[Linuxptp-users] How pch2sys calcs the offset

[Ledda W. E. <Wil...@it...>]

Hi all, 
I'm not sure if this is the right place to ask this question or if I have to use the devel mail list, but I have a question about phc2sys work. I was interested in how it calcs the offset between the system clock (destination) and the source clock (phc). Exploring the source code I found the read_phc function.

	struct timespec tdst1, tdst2, tsrc;
	int i;
	int64_t interval, best_interval = INT64_MAX;

	/* Pick the quickest clkid reading. */
	for (i = 0; i < readings; i++) {
		if (clock_gettime(sysclk, &tdst1) ||
				clock_gettime(clkid, &tsrc) ||
				clock_gettime(sysclk, &tdst2)) {
			perror("clock_gettime");
			return 0;
		}

		interval = (tdst2.tv_sec - tdst1.tv_sec) * NS_PER_SEC +
			tdst2.tv_nsec - tdst1.tv_nsec;

		if (best_interval > interval) {
			best_interval = interval;
			*offset = (tdst1.tv_sec - tsrc.tv_sec) * NS_PER_SEC +
				tdst1.tv_nsec - tsrc.tv_nsec + interval / 2;
			*ts = tdst2.tv_sec * NS_PER_SEC + tdst2.tv_nsec;

		}
	}

	return 1;

It's quite clear how it works but really I don't understand very well the following operation:

*offset = (tdst1.tv_sec - tsrc.tv_sec) * NS_PER_SEC + tdst1.tv_nsec - tsrc.tv_nsec + interval / 2;

Can someone explain to me why the half interval is added at the end?

Thanks

William