cpuidle: enter_state: Don't needlessly calculate diff time

Currently, ktime_us_delta() is invoked unconditionally to compute the
idle residency of the CPU, but it only makes sense to do that if a
valid idle state has been entered, so move the ktime_us_delta()
invocation after the entered_state >= 0 check.

While at it, merge two comment blocks in there into one and drop
a space between type casting of diff.

This patch has no functional changes.
Signed-off-by: Fieah Lim <kw@fieahl.im>
[ rjw: Changelog cleanup, comment format fix ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/cpuidle/cpuidle.c

@@ -247,17 +247,17 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
 	if (!cpuidle_state_is_coupled(drv, index))
 		local_irq_enable();
 
-	diff = ktime_us_delta(time_end, time_start);
-	if (diff > INT_MAX)
-		diff = INT_MAX;
-
-	dev->last_residency = (int) diff;
-
 	if (entered_state >= 0) {
-		/* Update cpuidle counters */
-		/* This can be moved to within driver enter routine
+		/*
+		 * Update cpuidle counters
+		 * This can be moved to within driver enter routine,
 		 * but that results in multiple copies of same code.
 		 */
+		diff = ktime_us_delta(time_end, time_start);
+		if (diff > INT_MAX)
+			diff = INT_MAX;
+
+		dev->last_residency = (int)diff;
 		dev->states_usage[entered_state].time += dev->last_residency;
 		dev->states_usage[entered_state].usage++;
 	} else {