hwmon: (ftsteutates) Fix scaling of measurements

[ Upstream commit ca8fd8c16a ]

A user complained that the ftsteutates driver was displaying
bogus values since its introduction. This happens because the
sensor measurements need to be scaled in order to produce
meaningful results:
- the fan speed needs to be multiplied by 60 since its in RPS
- the temperature is in degrees celsius and needs an offset of 64
- the voltage is in 1/256 of 3.3V

The offical datasheet says the voltage needs to be divided by 256,
but this is likely an off-by-one-error, since even the BIOS
devides by 255 (otherwise 3.3V could not be measured).

The voltage channels additionally need a board-specific multiplier,
however this can be done by the driver since its board-specific.

The reason the missing scaling of measurements is the way Fujitsu
used this driver when it was still out-of-tree. Back then, all
scaling was done in userspace by libsensors, even the generic one.

Tested on a Fujitsu DS3401-B1.

Fixes: 08426eda58 ("hwmon: Add driver for FTS BMC chip "Teutates"")
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://lore.kernel.org/r/20221224041855.83981-2-W_Armin@gmx.de
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Documentation/hwmon/ftsteutates.rst

@@ -22,6 +22,10 @@ enhancements. It can monitor up to 4 voltages, 16 temperatures and
 8 fans. It also contains an integrated watchdog which is currently
 implemented in this driver.
 
+The 4 voltages require a board-specific multiplier, since the BMC can
+only measure voltages up to 3.3V and thus relies on voltage dividers.
+Consult your motherboard manual for details.
+
 To clear a temperature or fan alarm, execute the following command with the
 correct path to the alarm file::
 

drivers/hwmon/ftsteutates.c

@@ -12,6 +12,7 @@
 #include <linux/i2c.h>
 #include <linux/init.h>
 #include <linux/jiffies.h>
+#include <linux/math.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
 #include <linux/slab.h>
@@ -347,13 +348,15 @@ static ssize_t in_value_show(struct device *dev,
 {
 	struct fts_data *data = dev_get_drvdata(dev);
 	int index = to_sensor_dev_attr(devattr)->index;
-	int err;
+	int value, err;
 
 	err = fts_update_device(data);
 	if (err < 0)
 		return err;
 
-	return sprintf(buf, "%u\n", data->volt[index]);
+	value = DIV_ROUND_CLOSEST(data->volt[index] * 3300, 255);
+
+	return sprintf(buf, "%d\n", value);
 }
 
 static ssize_t temp_value_show(struct device *dev,
@@ -361,13 +364,15 @@ static ssize_t temp_value_show(struct device *dev,
 {
 	struct fts_data *data = dev_get_drvdata(dev);
 	int index = to_sensor_dev_attr(devattr)->index;
-	int err;
+	int value, err;
 
 	err = fts_update_device(data);
 	if (err < 0)
 		return err;
 
-	return sprintf(buf, "%u\n", data->temp_input[index]);
+	value = (data->temp_input[index] - 64) * 1000;
+
+	return sprintf(buf, "%d\n", value);
 }
 
 static ssize_t temp_fault_show(struct device *dev,
@@ -436,13 +441,15 @@ static ssize_t fan_value_show(struct device *dev,
 {
 	struct fts_data *data = dev_get_drvdata(dev);
 	int index = to_sensor_dev_attr(devattr)->index;
-	int err;
+	int value, err;
 
 	err = fts_update_device(data);
 	if (err < 0)
 		return err;
 
-	return sprintf(buf, "%u\n", data->fan_input[index]);
+	value = data->fan_input[index] * 60;
+
+	return sprintf(buf, "%d\n", value);
 }
 
 static ssize_t fan_source_show(struct device *dev,