powerpc: Fix problem with time going backwards

The recent changes to keep gettimeofday in sync with xtime had the side
effect that it was occasionally possible for the time reported by
gettimeofday to go back by a microsecond.  There were two reasons:
(1) when we recalculated the offsets used by gettimeofday every 2^31
timebase ticks, we lost an accumulated fractional microsecond, and
(2) because the update is done some time after the notional start of
jiffy, if ntp is slowing the clock, it is possible to see time go backwards
when the timebase factor gets reduced.

This fixes it by (a) slowing the gettimeofday clock by about 1us in
2^31 timebase ticks (a factor of less than 1 in 3.7 million), and (b)
adjusting the timebase offsets in the rare case that the gettimeofday
result could possibly go backwards (i.e. when ntp is slowing the clock
and the timer interrupt is late).  In this case the adjustment will
reduce to zero eventually because of (a).

Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/time.c

@@ -283,9 +283,9 @@ static inline void update_gtod(u64 new_tb_stamp, u64 new_stamp_xsec,
 	 * the two values of tb_update_count match and are even then the
 	 * tb_to_xs and stamp_xsec values are consistent.  If not, then it
 	 * loops back and reads them again until this criteria is met.
+	 * We expect the caller to have done the first increment of
+	 * vdso_data->tb_update_count already.
 	 */
-	++(vdso_data->tb_update_count);
-	smp_wmb();
 	vdso_data->tb_orig_stamp = new_tb_stamp;
 	vdso_data->stamp_xsec = new_stamp_xsec;
 	vdso_data->tb_to_xs = new_tb_to_xs;
@@ -310,20 +310,15 @@ static __inline__ void timer_recalc_offset(u64 cur_tb)
 	unsigned long offset;
 	u64 new_stamp_xsec;
 	u64 tlen, t2x;
+	u64 tb, xsec_old, xsec_new;
+	struct gettimeofday_vars *varp;
 
 	if (__USE_RTC())
 		return;
 	tlen = current_tick_length();
 	offset = cur_tb - do_gtod.varp->tb_orig_stamp;
-	if (tlen == last_tick_len && offset < 0x80000000u) {
-		/* check that we're still in sync; if not, resync */
-		struct timeval tv;
-		__do_gettimeofday(&tv, cur_tb);
-		if (tv.tv_sec <= xtime.tv_sec &&
-		    (tv.tv_sec < xtime.tv_sec ||
-		     tv.tv_usec * 1000 <= xtime.tv_nsec))
-			return;
-	}
+	if (tlen == last_tick_len && offset < 0x80000000u)
+		return;
 	if (tlen != last_tick_len) {
 		t2x = mulhdu(tlen << TICKLEN_SHIFT, ticklen_to_xs);
 		last_tick_len = tlen;
@@ -332,6 +327,21 @@ static __inline__ void timer_recalc_offset(u64 cur_tb)
 	new_stamp_xsec = (u64) xtime.tv_nsec * XSEC_PER_SEC;
 	do_div(new_stamp_xsec, 1000000000);
 	new_stamp_xsec += (u64) xtime.tv_sec * XSEC_PER_SEC;
+
+	++vdso_data->tb_update_count;
+	smp_mb();
+
+	/*
+	 * Make sure time doesn't go backwards for userspace gettimeofday.
+	 */
+	tb = get_tb();
+	varp = do_gtod.varp;
+	xsec_old = mulhdu(tb - varp->tb_orig_stamp, varp->tb_to_xs)
+		+ varp->stamp_xsec;
+	xsec_new = mulhdu(tb - cur_tb, t2x) + new_stamp_xsec;
+	if (xsec_new < xsec_old)
+		new_stamp_xsec += xsec_old - xsec_new;
+
 	update_gtod(cur_tb, new_stamp_xsec, t2x);
 }
 
@@ -564,6 +574,10 @@ int do_settimeofday(struct timespec *tv)
 	}
 #endif
 
+	/* Make userspace gettimeofday spin until we're done. */
+	++vdso_data->tb_update_count;
+	smp_mb();
+
 	/*
 	 * Subtract off the number of nanoseconds since the
 	 * beginning of the last tick.
@@ -724,10 +738,16 @@ void __init time_init(void)
 	 * It is computed as:
 	 * ticklen_to_xs = 2^N / (tb_ticks_per_jiffy * 1e9)
 	 * where N = 64 + 20 - TICKLEN_SCALE - TICKLEN_SHIFT
-	 * so as to give the result as a 0.64 fixed-point fraction.
+	 * which turns out to be N = 51 - SHIFT_HZ.
+	 * This gives the result as a 0.64 fixed-point fraction.
+	 * That value is reduced by an offset amounting to 1 xsec per
+	 * 2^31 timebase ticks to avoid problems with time going backwards
+	 * by 1 xsec when we do timer_recalc_offset due to losing the
+	 * fractional xsec.  That offset is equal to ppc_tb_freq/2^51
+	 * since there are 2^20 xsec in a second.
 	 */
-	div128_by_32(1ULL << (64 + 20 - TICKLEN_SCALE - TICKLEN_SHIFT), 0,
-		     tb_ticks_per_jiffy, &res);
+	div128_by_32((1ULL << 51) - ppc_tb_freq, 0,
+		     tb_ticks_per_jiffy << SHIFT_HZ, &res);
 	div128_by_32(res.result_high, res.result_low, NSEC_PER_SEC, &res);
 	ticklen_to_xs = res.result_low;