timekeeping: Use mul_u64_u32_shr() instead of open coding it

The resume code must deal with a clocksource delta which is potentially big
enough to overflow the 64bit mult.

Replace the open coded handling with the proper function.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Parit Bhargava <prarit@redhat.com>
Cc: Laurent Vivier <lvivier@redhat.com>
Cc: "Christopher S. Hall" <christopher.s.hall@intel.com>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Liav Rehana <liavr@mellanox.com>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/20161208204228.921674404@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/time/timekeeping.c

@@ -1644,7 +1644,7 @@ void timekeeping_resume(void)
 	struct clocksource *clock = tk->tkr_mono.clock;
 	unsigned long flags;
 	struct timespec64 ts_new, ts_delta;
-	cycle_t cycle_now, cycle_delta;
+	cycle_t cycle_now;
 
 	sleeptime_injected = false;
 	read_persistent_clock64(&ts_new);
@@ -1670,27 +1670,11 @@ void timekeeping_resume(void)
 	cycle_now = tk->tkr_mono.read(clock);
 	if ((clock->flags & CLOCK_SOURCE_SUSPEND_NONSTOP) &&
 		cycle_now > tk->tkr_mono.cycle_last) {
-		u64 num, max = ULLONG_MAX;
-		u32 mult = clock->mult;
-		u32 shift = clock->shift;
-		s64 nsec = 0;
-
-		cycle_delta = clocksource_delta(cycle_now, tk->tkr_mono.cycle_last,
-						tk->tkr_mono.mask);
-
-		/*
-		 * "cycle_delta * mutl" may cause 64 bits overflow, if the
-		 * suspended time is too long. In that case we need do the
-		 * 64 bits math carefully
-		 */
-		do_div(max, mult);
-		if (cycle_delta > max) {
-			num = div64_u64(cycle_delta, max);
-			nsec = (((u64) max * mult) >> shift) * num;
-			cycle_delta -= num * max;
-		}
-		nsec += ((u64) cycle_delta * mult) >> shift;
+		u64 nsec, cyc_delta;
 
+		cyc_delta = clocksource_delta(cycle_now, tk->tkr_mono.cycle_last,
+					      tk->tkr_mono.mask);
+		nsec = mul_u64_u32_shr(cyc_delta, clock->mult, clock->shift);
 		ts_delta = ns_to_timespec64(nsec);
 		sleeptime_injected = true;
 	} else if (timespec64_compare(&ts_new, &timekeeping_suspend_time) > 0) {