thermal: core: Add Kconfig option to enable writable trips

Add a Kconfig option to allow system integrators to control whether
userspace tools can change trip temperatures. This option overrides
the thermal zone setup in the driver code and must be enabled for
platform specified writable trips to come into effect.

The original behaviour of requiring root privileges to change trip
temperatures remains unchanged.

Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>

drivers/thermal/Kconfig

@@ -42,6 +42,17 @@ config THERMAL_OF
 	  Say 'Y' here if you need to build thermal infrastructure
 	  based on device tree.
 
+config THERMAL_WRITABLE_TRIPS
+	bool "Enable writable trip points"
+	help
+	  This option allows the system integrator to choose whether
+	  trip temperatures can be changed from userspace. The
+	  writable trips need to be specified when setting up the
+	  thermal zone but the choice here takes precedence.
+
+	  Say 'Y' here if you would like to allow userspace tools to
+	  change trip temperatures.
+
 choice
 	prompt "Default Thermal governor"
 	default THERMAL_DEFAULT_GOV_STEP_WISE

drivers/thermal/thermal_core.c

@@ -1539,7 +1539,8 @@ static int create_trip_attrs(struct thermal_zone_device *tz, int mask)
 						tz->trip_temp_attrs[indx].name;
 		tz->trip_temp_attrs[indx].attr.attr.mode = S_IRUGO;
 		tz->trip_temp_attrs[indx].attr.show = trip_point_temp_show;
-		if (mask & (1 << indx)) {
+		if (IS_ENABLED(CONFIG_THERMAL_WRITABLE_TRIPS) &&
+		    mask & (1 << indx)) {
 			tz->trip_temp_attrs[indx].attr.attr.mode |= S_IWUSR;
 			tz->trip_temp_attrs[indx].attr.store =
 							trip_point_temp_store;