hwmon: (ftsteutates) Fix scaling of measurements

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: 08426eda58e0 ("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>

1 files changed, 4 insertions, 0 deletions

diff --git a/Documentation/hwmon/ftsteutates.rst b/Documentation/hwmon/ftsteutates.rst
index 58a2483d8d0d..198fa8e2819d 100644
--- a/Documentation/hwmon/ftsteutates.rst
+++ b/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::