[LinuxPPS] [PATCH 01/11] PPS: Don't waste time getting unnecessary timestamps.

[George Spelvin]

Rodolfo Giometti <giometti at enneenne.com> wrote:

> Nack.
>
> Function getnstimeofday() should be called as soon as possible since
> even a little delay may cause a large timing error! So is better
> calling getnstimeofday() even if not necessary then delay it.

My reasoning was that if you cared enough, you'd enable interrupt
timestamps; this is a fallback.

But really, one predicted untaken branch (forward branches are predicted
not taken if not in the prediction cache) is a truly negligible overhead.
(You can add a likely() hint to make the branch time deterministic.)

Far more critical is jitter, which is an issue becuase of the
possible looping inside getnstimeofday().
(/usr/src/linux/kernel/time/timekeeping.c)

If you don't like that, I'd at least simplify it to:

	/* First of all we get the time stamp... */
	getnstimeofday(&__ts);

-	/* Does caller give us a timestamp? */
-	if (ts) {	/* Yes. Let's use it! */
-		pps_ts.sec = ts->tv_sec;
-		pps_ts.nsec = ts->tv_nsec;
-	} else {	/* No. Do it ourself! */
-		pps_ts.sec = __ts.tv_sec;
-		pps_ts.nsec = __ts.tv_nsec;
+	/* Do we need this timestamp or have we got one already? */
+	if (!ts)
+		ts = &__ts;
+	pps_ts.sec = ts->tv_sec;
+	pps_ts.nsec = ts->tv_nsec;