thermal/of: support thermal zones w/o trips subnode

Although the current device tree binding of thermal zones require the
trips subnode, the binding in kernel v5.15 does not require it, and many
device trees shipped with the kernel, for example,
allwinner/sun50i-a64.dtsi and mediatek/mt8183-kukui.dtsi in ARM64, still
comply to the old binding and contain no trips subnode.

Allow the code to successfully register thermal zones w/o trips subnode
for DT binding compatibility now.

Furtherly, the inconsistency between DTs and bindings should be resolved
by either adding empty trips subnode or dropping the trips subnode
requirement.

Fixes: d0c75fa ("thermal/of: Initialize trip points separately")
Signed-off-by: Icenowy Zheng <[email protected]>
[[email protected]: Reworked logic and kernel log messages]
Signed-off-by: Chen-Yu Tsai <[email protected]>
Reviewed-by: Rafael J. Wysocki <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Daniel Lezcano <[email protected]>

Author: Icenowy

drivers/thermal/thermal_of.c

@@ -99,18 +99,15 @@ static struct thermal_trip *thermal_of_trips_init(struct device_node *np, int *n
 	struct device_node *trips;
 	int ret, count;
 
+	*ntrips = 0;
+	
 	trips = of_get_child_by_name(np, "trips");
-	if (!trips) {
-		pr_err("Failed to find 'trips' node\n");
-		return ERR_PTR(-EINVAL);
-	}
+	if (!trips)
+		return NULL;
 
 	count = of_get_child_count(trips);
-	if (!count) {
-		pr_err("No trip point defined\n");
-		ret = -EINVAL;
-		goto out_of_node_put;
-	}
+	if (!count)
+		return NULL;
 
 	tt = kzalloc(sizeof(*tt) * count, GFP_KERNEL);
 	if (!tt) {
@@ -133,7 +130,6 @@ static struct thermal_trip *thermal_of_trips_init(struct device_node *np, int *n
 
 out_kfree:
 	kfree(tt);
-	*ntrips = 0;
 out_of_node_put:
 	of_node_put(trips);
 
@@ -401,11 +397,14 @@ static struct thermal_zone_device *thermal_of_zone_register(struct device_node *
 
 	trips = thermal_of_trips_init(np, &ntrips);
 	if (IS_ERR(trips)) {
-		pr_err("Failed to find trip points for %pOFn id=%d\n", sensor, id);
+		pr_err("Failed to parse trip points for %pOFn id=%d\n", sensor, id);
 		ret = PTR_ERR(trips);
 		goto out_of_node_put;
 	}
 
+	if (!trips)
+		pr_info("No trip points found for %pOFn id=%d\n", sensor, id);
+
 	ret = thermal_of_monitor_init(np, &delay, &pdelay);
 	if (ret) {
 		pr_err("Failed to initialize monitoring delays from %pOFn\n", np);