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pps: Simplify the nsec calculation in pps_event()

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Let A be the current calculation of the frequency accumulator (pps_fcount)
update in pps_event()

  scale = (uint64_t)1 << 63;
  scale /= captc->tc_frequency;
  scale *= 2;
  bt.sec = 0;
  bt.frac = 0;
  bintime_addx(&bt, scale * tcount);
  bintime2timespec(&bt, &ts);
  hardpps(tsp, ts.tv_nsec + 1000000000 * ts.tv_sec);

and hardpps(..., delta_nsec):

  u_nsec = delta_nsec;
  if (u_nsec > (NANOSECOND >> 1))
          u_nsec -= NANOSECOND;
  else if (u_nsec < -(NANOSECOND >> 1))
          u_nsec += NANOSECOND;
  pps_fcount += u_nsec;

This change introduces a new calculation which is slightly simpler and more
straight forward.  Name it B.

Consider the following sample values with a tcount of 2000000100 and a
tc_frequency of 2000000000 (2GHz).

For A, the scale is 9223372036.  Then scale * tcount is 18446744994337203600
which is larger than UINT64_MAX (= 18446744073709551615).  The result is
920627651984 == 18446744994337203600 % UINT64_MAX.  Since all operands are
unsigned the result is well defined through modulo arithmetic.  The result of
bintime2timespec(&bt, &ts) is 49.  This is equal to the correct result
1000000049 % NANOSECOND.

In hardpps(), both conditional statements are not executed and pps_fcount is
incremented by 49.

For the new calculation B, we have 1000000000 * tcount is 2000000100000000000
which is less than UINT64_MAX. This yields after the division with tc_frequency
the correct result of 1000000050 for delta_nsec.

In hardpps(), the first conditional statement is executed and pps_fcount is
incremented by 50.

This shows that both methods yield roughly the same results.  However, method B
is easier to understand and requires fewer conditional statements.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/604
This commit is contained in:
Sebastian Huber 2023-02-27 14:49:09 -07:00 committed by Warner Losh
parent 8a142484d4
commit 1e48d9d336
2 changed files with 9 additions and 19 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
sys/kern/kern_ntptime.c
View File

@ -771,16 +771,10 @@ hardpps(struct timespec *tsp, long delta_nsec)
pps_tf[0].tv_nsec = u_nsec;
/*
* Compute the difference between the current and previous
* counter values. If the difference exceeds 0.5 s, assume it
* has wrapped around, so correct 1.0 s.
* Update the frequency accumulator using the difference between the
* current and previous PPS event measured directly by the timecounter.
*/
u_nsec = delta_nsec;
if (u_nsec > (NANOSECOND >> 1))
u_nsec -= NANOSECOND;
else if (u_nsec < -(NANOSECOND >> 1))
u_nsec += NANOSECOND;
pps_fcount += u_nsec;
pps_fcount += delta_nsec - NANOSECOND;
if (v_nsec > MAXFREQ || v_nsec < -MAXFREQ)
goto out;
time_status &= ~STA_PPSJITTER;

16
sys/kern/kern_tc.c
View File

@ -1781,7 +1781,7 @@ pps_event(struct pps_state *pps, int event)
struct timecounter *captc;
uint64_t capth_scale;
struct bintime bt;
struct timespec ts, *tsp, *osp;
struct timespec *tsp, *osp;
u_int tcount, *pcount;
int foff;
pps_seq_t *pseq;
@ -1881,7 +1881,7 @@ pps_event(struct pps_state *pps, int event)
#ifdef PPS_SYNC
if (fhard) {
uint64_t scale;
uint64_t delta_nsec;
/*
* Feed the NTP PLL/FLL.
@ -1891,14 +1891,10 @@ pps_event(struct pps_state *pps, int event)
tcount = pps->capcount - pps->ppscount[2];
pps->ppscount[2] = pps->capcount;
tcount &= captc->tc_counter_mask;
scale = (uint64_t)1 << 63;
scale /= captc->tc_frequency;
scale *= 2;
bt.sec = 0;
bt.frac = 0;
bintime_addx(&bt, scale * tcount);
bintime2timespec(&bt, &ts);
hardpps(tsp, ts.tv_nsec + 1000000000 * ts.tv_sec);
delta_nsec = 1000000000;
delta_nsec *= tcount;
delta_nsec /= captc->tc_frequency;
hardpps(tsp, (long)delta_nsec);
}
#endif