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pps: Round to closest integer in pps_event()

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The comment above bintime2timespec() says:

  When converting between timestamps on parallel timescales of differing
  resolutions it is historical and scientific practice to round down.

However, the delta_nsec value is a time difference and not a timestamp.  Also
the rounding errors accumulate in the frequency accumulator, see hardpps().
So, rounding to the closest integer is probably slightly better.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/604
This commit is contained in:
Sebastian Huber 2023-02-27 14:49:10 -07:00 committed by Warner Losh
parent 1e48d9d336
commit 28ed159f26
1 changed files with 3 additions and 1 deletions
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4
sys/kern/kern_tc.c
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@ -1882,6 +1882,7 @@ pps_event(struct pps_state *pps, int event)
#ifdef PPS_SYNC
if (fhard) {
uint64_t delta_nsec;
uint64_t freq;
/*
* Feed the NTP PLL/FLL.
@ -1893,7 +1894,8 @@ pps_event(struct pps_state *pps, int event)
tcount &= captc->tc_counter_mask;
delta_nsec = 1000000000;
delta_nsec *= tcount;
delta_nsec /= captc->tc_frequency;
freq = captc->tc_frequency;
delta_nsec = (delta_nsec + freq / 2) / freq;
hardpps(tsp, (long)delta_nsec);
}
#endif