[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: NJohn Stultz <johnstul@us.ibm.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

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;

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 *);
 

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 */