timekeeping: Let timekeeping_cycles_to_ns() handle both under and overflow

For the case !CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE, forego overflow
protection in the range (mask << 1) < delta <= mask, and interpret it
always as an inconsistency between CPU clock values. That allows
slightly neater code, and it is on a slow path so has no effect on
performance.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240325064023.2997-19-adrian.hunter@intel.com

Author: ahunter6

kernel/time/timekeeping.c

@@ -266,17 +266,14 @@ static inline u64 timekeeping_debug_get_ns(const struct tk_read_base *tkr)
 	 * Try to catch underflows by checking if we are seeing small
 	 * mask-relative negative values.
 	 */
-	if (unlikely((~delta & mask) < (mask >> 3))) {
+	if (unlikely((~delta & mask) < (mask >> 3)))
 		tk->underflow_seen = 1;
-		now = last;
-	}
 
-	/* Cap delta value to the max_cycles values to avoid mult overflows */
-	if (unlikely(delta > max)) {
+	/* Check for multiplication overflows */
+	if (unlikely(delta > max))
 		tk->overflow_seen = 1;
-		now = last + max;
-	}
 
+	/* timekeeping_cycles_to_ns() handles both under and overflow */
 	return timekeeping_cycles_to_ns(tkr, now);
 }
 #else
@@ -375,19 +372,17 @@ static inline u64 timekeeping_cycles_to_ns(const struct tk_read_base *tkr, u64 c
 	u64 mask = tkr->mask, delta = (cycles - tkr->cycle_last) & mask;
 
 	/*
-	 * This detects the case where the delta overflows the multiplication
-	 * with tkr->mult.
+	 * This detects both negative motion and the case where the delta
+	 * overflows the multiplication with tkr->mult.
 	 */
 	if (unlikely(delta > tkr->clock->max_cycles)) {
-		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;
-		}
+		/*
+		 * 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 (delta & ~(mask >> 1))
+			return tkr->xtime_nsec >> tkr->shift;
 
 		return delta_to_ns_safe(tkr, delta);
 	}