Linux time routines

["Ulrich Windl" <ulrich.windl@rz.uni-regensburg.de>]

(Please CC: replies to me, not subscribed)
Hi,

first of all I must conferss that I was wrong when presenting
1) a patch for getnstimeofday()
2) measurements based on 1)

My patch, even when looking right at the first glance, it was ignoring the tick-
interpolation when no time interpolator was defined. It clearly indicates that the 
the variety on non-coherent time routines is a mess. A revised fix would look like 
this (other junks unaffected):

-#ifdef CONFIG_TIME_INTERPOLATION
+/* get system time with nanosecond accuracy */
 void getnstimeofday (struct timespec *tv)
 {
-	unsigned long seq,sec,nsec;
-
+	unsigned long seq, nsec, sec;
+#ifdef CONFIG_TIME_INTERPOLATION
 	do {
 		seq = read_seqbegin(&xtime_lock);
 		sec = xtime.tv_sec;
-		nsec = xtime.tv_nsec+time_interpolator_get_offset();
+		nsec = xtime.tv_nsec + time_interpolator_get_offset();
 	} while (unlikely(read_seqretry(&xtime_lock, seq)));
 
 	while (unlikely(nsec >= NSEC_PER_SEC)) {
 		nsec -= NSEC_PER_SEC;
 		++sec;
 	}
+#else
+	{	/*FIXME: Try not to loose nanoseconds */
+		struct timeval tv;
+
+		do_gettimeofday(&tv);
+		sec = tv.tv_sec;
+		nsec = tv.tv_usec * 1000 + xtime.tv_nsec % 1000;
+	}
+#endif
 	tv->tv_sec = sec;
 	tv->tv_nsec = nsec;
 }


--

Another important issue is this:
do_gettimeofday() (and related routines) currently does not return anything. 
However occasionally one needs the "raw" (more or less) value of the tick 
interpolation (e.g. for PPS frequency calibration). To get the current time and 
the interpolation value in a coherent way, it seems easiest to make 
do_gettimeofday() (and related) routines to return the interpolation value (they 
have it anyway).

(Explanation: NTP offset correction plus frequency correction do already add to 
the kernel clock. So when trying to estimate the absolute frequency error, you 
must measure the timing without such corrections.)

Thus something like:
unsigned long do_gettimeofday (struct timeval *tv)
{
        unsigned long seq, nsec, usec, sec, offset;
        do {
                seq = read_seqbegin(&xtime_lock);
                offset = time_interpolator_get_offset();
                sec = xtime.tv_sec;
                nsec = xtime.tv_nsec;
        } while (unlikely(read_seqretry(&xtime_lock, seq)));

        usec = (nsec + offset) / 1000;

        while (unlikely(usec >= USEC_PER_SEC)) {
                usec -= USEC_PER_SEC;
                ++sec;
        }

        tv->tv_sec = sec;
        tv->tv_usec = usec;
        return offset / 1000;
}

or

unsigned long do_gettimeofday(struct timeval *tv)
{
        unsigned long seq;
        unsigned long result, usec, sec;
        unsigned long max_ntp_tick;

        do {
                unsigned long lost;

                seq = read_seqbegin(&xtime_lock);

                result = usec = cur_timer->get_offset();
                lost = jiffies - wall_jiffies;

                /*
                 * If time_adjust is negative then NTP is slowing the clock
                 * so make sure not to go into next possible interval.
                 * Better to lose some accuracy than have time go backwards..
                 */
                if (unlikely(time_adjust < 0)) {
                        max_ntp_tick = (USEC_PER_SEC / HZ) - tickadj;
                        usec = min(usec, max_ntp_tick);

                        if (lost)
                                usec += lost * max_ntp_tick;
                }
                else if (unlikely(lost))
                        usec += lost * (USEC_PER_SEC / HZ);

                sec = xtime.tv_sec;
                usec += (xtime.tv_nsec / 1000);
        } while (read_seqretry(&xtime_lock, seq));

        while (usec >= 1000000) {
                usec -= 1000000;
                sec++;
        }

        tv->tv_sec = sec;
        tv->tv_usec = usec;
        return result;
}

Similarly

static int do_posix_gettime(struct k_clock *clock, struct timespec *tp)
{
        if (clock->clock_get)
                return clock->clock_get(tp);

        return getnstimeofday(tp);
}

with

int do_posix_clock_monotonic_gettime(struct timespec *tp) returning the proper 
interpolation offset, etc.

Would such a change in the interface be acceptable?

Regards,
Ulrich

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