thermal: add sanity check for the passive attribute

Values below 1000 milli-celsius don't make sense and can cause the
system to go into a thermal heart attack: the actual temperature
will always be lower and thus the system will be throttled down to
its lowest setting.

An additional problem is that values below 1000 will show as 0 in
/proc/acpi/thermal/TZx/trip_points:passive.

cat passive
0
echo -n 90 >passive
bash: echo: write error: Invalid argument
echo -n 90000 >passive
cat passive
90000
Signed-off-by: Frans Pop <elendil@planet.nl>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>

Documentation/thermal/sysfs-api.txt

@@ -206,6 +206,7 @@ passive
 	passive trip point for the zone. Activation is done by polling with
 	an interval of 1 second.
 	Unit: millidegrees Celsius
+	Valid values: 0 (disabled) or greater than 1000
 	RW, Optional
 
 *****************************

drivers/thermal/thermal_sys.c

@@ -225,6 +225,12 @@ passive_store(struct device *dev, struct device_attribute *attr,
 	if (!sscanf(buf, "%d\n", &state))
 		return -EINVAL;
 
+	/* sanity check: values below 1000 millicelcius don't make sense
+	 * and can cause the system to go into a thermal heart attack
+	 */
+	if (state && state < 1000)
+		return -EINVAL;
+
 	if (state && !tz->forced_passive) {
 		mutex_lock(&thermal_list_lock);
 		list_for_each_entry(cdev, &thermal_cdev_list, node) {