[PATCH 1/3] clocksource: Also tweak ->maxadj in clocksource_calc_mult_shift()

[Yong Zhang <yong.zhang0@gmail.com>]

There is still user of clocksource_calc_mult_shift(), and it's
complement to commit d65670a7 [clocksource: Avoid selecting mult
values that might overflow when adjusted]

Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
 include/linux/clocksource.h |    8 ++------
 kernel/time/clocksource.c   |   18 ++++++++++++++++++
 2 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index c86c940..a02a8d7 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -314,12 +314,8 @@ static inline void __clocksource_updatefreq_khz(struct clocksource *cs, u32 khz)
 	__clocksource_updatefreq_scale(cs, 1000, khz);
 }
 
-static inline void
-clocksource_calc_mult_shift(struct clocksource *cs, u32 freq, u32 minsec)
-{
-	return clocks_calc_mult_shift(&cs->mult, &cs->shift, freq,
-				      NSEC_PER_SEC, minsec);
-}
+extern void
+clocksource_calc_mult_shift(struct clocksource *cs, u32 freq, u32 minsec);
 
 #ifdef CONFIG_GENERIC_TIME_VSYSCALL
 extern void
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index cfc65e1..5119042 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -724,6 +724,24 @@ int __clocksource_register_scale(struct clocksource *cs, u32 scale, u32 freq)
 }
 EXPORT_SYMBOL_GPL(__clocksource_register_scale);
 
+void clocksource_calc_mult_shift(struct clocksource *cs, u32 freq, u32 minsec)
+{
+	clocks_calc_mult_shift(&cs->mult, &cs->shift, freq,
+				NSEC_PER_SEC, minsec);
+
+	/*
+	 * for clocksources that have large mults, to avoid overflow.
+	 * Since mult may be adjusted by ntp, add an safety extra margin
+	 *
+	 */
+	cs->maxadj = clocksource_max_adjustment(cs);
+	while ((cs->mult + cs->maxadj < cs->mult)
+		|| (cs->mult - cs->maxadj > cs->mult)) {
+		cs->mult >>= 1;
+		cs->shift--;
+		cs->maxadj = clocksource_max_adjustment(cs);
+	}
+}
 
 /**
  * clocksource_register - Used to install new clocksources
-- 
1.7.5.4