regulator: Optimise out noop voltage changes

If a consumer sets the same voltage range as is currently configured
for that consumer there's no need to run through setting the voltage
again. This pattern may occur with some CPUfreq implementations where
the same voltage range is used for multiple frequencies.
Reported-by: Saravana Kannan <skannan@codeaurora.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>

drivers/regulator/core.c

@@ -1697,10 +1697,17 @@ static int _regulator_do_set_voltage(struct regulator_dev *rdev,
 int regulator_set_voltage(struct regulator *regulator, int min_uV, int max_uV)
 {
 	struct regulator_dev *rdev = regulator->rdev;
-	int ret;
+	int ret = 0;
 
 	mutex_lock(&rdev->mutex);
 
+	/* If we're setting the same range as last time the change
+	 * should be a noop (some cpufreq implementations use the same
+	 * voltage for multiple frequencies, for example).
+	 */
+	if (regulator->min_uV == min_uV && regulator->max_uV == max_uV)
+		goto out;
+
 	/* sanity check */
 	if (!rdev->desc->ops->set_voltage &&
 	    !rdev->desc->ops->set_voltage_sel) {