ntp: correct inconsistent interval/tick_length usage

clocksource initialization and error accumulation. This corrects a 280ppm
drift seen on some systems using acpi_pm, and affects other clocksources as
well (likely to a lesser degree).

Signed-off-by: John Stultz <[e-mail]>
Cc: Roman Zippel <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>

include/linux/timex.h

···
 #else
 #define NTP_INTERVAL_FREQ  (HZ)
 #endif
-#define NTP_INTERVAL_LENGTH (NSEC_PER_SEC/NTP_INTERVAL_FREQ)
+
+#define CLOCK_TICK_OVERFLOW	(LATCH * HZ - CLOCK_TICK_RATE)
+#define CLOCK_TICK_ADJUST	(((s64)CLOCK_TICK_OVERFLOW * NSEC_PER_SEC) / \
+					(s64)CLOCK_TICK_RATE)
+
+/* Because using NSEC_PER_SEC would be too easy */
+#define NTP_INTERVAL_LENGTH ((((s64)TICK_USEC * NSEC_PER_USEC * USER_HZ) + \
+			      CLOCK_TICK_ADJUST) / NTP_INTERVAL_FREQ)
 
 /* Returns how long ticks are at present, in ns / 2^(SHIFT_SCALE-10). */
 extern u64 current_tick_length(void);

kernel/time/ntp.c

···
 static long time_reftime;		/* time at last adjustment (s)	*/
 long time_adjust;
 
-#define CLOCK_TICK_OVERFLOW	(LATCH * HZ - CLOCK_TICK_RATE)
-#define CLOCK_TICK_ADJUST	(((s64)CLOCK_TICK_OVERFLOW * NSEC_PER_SEC) / \
-					(s64)CLOCK_TICK_RATE)
-
 static void ntp_update_frequency(void)
 {
 	u64 second_length = (u64)(tick_usec * NSEC_PER_USEC * USER_HZ)