3 files changed, 11 insertions, 4 deletions

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index 55e434feec99..f0a7fb984413 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -45,7 +45,8 @@ struct clocksource;
  * @read:               returns a cycle value
  * @mask:               bitmask for two's complement
  *                      subtraction of non 64 bit counters
- * @mult:               cycle to nanosecond multiplier
+ * @mult:               cycle to nanosecond multiplier (adjusted by NTP)
+ * @mult_orig:          cycle to nanosecond multiplier (unadjusted by NTP)
  * @shift:              cycle to nanosecond divisor (power of two)
  * @flags:              flags describing special properties
  * @vread:              vsyscall based read
@@ -63,6 +64,7 @@ struct clocksource {
         cycle_t (*read)(void);
         cycle_t mask;
         u32 mult;
+        u32 mult_orig;
         u32 shift;
         unsigned long flags;
         cycle_t (*vread)(void);
@@ -201,16 +203,17 @@ static inline void clocksource_calculate_interval(struct clocksource *c,
 {
         u64 tmp;
 
-        /* XXX - All of this could use a whole lot of optimization */
+        /* Do the ns -> cycle conversion first, using original mult */
         tmp = length_nsec;
         tmp <<= c->shift;
-        tmp += c->mult/2;
-        do_div(tmp, c->mult);
+        tmp += c->mult_orig/2;
+        do_div(tmp, c->mult_orig);
 
         c->cycle_interval = (cycle_t)tmp;
         if (c->cycle_interval == 0)
                 c->cycle_interval = 1;
 
+        /* Go back from cycles -> shifted ns, this time use ntp adjused mult */
         c->xtime_interval = (u64)c->cycle_interval * c->mult;
 }
 
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 093d4acf993b..9ed2eec97526 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -325,6 +325,9 @@ int clocksource_register(struct clocksource *c)
         unsigned long flags;
         int ret;
 
+        /* save mult_orig on registration */
+        c->mult_orig = c->mult;
+
         spin_lock_irqsave(&clocksource_lock, flags);
         ret = clocksource_enqueue(c);
         if (!ret)
diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c
index 4c256fdb8875..1ca99557e929 100644
--- a/kernel/time/jiffies.c
+++ b/kernel/time/jiffies.c
@@ -61,6 +61,7 @@ struct clocksource clocksource_jiffies = {
         .read           = jiffies_read,
         .mask           = 0xffffffff, /*32bits*/
         .mult           = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
+        .mult_orig      = NSEC_PER_JIFFY << JIFFIES_SHIFT,
         .shift          = JIFFIES_SHIFT,
 };