1 files changed, 34 insertions, 7 deletions

diff --git a/drivers/acpi/thermal.c b/drivers/acpi/thermal.c
index e8c143caf0fd..ddbb7c8f994d 100644
--- a/drivers/acpi/thermal.c
+++ b/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;