clocksource: Make clocksource validation work for all clocksources

BugLink: http://bugs.launchpad.net/bugs/1519625

The clocksource validation which makes sure that the newly read value
is not smaller than the last value only works if the clocksource mask
is 64bit, i.e. the counter is 64bit wide. But we want to use that
mechanism also for clocksources which are less than 64bit wide.

So instead of checking whether bit 63 is set, we check whether the
most significant bit of the clocksource mask is set in the delta
result. If it is set, we return 0.

[ tglx: Simplified the implementation, added a comment and massaged
  	the commit message ]
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Cc: <linux-arm-kernel@lists.infradead.org>
Link: http://lkml.kernel.org/r/56349607.6070708@huawei.comSigned-off-by: Thomas Gleixner <tglx@linutronix.de>
(cherry picked from commit 1f45f1f3)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

kernel/time/timekeeping_internal.h

@@ -17,7 +17,11 @@ static inline cycle_t clocksource_delta(cycle_t now, cycle_t last, cycle_t mask)
 {
 	cycle_t ret = (now - last) & mask;
 
-	return (s64) ret > 0 ? ret : 0;
+	/*
+	 * Prevent time going backwards by checking the MSB of mask in
+	 * the result. If set, return 0.
+	 */
+	return ret & ~(mask >> 1) ? 0 : ret;
 }
 #else
 static inline cycle_t clocksource_delta(cycle_t now, cycle_t last, cycle_t mask)