clocksource: Get rid of the hardcoded 5 seconds sleep time limit

Slow clocksources can have a way longer sleep time than 5 seconds and
even fast ones can easily cope with 600 seconds and still maintain
proper accuracy.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Reviewed-by: Ingo Molnar <mingo@elte.hu>
Link: http://lkml.kernel.org/r/%3C20110518210136.109811585%40linutronix.de%3E

kernel/time/clocksource.c

@@ -626,19 +626,6 @@ static void clocksource_enqueue(struct clocksource *cs)
 	list_add(&cs->list, entry);
 }
 
-
-/*
- * Maximum time we expect to go between ticks. This includes idle
- * tickless time. It provides the trade off between selecting a
- * mult/shift pair that is very precise but can only handle a short
- * period of time, vs. a mult/shift pair that can handle long periods
- * of time but isn't as precise.
- *
- * This is a subsystem constant, and actual hardware limitations
- * may override it (ie: clocksources that wrap every 3 seconds).
- */
-#define MAX_UPDATE_LENGTH 5 /* Seconds */
-
 /**
  * __clocksource_updatefreq_scale - Used update clocksource with new freq
  * @t:		clocksource to be registered
@@ -652,15 +639,28 @@ static void clocksource_enqueue(struct clocksource *cs)
  */
 void __clocksource_updatefreq_scale(struct clocksource *cs, u32 scale, u32 freq)
 {
+	unsigned long sec;
+
 	/*
-	 * Ideally we want to use  some of the limits used in
-	 * clocksource_max_deferment, to provide a more informed
-	 * MAX_UPDATE_LENGTH. But for now this just gets the
-	 * register interface working properly.
+	 * Calc the maximum number of seconds which we can run before
+	 * wrapping around. For clocksources which have a mask > 32bit
+	 * we need to limit the max sleep time to have a good
+	 * conversion precision. 10 minutes is still a reasonable
+	 * amount. That results in a shift value of 24 for a
+	 * clocksource with mask >= 40bit and f >= 4GHz. That maps to
+	 * ~ 0.06ppm granularity for NTP. We apply the same 12.5%
+	 * margin as we do in clocksource_max_deferment()
 	 */
+	sec = (cs->mask - (cs->mask >> 5));
+	do_div(sec, freq);
+	do_div(sec, scale);
+	if (!sec)
+		sec = 1;
+	else if (sec > 600 && cs->mask > UINT_MAX)
+		sec = 600;
+
 	clocks_calc_mult_shift(&cs->mult, &cs->shift, freq,
-				      NSEC_PER_SEC/scale,
-				      MAX_UPDATE_LENGTH*scale);
+			       NSEC_PER_SEC / scale, sec * scale);
 	cs->max_idle_ns = clocksource_max_deferment(cs);
 }
 EXPORT_SYMBOL_GPL(__clocksource_updatefreq_scale);