[PATCH 1/2] hwmon: (cros_ec) Implement custom kelvin to celsius conversions

The ChromeOS EC APIs use integers representing degrees kelvin for
temperatures. The default conversions from linux/units.h will then
always convert these integer degrees celsius with a 150 millidegree
offset. This is a bit confusing, as it also differs from other CrOS EC
tooling. Internally the EC uses a kelvin to celsius offset of a round
273, so the current conversion is also not entirely accurate.

Implement a custom conversion which preserves round values.

Signed-off-by: Thomas Weißschuh <[email protected]>
---
 drivers/hwmon/cros_ec_hwmon.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/hwmon/cros_ec_hwmon.c b/drivers/hwmon/cros_ec_hwmon.c
index ea24056ae646..44291799dd8e 100644
--- a/drivers/hwmon/cros_ec_hwmon.c
+++ b/drivers/hwmon/cros_ec_hwmon.c
@@ -147,9 +147,17 @@ static bool cros_ec_hwmon_is_error_temp(u8 temp)
 	       temp == EC_TEMP_SENSOR_NOT_CALIBRATED;
 }
 
+/* This differs slightly from the variant in units.h to avoid rounding inconsistencies. */
+#define CROS_EC_HWMON_ABSOLUTE_ZERO_MILLICELSIUS (-273000)
+
+static long cros_ec_hwmon_kelvin_to_millicelsius(long t)
+{
+	return t * MILLIDEGREE_PER_DEGREE + CROS_EC_HWMON_ABSOLUTE_ZERO_MILLICELSIUS;
+}
+
 static long cros_ec_hwmon_temp_to_millicelsius(u8 temp)
 {
-	return kelvin_to_millicelsius((((long)temp) + EC_TEMP_SENSOR_OFFSET));
+	return cros_ec_hwmon_kelvin_to_millicelsius((((long)temp) + EC_TEMP_SENSOR_OFFSET));
 }
 
 static bool cros_ec_hwmon_attr_is_temp_threshold(u32 attr)
@@ -228,7 +236,7 @@ static int cros_ec_hwmon_read(struct device *dev, enum hwmon_sensor_types type,
 								cros_ec_hwmon_attr_to_thres(attr),
 								&threshold);
 			if (ret == 0)
-				*val = kelvin_to_millicelsius(threshold);
+				*val = cros_ec_hwmon_kelvin_to_millicelsius(threshold);
 		}
 	}
 

-- 
2.55.0