gpio: Add helpers for optional GPIOs

Introduce gpiod_get_optional() and gpiod_get_index_optional() helpers
that make it easier for drivers to handle optional GPIOs.

Currently in order to handle optional GPIOs, a driver needs to special
case error handling for -ENOENT, such as this:

	gpio = gpiod_get(dev, "foo");
	if (IS_ERR(gpio)) {
		if (PTR_ERR(gpio) != -ENOENT)
			return PTR_ERR(gpio);

		gpio = NULL;
	}

	if (gpio) {
		/* set up GPIO */
	}

With these new helpers the above is reduced to:

	gpio = gpiod_get_optional(dev, "foo");
	if (IS_ERR(gpio))
		return PTR_ERR(gpio);

	if (gpio) {
		/* set up GPIO */
	}

While at it, device-managed variants of these functions are also
provided.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/driver-model/devres.txt

@@ -312,4 +312,6 @@ SPI
 GPIO
   devm_gpiod_get()
   devm_gpiod_get_index()
+  devm_gpiod_get_optional()
+  devm_gpiod_get_index_optional()
   devm_gpiod_put()

drivers/gpio/devres.c

@@ -51,6 +51,22 @@ struct gpio_desc *__must_check devm_gpiod_get(struct device *dev,
 }
 EXPORT_SYMBOL(devm_gpiod_get);
 
+/**
+ * devm_gpiod_get_optional - Resource-managed gpiod_get_optional()
+ * @dev: GPIO consumer
+ * @con_id: function within the GPIO consumer
+ *
+ * Managed gpiod_get_optional(). GPIO descriptors returned from this function
+ * are automatically disposed on driver detach. See gpiod_get_optional() for
+ * detailed information about behavior and return values.
+ */
+struct gpio_desc *__must_check devm_gpiod_get_optional(struct device *dev,
+						       const char *con_id)
+{
+	return devm_gpiod_get_index_optional(dev, con_id, 0);
+}
+EXPORT_SYMBOL(devm_gpiod_get_optional);
+
 /**
  * devm_gpiod_get_index - Resource-managed gpiod_get_index()
  * @dev:	GPIO consumer
@@ -86,6 +102,33 @@ struct gpio_desc *__must_check devm_gpiod_get_index(struct device *dev,
 }
 EXPORT_SYMBOL(devm_gpiod_get_index);
 
+/**
+ * devm_gpiod_get_index_optional - Resource-managed gpiod_get_index_optional()
+ * @dev: GPIO consumer
+ * @con_id: function within the GPIO consumer
+ * @index: index of the GPIO to obtain in the consumer
+ *
+ * Managed gpiod_get_index_optional(). GPIO descriptors returned from this
+ * function are automatically disposed on driver detach. See
+ * gpiod_get_index_optional() for detailed information about behavior and
+ * return values.
+ */
+struct gpio_desc *__must_check devm_gpiod_get_index_optional(struct device *dev,
+							     const char *con_id,
+							     unsigned int index)
+{
+	struct gpio_desc *desc;
+
+	desc = devm_gpiod_get_index(dev, con_id, index);
+	if (IS_ERR(desc)) {
+		if (PTR_ERR(desc) == -ENOENT)
+			return NULL;
+	}
+
+	return desc;
+}
+EXPORT_SYMBOL(devm_gpiod_get_index_optional);
+
 /**
  * devm_gpiod_put - Resource-managed gpiod_put()
  * @desc:	GPIO descriptor to dispose of

drivers/gpio/gpiolib.c

@@ -2737,6 +2737,22 @@ struct gpio_desc *__must_check gpiod_get(struct device *dev, const char *con_id)
 }
 EXPORT_SYMBOL_GPL(gpiod_get);
 
+/**
+ * gpiod_get_optional - obtain an optional GPIO for a given GPIO function
+ * @dev: GPIO consumer, can be NULL for system-global GPIOs
+ * @con_id: function within the GPIO consumer
+ *
+ * This is equivalent to gpiod_get(), except that when no GPIO was assigned to
+ * the requested function it will return NULL. This is convenient for drivers
+ * that need to handle optional GPIOs.
+ */
+struct gpio_desc *__must_check gpiod_get_optional(struct device *dev,
+						  const char *con_id)
+{
+	return gpiod_get_index_optional(dev, con_id, 0);
+}
+EXPORT_SYMBOL_GPL(gpiod_get_optional);
+
 /**
  * gpiod_get_index - obtain a GPIO from a multi-index GPIO function
  * @dev:	GPIO consumer, can be NULL for system-global GPIOs
@@ -2799,6 +2815,33 @@ struct gpio_desc *__must_check gpiod_get_index(struct device *dev,
 }
 EXPORT_SYMBOL_GPL(gpiod_get_index);
 
+/**
+ * gpiod_get_index_optional - obtain an optional GPIO from a multi-index GPIO
+ *                            function
+ * @dev: GPIO consumer, can be NULL for system-global GPIOs
+ * @con_id: function within the GPIO consumer
+ * @index: index of the GPIO to obtain in the consumer
+ *
+ * This is equivalent to gpiod_get_index(), except that when no GPIO with the
+ * specified index was assigned to the requested function it will return NULL.
+ * This is convenient for drivers that need to handle optional GPIOs.
+ */
+struct gpio_desc *__must_check gpiod_get_index_optional(struct device *dev,
+							const char *con_id,
+							unsigned int index)
+{
+	struct gpio_desc *desc;
+
+	desc = gpiod_get_index(dev, con_id, index);
+	if (IS_ERR(desc)) {
+		if (PTR_ERR(desc) == -ENOENT)
+			return NULL;
+	}
+
+	return desc;
+}
+EXPORT_SYMBOL_GPL(gpiod_get_index_optional);
+
 /**
  * gpiod_put - dispose of a GPIO descriptor
  * @desc:	GPIO descriptor to dispose of

include/linux/gpio/consumer.h

@@ -23,6 +23,12 @@ struct gpio_desc *__must_check gpiod_get(struct device *dev,
 struct gpio_desc *__must_check gpiod_get_index(struct device *dev,
 					       const char *con_id,
 					       unsigned int idx);
+struct gpio_desc *__must_check gpiod_get_optional(struct device *dev,
+						  const char *con_id);
+struct gpio_desc *__must_check gpiod_get_index_optional(struct device *dev,
+							const char *con_id,
+							unsigned int index);
+
 void gpiod_put(struct gpio_desc *desc);
 
 struct gpio_desc *__must_check devm_gpiod_get(struct device *dev,
@@ -30,6 +36,12 @@ struct gpio_desc *__must_check devm_gpiod_get(struct device *dev,
 struct gpio_desc *__must_check devm_gpiod_get_index(struct device *dev,
 						    const char *con_id,
 						    unsigned int idx);
+struct gpio_desc *__must_check devm_gpiod_get_optional(struct device *dev,
+						       const char *con_id);
+struct gpio_desc *__must_check
+devm_gpiod_get_index_optional(struct device *dev, const char *con_id,
+			      unsigned int index);
+
 void devm_gpiod_put(struct device *dev, struct gpio_desc *desc);
 
 int gpiod_get_direction(const struct gpio_desc *desc);
@@ -73,6 +85,20 @@ static inline struct gpio_desc *__must_check gpiod_get_index(struct device *dev,
 {
 	return ERR_PTR(-ENOSYS);
 }
+
+static inline struct gpio_desc *__must_check
+gpiod_get_optional(struct device *dev, const char *con_id)
+{
+	return ERR_PTR(-ENOSYS);
+}
+
+static inline struct gpio_desc *__must_check
+gpiod_get_index_optional(struct device *dev, const char *con_id,
+			 unsigned int index)
+{
+	return ERR_PTR(-ENOSYS);
+}
+
 static inline void gpiod_put(struct gpio_desc *desc)
 {
 	might_sleep();
@@ -93,6 +119,20 @@ struct gpio_desc *__must_check devm_gpiod_get_index(struct device *dev,
 {
 	return ERR_PTR(-ENOSYS);
 }
+
+static inline struct gpio_desc *__must_check
+devm_gpiod_get_optional(struct device *dev, const char *con_id)
+{
+	return ERR_PTR(-ENOSYS);
+}
+
+static inline struct gpio_desc *__must_check
+devm_gpiod_get_index_optional(struct device *dev, const char *con_id,
+			      unsigned int index)
+{
+	return ERR_PTR(-ENOSYS);
+}
+
 static inline void devm_gpiod_put(struct device *dev, struct gpio_desc *desc)
 {
 	might_sleep();