[PATCH V2 16/19] timekeeping: Prepare timekeeping_cycles_to_ns() for overflow safety

[Adrian Hunter <adrian.hunter@intel.com>]

Open code clocksource_delta() in timekeeping_cycles_to_ns() so that
overflow safety can be added efficiently.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
---
 kernel/time/timekeeping.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 749387f22f0f..d17484082e2c 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -367,7 +367,17 @@ static void tk_setup_internals(struct timekeeper *tk, struct clocksource *clock)
 static inline u64 timekeeping_cycles_to_ns(const struct tk_read_base *tkr, u64 cycles)
 {
 	/* Calculate the delta since the last update_wall_time() */
-	u64 delta = clocksource_delta(cycles, tkr->cycle_last, tkr->mask);
+	u64 mask = tkr->mask, delta = (cycles - tkr->cycle_last) & mask;
+
+	if (IS_ENABLED(CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE)) {
+		/*
+		 * Handle clocksource inconsistency between CPUs to prevent
+		 * time from going backwards by checking for the MSB of the
+		 * mask being set in the delta.
+		 */
+		if (unlikely(delta & ~(mask >> 1)))
+			return tkr->xtime_nsec >> tkr->shift;
+	}
 
 	return ((delta * tkr->mult) + tkr->xtime_nsec) >> tkr->shift;
 }
-- 
2.34.1