ACPI: Adjust Kelvin offset to match local implementation

The exact offset between Kelvin and degree Celsius is 273.15. However
ACPI handles temperature values with a single decimal place. As a
consequence, some implementations use an offset of 273.1 and others
use an offset of 273.2. Try to find out which one is being used, to
present the most accurate and visually appealing number.

Tested on a Sony Vaio PGC-GR214EP (which uses 273.1) and a Lenovo
Thinkpad T60p (which uses 273.2).
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/thermal.c

@@ -192,6 +192,7 @@ struct acpi_thermal {
 	struct acpi_handle_list devices;
 	struct thermal_zone_device *thermal_zone;
 	int tz_enabled;
+	int kelvin_offset;
 	struct mutex lock;
 };
 
@@ -581,7 +582,7 @@ static void acpi_thermal_check(void *data)
 }
 
 /* sys I/F for generic thermal sysfs support */
-#define KELVIN_TO_MILLICELSIUS(t) (t * 100 - 273200)
+#define KELVIN_TO_MILLICELSIUS(t, off) (((t) - (off)) * 100)
 
 static int thermal_get_temp(struct thermal_zone_device *thermal,
 			    unsigned long *temp)
@@ -596,7 +597,7 @@ static int thermal_get_temp(struct thermal_zone_device *thermal,
 	if (result)
 		return result;
 
-	*temp = KELVIN_TO_MILLICELSIUS(tz->temperature);
+	*temp = KELVIN_TO_MILLICELSIUS(tz->temperature, tz->kelvin_offset);
 	return 0;
 }
 
@@ -702,7 +703,8 @@ static int thermal_get_trip_temp(struct thermal_zone_device *thermal,
 	if (tz->trips.critical.flags.valid) {
 		if (!trip) {
 			*temp = KELVIN_TO_MILLICELSIUS(
-				tz->trips.critical.temperature);
+				tz->trips.critical.temperature,
+				tz->kelvin_offset);
 			return 0;
 		}
 		trip--;
@@ -711,7 +713,8 @@ static int thermal_get_trip_temp(struct thermal_zone_device *thermal,
 	if (tz->trips.hot.flags.valid) {
 		if (!trip) {
 			*temp = KELVIN_TO_MILLICELSIUS(
-				tz->trips.hot.temperature);
+				tz->trips.hot.temperature,
+				tz->kelvin_offset);
 			return 0;
 		}
 		trip--;
@@ -720,7 +723,8 @@ static int thermal_get_trip_temp(struct thermal_zone_device *thermal,
 	if (tz->trips.passive.flags.valid) {
 		if (!trip) {
 			*temp = KELVIN_TO_MILLICELSIUS(
-				tz->trips.passive.temperature);
+				tz->trips.passive.temperature,
+				tz->kelvin_offset);
 			return 0;
 		}
 		trip--;
@@ -730,7 +734,8 @@ static int thermal_get_trip_temp(struct thermal_zone_device *thermal,
 		tz->trips.active[i].flags.valid; i++) {
 		if (!trip) {
 			*temp = KELVIN_TO_MILLICELSIUS(
-				tz->trips.active[i].temperature);
+				tz->trips.active[i].temperature,
+				tz->kelvin_offset);
 			return 0;
 		}
 		trip--;
@@ -745,7 +750,8 @@ static int thermal_get_crit_temp(struct thermal_zone_device *thermal,
 
 	if (tz->trips.critical.flags.valid) {
 		*temperature = KELVIN_TO_MILLICELSIUS(
-				tz->trips.critical.temperature);
+				tz->trips.critical.temperature,
+				tz->kelvin_offset);
 		return 0;
 	} else
 		return -EINVAL;
@@ -1334,6 +1340,25 @@ static int acpi_thermal_get_info(struct acpi_thermal *tz)
 	return 0;
 }
 
+/*
+ * The exact offset between Kelvin and degree Celsius is 273.15. However ACPI
+ * handles temperature values with a single decimal place. As a consequence,
+ * some implementations use an offset of 273.1 and others use an offset of
+ * 273.2. Try to find out which one is being used, to present the most
+ * accurate and visually appealing number.
+ *
+ * The heuristic below should work for all ACPI thermal zones which have a
+ * critical trip point with a value being a multiple of 0.5 degree Celsius.
+ */
+static void acpi_thermal_guess_offset(struct acpi_thermal *tz)
+{
+	if (tz->trips.critical.flags.valid &&
+	    (tz->trips.critical.temperature % 5) == 1)
+		tz->kelvin_offset = 2731;
+	else
+		tz->kelvin_offset = 2732;
+}
+
 static int acpi_thermal_add(struct acpi_device *device)
 {
 	int result = 0;
@@ -1360,6 +1385,8 @@ static int acpi_thermal_add(struct acpi_device *device)
 	if (result)
 		goto free_memory;
 
+	acpi_thermal_guess_offset(tz);
+
 	result = acpi_thermal_register_thermal_zone(tz);
 	if (result)
 		goto free_memory;