time: ntp: fix bug in ntp_update_offset() & do_adjtimex()

Impact: change (fix) the way the NTP PLL seconds offset is initialized/tracked

Fix a bug and do a micro-optimization:

When PLL is enabled we do not reset time_reftime. If the PLL
was off for a long time (for example after bootup), this is
arguably the wrong thing to do.

We already had a hack for the common boot-time case in
ntp_update_offset(), in form of:

	if (unlikely(time_status & STA_FREQHOLD || time_reftime == 0))
 		secs = 0;

But the update delta should be reset later on too - not just when
the PLL is enabled for the first time after bootup.

So do it on !STA_PLL -> STA_PLL transitions.

This changes behavior, as previously if ntpd was disabled for
a long time and we restarted it, we'd run from that last update,
with a very large delta.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/time/ntp.c

@@ -142,7 +142,7 @@ static void ntp_update_offset(long offset)
 	 * and in which mode (PLL or FLL).
 	 */
 	secs = xtime.tv_sec - time_reftime;
-	if (unlikely(time_status & STA_FREQHOLD || time_reftime == 0))
+	if (unlikely(time_status & STA_FREQHOLD))
 		secs = 0;
 
 	time_reftime = xtime.tv_sec;
@@ -394,6 +394,13 @@ int do_adjtimex(struct timex *txc)
 			}
 			/* only set allowed bits */
 			time_status &= STA_RONLY;
+			/*
+			 * If we turn on PLL adjustments then reset the
+			 * reference time to current time.
+			 */
+			if (!(time_status & STA_PLL) && (txc->status & STA_PLL))
+				time_reftime = xtime.tv_sec;
+
 			time_status |= txc->status & ~STA_RONLY;
 
 			switch (time_state) {