gpiolib: convert 'devprop_gpiochip_set_names' to support multiple gpiochip banks per device

The default gpiolib-of implementation does not work with the multiple
gpiochip banks per device structure used for example by the gpio-mt7621
and gpio-brcmstb drivers. To fix these kind of situations driver code
is forced to fill the names to avoid the gpiolib code to set names
repeated along the banks. Instead of continue with that antipattern
fix the gpiolib core function to get expected behaviour for every
single situation adding a field 'offset' in the gpiochip structure.
Doing in this way, we can assume this offset will be zero for normal
driver code where only one gpiochip bank per device is used but
can be set explicitly in those drivers that really need more than
one gpiochip.
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Gregory Fong <gregory.0xf0@gmail.com>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>

drivers/gpio/gpiolib.c

@@ -382,10 +382,18 @@ static int devprop_gpiochip_set_names(struct gpio_chip *chip)
 	if (count < 0)
 		return 0;
 
-	if (count > gdev->ngpio) {
-		dev_warn(&gdev->dev, "gpio-line-names is length %d but should be at most length %d",
-			 count, gdev->ngpio);
-		count = gdev->ngpio;
+	/*
+	 * When offset is set in the driver side we assume the driver internally
+	 * is using more than one gpiochip per the same device. We have to stop
+	 * setting friendly names if the specified ones with 'gpio-line-names'
+	 * are less than the offset in the device itself. This means all the
+	 * lines are not present for every single pin within all the internal
+	 * gpiochips.
+	 */
+	if (count <= chip->offset) {
+		dev_warn(&gdev->dev, "gpio-line-names too short (length %d), cannot map names for the gpiochip at offset %u\n",
+			 count, chip->offset);
+		return 0;
 	}
 
 	names = kcalloc(count, sizeof(*names), GFP_KERNEL);
@@ -400,8 +408,22 @@ static int devprop_gpiochip_set_names(struct gpio_chip *chip)
 		return ret;
 	}
 
+	/*
+	 * When more that one gpiochip per device is used, 'count' can
+	 * contain at most number gpiochips x chip->ngpio. We have to
+	 * correctly distribute all defined lines taking into account
+	 * chip->offset as starting point from where we will assign
+	 * the names to pins from the 'names' array. Since property
+	 * 'gpio-line-names' cannot contains gaps, we have to be sure
+	 * we only assign those pins that really exists since chip->ngpio
+	 * can be different of the chip->offset.
+	 */
+	count = (count > chip->offset) ? count - chip->offset : count;
+	if (count > chip->ngpio)
+		count = chip->ngpio;
+
 	for (i = 0; i < count; i++)
-		gdev->descs[i].name = names[i];
+		gdev->descs[i].name = names[chip->offset + i];
 
 	kfree(names);
 

include/linux/gpio/driver.h

@@ -312,6 +312,9 @@ struct gpio_irq_chip {
  *	get rid of the static GPIO number space in the long run.
  * @ngpio: the number of GPIOs handled by this controller; the last GPIO
  *	handled is (base + ngpio - 1).
+ * @offset: when multiple gpio chips belong to the same device this
+ *	can be used as offset within the device so friendly names can
+ *	be properly assigned.
  * @names: if set, must be an array of strings to use as alternative
  *      names for the GPIOs in this chip. Any entry in the array
  *      may be NULL if there is no alias for the GPIO, however the
@@ -398,6 +401,7 @@ struct gpio_chip {
 
 	int			base;
 	u16			ngpio;
+	u16			offset;
 	const char		*const *names;
 	bool			can_sleep;