD's assumptions about time
From: |
Eric S. Raymond |
Subject: |
Re: [gpsd-dev] GPSD's assumptions about time |
Date: |
Tue, 26 Nov 2013 11:37:19 -0500 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Sanjeev Gupta <address@hidden>:
> Wouldn't an fstat on the gpsd binary give us a good idea of the
> century we are in? the st_mtime returned will give us the time that
> gpsd was installed, and we can assume that we are at some later
> point than that.
That's an interesting idea. That is, using external file timestamps as
a reference in case the system clock has been lost. That might enable
us to detect that a GPS rollover has occurred.
A more self-contained version of this idea would be to include the
current GPS week in the build; if we ever see GPS time resolving to
a week earlier than that, we can bump the rollover counter. That
will work unless we're cold-starting so long after the build that
we're in the *second* following rollover period. Since GPS is going
to 13-bit week counters that would be upwards of 157 years, which is
probably well over the design lifetime of any of our deployments.
> I think we (and by "we", I mean "you" are over-thinking the issue. No
> amount of effort and design may be enough to solve a problem where the
> inputs are wron, misleading, or being kept confidential.
The stance from which that was written was that I was trying to figure out
how to make GPSD an entirely autonomous time source so that it could be
used to *set* the system clock, e.g. on reboot of a navigation system on
an autonomous sub a thousand miles from the nearest Internet.
For GPSD, this is is a no-shit real deployment case.
--
<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>
Re: [gpsd-dev] GPSD's assumptions about time, Hal Murray, 2013/11/28