timekeeping: Store cycle_last value in timekeeper struct as well

For implementing a shadow timekeeper and a split calculation/update
region we need to store the cycle_last value in the timekeeper and
update the value in the clocksource struct only in the update region.

Add the extra storage to the timekeeper.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>

include/linux/timekeeper_internal.h

@@ -20,6 +20,8 @@ struct timekeeper {
 	u32			shift;
 	/* Number of clock cycles in one NTP interval. */
 	cycle_t			cycle_interval;
+	/* Last cycle value (also stored in clock->cycle_last) */
+	cycle_t			cycle_last;
 	/* Number of clock shifted nano seconds in one NTP interval. */
 	u64			xtime_interval;
 	/* shifted nano seconds left over when rounding cycle_interval */

kernel/time/timekeeping.c

@@ -101,7 +101,7 @@ static void tk_setup_internals(struct timekeeper *tk, struct clocksource *clock)
 
 	old_clock = tk->clock;
 	tk->clock = clock;
-	clock->cycle_last = clock->read(clock);
+	tk->cycle_last = clock->cycle_last = clock->read(clock);
 
 	/* Do the ns -> cycle conversion first, using original mult */
 	tmp = NTP_INTERVAL_LENGTH;
@@ -266,7 +266,7 @@ static void timekeeping_forward_now(struct timekeeper *tk)
 	clock = tk->clock;
 	cycle_now = clock->read(clock);
 	cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;
-	clock->cycle_last = cycle_now;
+	tk->cycle_last = clock->cycle_last = cycle_now;
 
 	tk->xtime_nsec += cycle_delta * tk->mult;