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 <johnstul@us.ibm.com>
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

2 files changed, 8 insertions, 5 deletions

diff --git a/include/linux/timex.h b/include/linux/timex.h
index 8ea3e71ba7fa..c3f374786a43 100644
--- a/include/linux/timex.h
+++ b/include/linux/timex.h
@@ -232,7 +232,14 @@ static inline int ntp_synced(void)
 #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);
diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index e64efaf957e8..c88b5910e7ab 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -43,10 +43,6 @@ long time_freq;				/* frequency offset (scaled ppm)*/
 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)