gpio: of: make it possible to name GPIO lines

Make it possible to name the producer side of a GPIO line using
a "gpio-line-names" property array, modeled on the
"clock-output-names" property from the clock bindings.

This naming is especially useful for:

- Debugging: lines are named after function, not just opaque
  offset numbers.

- Exploration: systems where some or all GPIO lines are available
  to end users, such as prototyping, one-off's "makerspace usecases"
  users are helped by the names of the GPIO lines when tinkering.
  This usecase has been surfacing recently.

The gpio-line-names attribute is completely optional.

Example output from lsgpio on a patched Snowball tree:

GPIO chip: gpiochip6, "8000e180.gpio", 32 GPIO lines
        line  0: unnamed unused
        line  1: "AP_GPIO161" "extkb3" [kernel]
        line  2: "AP_GPIO162" "extkb4" [kernel]
        line  3: "ACCELEROMETER_INT1_RDY" unused [kernel]
        line  4: "ACCELEROMETER_INT2" unused
        line  5: "MAG_DRDY" unused [kernel]
        line  6: "GYRO_DRDY" unused [kernel]
        line  7: "RSTn_MLC" unused
        line  8: "RSTn_SLC" unused
        line  9: "GYRO_INT" unused
        line 10: "UART_WAKE" unused
        line 11: "GBF_RESET" unused
        line 12: unnamed unused

Cc: Grant Likely <[email protected]>
Cc: Amit Kucheria <[email protected]>
Cc: David Mandala <[email protected]>
Cc: Lee Campbell <[email protected]>
Cc: [email protected]
Acked-by: Rob Herring <[email protected]>
Reviewed-by: Michael Welling <[email protected]>
Signed-off-by: Linus Walleij <[email protected]>

drivers/gpio/gpiolib-of.c

@@ -195,6 +195,51 @@ static struct gpio_desc *of_parse_own_gpio(struct device_node *np,
 	return gg_data.out_gpio;
 }
 
+/**
+ * of_gpiochip_set_names() - set up the names of the lines
+ * @chip: GPIO chip whose lines should be named, if possible
+ */
+static void of_gpiochip_set_names(struct gpio_chip *gc)
+{
+	struct gpio_device *gdev = gc->gpiodev;
+	struct device_node *np = gc->of_node;
+	int i;
+	int nstrings;
+
+	nstrings = of_property_count_strings(np, "gpio-line-names");
+	if (nstrings <= 0)
+		/* Lines names not present */
+		return;
+
+	/* This is normally not what you want */
+	if (gdev->ngpio != nstrings)
+		dev_info(&gdev->dev, "gpio-line-names specifies %d line "
+			 "names but there are %d lines on the chip\n",
+			 nstrings, gdev->ngpio);
+
+	/*
+	 * Make sure to not index beyond the end of the number of descriptors
+	 * of the GPIO device.
+	 */
+	for (i = 0; i < gdev->ngpio; i++) {
+		const char *name;
+		int ret;
+
+		ret = of_property_read_string_index(np,
+						    "gpio-line-names",
+						    i,
+						    &name);
+		if (ret) {
+			if (ret != -ENODATA)
+                                dev_err(&gdev->dev,
+                                        "unable to name line %d: %d\n",
+                                        i, ret);
+			break;
+		}
+		gdev->descs[i].name = name;
+	}
+}
+
 /**
  * of_gpiochip_scan_gpios - Scan gpio-controller for gpio definitions
  * @chip:	gpio chip to act on
@@ -445,6 +490,10 @@ int of_gpiochip_add(struct gpio_chip *chip)
 	if (status)
 		return status;
 
+	/* If the chip defines names itself, these take precedence */
+	if (!chip->names)
+		of_gpiochip_set_names(chip);
+
 	of_node_get(chip->of_node);
 
 	return of_gpiochip_scan_gpios(chip);