regulator: core: Reduce busy-wait looping

Keep busy-wait looping to a minimum while waiting for a regulator to
ramp-up to the target voltage. This follows the guidelines set forth
in Documentation/timers/timers-howto.txt and assumes that regulators
are never enabled in atomic context.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/regulator/core.c

@@ -1740,11 +1740,39 @@ static int _regulator_do_enable(struct regulator_dev *rdev)
 	 * together.  */
 	trace_regulator_enable_delay(rdev_get_name(rdev));
 
-	if (delay >= 1000) {
-		mdelay(delay / 1000);
-		udelay(delay % 1000);
-	} else if (delay) {
-		udelay(delay);
+	/*
+	 * Delay for the requested amount of time as per the guidelines in:
+	 *
+	 *     Documentation/timers/timers-howto.txt
+	 *
+	 * The assumption here is that regulators will never be enabled in
+	 * atomic context and therefore sleeping functions can be used.
+	 */
+	if (delay) {
+		unsigned int ms = delay / 1000;
+		unsigned int us = delay % 1000;
+
+		if (ms > 0) {
+			/*
+			 * For small enough values, handle super-millisecond
+			 * delays in the usleep_range() call below.
+			 */
+			if (ms < 20)
+				us += ms * 1000;
+			else
+				msleep(ms);
+		}
+
+		/*
+		 * Give the scheduler some room to coalesce with any other
+		 * wakeup sources. For delays shorter than 10 us, don't even
+		 * bother setting up high-resolution timers and just busy-
+		 * loop.
+		 */
+		if (us >= 10)
+			usleep_range(us, us + 100);
+		else
+			udelay(us);
 	}
 
 	trace_regulator_enable_complete(rdev_get_name(rdev));