[PATCH] Time: Let user request precision from current_tick_length()

Change the current_tick_length() function so it takes an argument which
specifies how much precision to return in shifted nanoseconds.  This provides
a simple way to convert between NTPs internal nanoseconds shifted by
(SHIFT_SCALE - 10) to other shifted nanosecond units that are used by the
clocksource abstraction.

Signed-off-by: John Stultz <johnstul@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

3 files changed, 19 insertions, 6 deletions

diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index d20907561f46..742f07a63161 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -534,7 +534,7 @@ static __inline__ void timer_recalc_offset(u64 cur_tb)
 
         if (__USE_RTC())
                 return;
-        tlen = current_tick_length();
+        tlen = current_tick_length(SHIFT_SCALE - 10);
         offset = cur_tb - do_gtod.varp->tb_orig_stamp;
         if (tlen == last_tick_len && offset < 0x80000000u)
                 return;
diff --git a/include/linux/timex.h b/include/linux/timex.h
index 34d3ccff7bbb..1ba3071fcb82 100644
--- a/include/linux/timex.h
+++ b/include/linux/timex.h
@@ -304,7 +304,7 @@ time_interpolator_reset(void)
 #endif /* !CONFIG_TIME_INTERPOLATION */
 
 /* Returns how long ticks are at present, in ns / 2^(SHIFT_SCALE-10). */
-extern u64 current_tick_length(void);
+extern u64 current_tick_length(long);
 
 extern int do_adjtimex(struct timex *);
 
diff --git a/kernel/timer.c b/kernel/timer.c
index 524c7f638365..623f9ea198d8 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -780,16 +780,29 @@ static void update_wall_time_one_tick(void)
  * Return how long ticks are at the moment, that is, how much time
  * update_wall_time_one_tick will add to xtime next time we call it
  * (assuming no calls to do_adjtimex in the meantime).
- * The return value is in fixed-point nanoseconds with SHIFT_SCALE-10
- * bits to the right of the binary point.
+ * The return value is in fixed-point nanoseconds shifted by the
+ * specified number of bits to the right of the binary point.
  * This function has no side-effects.
  */
-u64 current_tick_length(void)
+u64 current_tick_length(long shift)
 {
         long delta_nsec;
+        u64 ret;
 
+        /* calculate the finest interval NTP will allow.
+         *    ie: nanosecond value shifted by (SHIFT_SCALE - 10)
+         */
         delta_nsec = tick_nsec + adjtime_adjustment() * 1000;
-        return ((u64) delta_nsec << (SHIFT_SCALE - 10)) + time_adj;
+        ret = ((u64) delta_nsec << (SHIFT_SCALE - 10)) + time_adj;
+
+        /* convert from (SHIFT_SCALE - 10) to specified shift scale: */
+        shift = shift - (SHIFT_SCALE - 10);
+        if (shift < 0)
+                ret >>= -shift;
+        else
+                ret <<= shift;
+
+        return ret;
 }
 
 /* XXX - all of this timekeeping code should be later moved to time.c */