[PATCH 07/10] time: Try to catch clocksource delta underflows

[John Stultz <john.stultz@linaro.org>]

In the case where there is a broken clocksource
where there are multiple actual clocks that
aren't perfectly aligned, we may see small "negative"
deltas when we subtract now from cycle_last.

The values are actually negative with respect to the
clocksource mask value, not necessarily negative
if cast to a s64, but we can check by checking the
delta see if it is a small (relative to the mask)
negative value (again negative relative to the mask).

If so, we assume we jumped backwards somehow and
instead use zero for our delta.

Cc: Dave Jones <davej@codemonkey.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
 kernel/time/timekeeping.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 9740fd8..4acfc7f 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -224,6 +224,13 @@ static inline s64 timekeeping_get_ns_raw(struct timekeeper *tk)
 	/* calculate the delta since the last update_wall_time: */
 	delta = clocksource_delta(cycle_now, tk->tkr.cycle_last, tk->tkr.mask);
 
+	/*
+	 * Try to catch underflows by checking if we are seeing small
+	 * mask-relative negative values.
+	 */
+	if (((~delta+1) & tk->tkr.mask) < (tk->tkr.mask >> 3))
+		delta = 0;
+
 	/* Cap delta value to the max_cycles values to avoid mult overflows */
 	delta = min(delta, tk->tkr.clock->max_cycles);
 
-- 
1.9.1