gpiolib: return -ENOENT if no GPIO mapping exists

Some devices drivers make use of optional GPIO parameters. For such
drivers, it is important to discriminate between the case where no
GPIO mapping has been defined for the function they are requesting, and
the case where a mapping exists but an error occured while resolving it
or when acquiring the GPIO.

This patch changes the family of gpiod_get() functions such that they
will return -ENOENT if and only if no GPIO mapping is defined for the
requested function. Other error codes are used when an actual error
occured during the GPIO resolution.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/gpio/consumer.txt

@@ -38,7 +38,11 @@ device that displays digits), an additional index argument can be specified:
 					  const char *con_id, unsigned int idx)
 
 Both functions return either a valid GPIO descriptor, or an error code checkable
-with IS_ERR(). They will never return a NULL pointer.
+with IS_ERR() (they will never return a NULL pointer). -ENOENT will be returned
+if and only if no GPIO has been assigned to the device/function/index triplet,
+other error codes are used for cases where a GPIO has been assigned but an error
+occured while trying to acquire it. This is useful to discriminate between mere
+errors and an absence of GPIO for optional GPIO parameters.
 
 Device-managed variants of these functions are also defined:
 

drivers/gpio/gpiolib.c

@@ -2367,7 +2367,7 @@ static struct gpio_desc *gpiod_find(struct device *dev, const char *con_id,
 				    unsigned int idx,
 				    enum gpio_lookup_flags *flags)
 {
-	struct gpio_desc *desc = ERR_PTR(-ENODEV);
+	struct gpio_desc *desc = ERR_PTR(-ENOENT);
 	struct gpiod_lookup_table *table;
 	struct gpiod_lookup *p;
 
@@ -2389,19 +2389,22 @@ static struct gpio_desc *gpiod_find(struct device *dev, const char *con_id,
 		chip = find_chip_by_name(p->chip_label);
 
 		if (!chip) {
-			dev_warn(dev, "cannot find GPIO chip %s\n",
-				 p->chip_label);
-			continue;
+			dev_err(dev, "cannot find GPIO chip %s\n",
+				p->chip_label);
+			return ERR_PTR(-ENODEV);
 		}
 
 		if (chip->ngpio <= p->chip_hwnum) {
-			dev_warn(dev, "GPIO chip %s has %d GPIOs\n",
-				 chip->label, chip->ngpio);
-			continue;
+			dev_err(dev,
+				"requested GPIO %d is out of range [0..%d] for chip %s\n",
+				idx, chip->ngpio, chip->label);
+			return ERR_PTR(-EINVAL);
 		}
 
 		desc = gpiochip_offset_to_desc(chip, p->chip_hwnum);
 		*flags = p->flags;
+
+		return desc;
 	}
 
 	return desc;
@@ -2413,7 +2416,8 @@ static struct gpio_desc *gpiod_find(struct device *dev, const char *con_id,
  * @con_id:	function within the GPIO consumer
  *
  * Return the GPIO descriptor corresponding to the function con_id of device
- * dev, or an IS_ERR() condition if an error occured.
+ * dev, -ENOENT if no GPIO has been assigned to the requested function, or
+ * another IS_ERR() code if an error occured while trying to acquire the GPIO.
  */
 struct gpio_desc *__must_check gpiod_get(struct device *dev, const char *con_id)
 {
@@ -2430,7 +2434,9 @@ EXPORT_SYMBOL_GPL(gpiod_get);
  * This variant of gpiod_get() allows to access GPIOs other than the first
  * defined one for functions that define several GPIOs.
  *
- * Return a valid GPIO descriptor, or an IS_ERR() condition in case of error.
+ * Return a valid GPIO descriptor, -ENOENT if no GPIO has been assigned to the
+ * requested function and/or index, or another IS_ERR() code if an error
+ * occured while trying to acquire the GPIO.
  */
 struct gpio_desc *__must_check gpiod_get_index(struct device *dev,
 					       const char *con_id,
@@ -2455,15 +2461,9 @@ struct gpio_desc *__must_check gpiod_get_index(struct device *dev,
 	 * Either we are not using DT or ACPI, or their lookup did not return
 	 * a result. In that case, use platform lookup as a fallback.
 	 */
-	if (!desc || IS_ERR(desc)) {
-		struct gpio_desc *pdesc;
-
+	if (!desc || desc == ERR_PTR(-ENOENT)) {
 		dev_dbg(dev, "using lookup tables for GPIO lookup");
-		pdesc = gpiod_find(dev, con_id, idx, &flags);
-
-		/* If used as fallback, do not replace the previous error */
-		if (!IS_ERR(pdesc) || !desc)
-			desc = pdesc;
+		desc = gpiod_find(dev, con_id, idx, &flags);
 	}
 
 	if (IS_ERR(desc)) {