regulator: devres: add API for reference voltage supplies

A common use case for regulators is to supply a reference voltage to an
analog input or output device. This adds a new devres API to get,
enable, and get the voltage in a single call. This allows eliminating
boilerplate code in drivers that use reference supplies in this way.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240429-regulator-get-enable-get-votlage-v2-1-b1f11ab766c1@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/driver-api/driver-model/devres.rst

@@ -433,6 +433,7 @@ REGULATOR
   devm_regulator_bulk_put()
   devm_regulator_get()
   devm_regulator_get_enable()
+  devm_regulator_get_enable_read_voltage()
   devm_regulator_get_enable_optional()
   devm_regulator_get_exclusive()
   devm_regulator_get_optional()

drivers/regulator/devres.c

@@ -145,6 +145,65 @@ struct regulator *devm_regulator_get_optional(struct device *dev,
 }
 EXPORT_SYMBOL_GPL(devm_regulator_get_optional);
 
+/**
+ * devm_regulator_get_enable_read_voltage - Resource managed regulator get and
+ *                                          enable that returns the voltage
+ * @dev: device to supply
+ * @id:  supply name or regulator ID.
+ *
+ * Get and enable regulator for duration of the device life-time.
+ * regulator_disable() and regulator_put() are automatically called on driver
+ * detach. See regulator_get_optional(), regulator_enable(), and
+ * regulator_get_voltage() for more information.
+ *
+ * This is a convenience function for supplies that provide a reference voltage
+ * where the consumer driver just needs to know the voltage and keep the
+ * regulator enabled.
+ *
+ * In cases where the supply is not strictly required, callers can check for
+ * -ENODEV error and handle it accordingly.
+ *
+ * Returns: voltage in microvolts on success, or an error code on failure.
+ */
+int devm_regulator_get_enable_read_voltage(struct device *dev, const char *id)
+{
+	struct regulator *r;
+	int ret;
+
+	/*
+	 * Since we need a real voltage, we use devm_regulator_get_optional()
+	 * rather than getting a dummy regulator with devm_regulator_get() and
+	 * then letting regulator_get_voltage() fail with -EINVAL. This way, the
+	 * caller can handle the -ENODEV error code if needed instead of the
+	 * ambiguous -EINVAL.
+	 */
+	r = devm_regulator_get_optional(dev, id);
+	if (IS_ERR(r))
+		return PTR_ERR(r);
+
+	ret = regulator_enable(r);
+	if (ret)
+		goto err_regulator_put;
+
+	ret = devm_add_action_or_reset(dev, regulator_action_disable, r);
+	if (ret)
+		goto err_regulator_put;
+
+	ret = regulator_get_voltage(r);
+	if (ret < 0)
+		goto err_release_action;
+
+	return 0;
+
+err_release_action:
+	devm_release_action(dev, regulator_action_disable, r);
+err_regulator_put:
+	devm_regulator_put(r);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(devm_regulator_get_enable_read_voltage);
+
 static int devm_regulator_match(struct device *dev, void *res, void *data)
 {
 	struct regulator **r = res;

include/linux/regulator/consumer.h

@@ -164,6 +164,7 @@ struct regulator *__must_check devm_regulator_get_optional(struct device *dev,
 							   const char *id);
 int devm_regulator_get_enable(struct device *dev, const char *id);
 int devm_regulator_get_enable_optional(struct device *dev, const char *id);
+int devm_regulator_get_enable_read_voltage(struct device *dev, const char *id);
 void regulator_put(struct regulator *regulator);
 void devm_regulator_put(struct regulator *regulator);
 
@@ -329,6 +330,12 @@ static inline int devm_regulator_get_enable_optional(struct device *dev,
 	return -ENODEV;
 }
 
+static inline int devm_regulator_get_enable_read_voltage(struct device *dev,
+							 const char *id)
+{
+	return -ENODEV;
+}
+
 static inline struct regulator *__must_check
 regulator_get_optional(struct device *dev, const char *id)
 {