gpio: of_get_named_gpio_flags() return -EPROBE_DEFER if GPIO not yet available

of_get_named_gpio_flags() and of_get_named_gpio() return -EPROBE_DEFER if the
respective GPIO is not (yet) available. This is useful if driver's probe()
functions try to get a GPIO whose controller isn't probed yet. Thus, the driver
can be probed again later on.

The function still returns -EINVAL on other errors (parse error or node doesn't
exist). This way, the case of an optional/intentionally missing GPIO is handled
appropriately.
Signed-off-by: Roland Stigge <stigge@antcom.de>
Acked-by: Alexandre Pereira da Silva <aletes.xgr@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib-of.c

@@ -62,7 +62,10 @@ static int of_gpiochip_find_and_xlate(struct gpio_chip *gc, void *data)
 int of_get_named_gpio_flags(struct device_node *np, const char *propname,
                            int index, enum of_gpio_flags *flags)
 {
-	struct gg_data gg_data = { .flags = flags, .out_gpio = -ENODEV };
+	/* Return -EPROBE_DEFER to support probe() functions to be called
+	 * later when the GPIO actually becomes available
+	 */
+	struct gg_data gg_data = { .flags = flags, .out_gpio = -EPROBE_DEFER };
 	int ret;
 
 	/* .of_xlate might decide to not fill in the flags, so clear it. */