mfd: syscon: Restore device_node_to_regmap() for non-syscon nodes

Commit ba5095e ("mfd: syscon: Allow syscon nodes without a
"syscon" compatible") broke drivers which call device_node_to_regmap()
on nodes without a "syscon" compatible. Restore the prior behavior for
device_node_to_regmap().

This also makes using device_node_to_regmap() incompatible with
of_syscon_register_regmap() again, so add kerneldoc for
device_node_to_regmap() and syscon_node_to_regmap() to make it clear
how and when each one should be used.

Fixes: ba5095e ("mfd: syscon: Allow syscon nodes without a "syscon" compatible")
Reported-by: Vaishnav Achath <vaishnav.a@ti.com>
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: Chen-Yu Tsai <wenst@chromium.org>
Tested-by: Nishanth Menon <nm@ti.com>
Tested-by: Daniel Golle <daniel@makrotopia.org>
Tested-by: Frank Wunderlich <frank-w@public-files.de>
Tested-by: Dhruva Gole <d-gole@ti.com>
Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20250124191644.2309790-1-robh@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>

drivers/mfd/syscon.c

@@ -159,6 +159,7 @@ static struct syscon *of_syscon_register(struct device_node *np, bool check_res)
 }
 
 static struct regmap *device_node_get_regmap(struct device_node *np,
+					     bool create_regmap,
 					     bool check_res)
 {
 	struct syscon *entry, *syscon = NULL;
@@ -172,7 +173,7 @@ static struct regmap *device_node_get_regmap(struct device_node *np,
 		}
 
 	if (!syscon) {
-		if (of_device_is_compatible(np, "syscon"))
+		if (create_regmap)
 			syscon = of_syscon_register(np, check_res);
 		else
 			syscon = ERR_PTR(-EINVAL);
@@ -233,15 +234,37 @@ int of_syscon_register_regmap(struct device_node *np, struct regmap *regmap)
 }
 EXPORT_SYMBOL_GPL(of_syscon_register_regmap);
 
+/**
+ * device_node_to_regmap() - Get or create a regmap for specified device node
+ * @np: Device tree node
+ *
+ * Get a regmap for the specified device node. If there's not an existing
+ * regmap, then one is instantiated. This function should not be used if the
+ * device node has a custom regmap driver or has resources (clocks, resets) to
+ * be managed. Use syscon_node_to_regmap() instead for those cases.
+ *
+ * Return: regmap ptr on success, negative error code on failure.
+ */
 struct regmap *device_node_to_regmap(struct device_node *np)
 {
-	return device_node_get_regmap(np, false);
+	return device_node_get_regmap(np, true, false);
 }
 EXPORT_SYMBOL_GPL(device_node_to_regmap);
 
+/**
+ * syscon_node_to_regmap() - Get or create a regmap for specified syscon device node
+ * @np: Device tree node
+ *
+ * Get a regmap for the specified device node. If there's not an existing
+ * regmap, then one is instantiated if the node is a generic "syscon". This
+ * function is safe to use for a syscon registered with
+ * of_syscon_register_regmap().
+ *
+ * Return: regmap ptr on success, negative error code on failure.
+ */
 struct regmap *syscon_node_to_regmap(struct device_node *np)
 {
-	return device_node_get_regmap(np, true);
+	return device_node_get_regmap(np, of_device_is_compatible(np, "syscon"), true);
 }
 EXPORT_SYMBOL_GPL(syscon_node_to_regmap);