pinctrl: implement pinctrl deferred probing

If drivers try to obtain pinctrl handles for a pin controller that
has not yet registered to the subsystem, we need to be able to
back out and retry with deferred probing. So let's return
-EPROBE_DEFER whenever this location fails. Also downgrade the
errors to info, maybe we will even set them to debug once the
deferred probing is commonplace.

Cc: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/pinctrl.txt

@@ -1043,6 +1043,11 @@ quickly poking some registers.
 The pins are allocated for your device when you issue the pinctrl_get() call,
 after this you should be able to see this in the debugfs listing of all pins.
 
+NOTE: the pinctrl system will return -EPROBE_DEFER if it cannot find the
+requested pinctrl handles, for example if the pinctrl driver has not yet
+registered. Thus make sure that the error path in your driver gracefully
+cleans up and is ready to retry the probing later in the startup process.
+
 
 System pin control hogging
 ==========================

drivers/pinctrl/core.c

@@ -518,11 +518,14 @@ static int add_setting(struct pinctrl *p, struct pinctrl_map const *map)
 
 	setting->pctldev = get_pinctrl_dev_from_devname(map->ctrl_dev_name);
 	if (setting->pctldev == NULL) {
-		dev_err(p->dev, "unknown pinctrl device %s in map entry",
+		dev_info(p->dev, "unknown pinctrl device %s in map entry, deferring probe",
 			map->ctrl_dev_name);
 		kfree(setting);
-		/* Eventually, this should trigger deferred probe */
-		return -ENODEV;
+		/*
+		 * OK let us guess that the driver is not there yet, and
+		 * let's defer obtaining this pinctrl handle to later...
+		 */
+		return -EPROBE_DEFER;
 	}
 
 	switch (map->type) {

drivers/pinctrl/devicetree.c

@@ -121,11 +121,11 @@ static int dt_to_map_one_config(struct pinctrl *p, const char *statename,
 	for (;;) {
 		np_pctldev = of_get_next_parent(np_pctldev);
 		if (!np_pctldev || of_node_is_root(np_pctldev)) {
-			dev_err(p->dev, "could not find pctldev for node %s\n",
+			dev_info(p->dev, "could not find pctldev for node %s, deferring probe\n",
 				np_config->full_name);
 			of_node_put(np_pctldev);
-			/* FIXME: This should trigger deferrered probe */
-			return -ENODEV;
+			/* OK let's just assume this will appear later then */
+			return -EPROBE_DEFER;
 		}
 		pctldev = find_pinctrl_by_of_node(np_pctldev);
 		if (pctldev)