hwmon: (scpi-hwmon) shows the negative temperature properly

The scpi hwmon shows the sub-zero temperature in an unsigned integer,
which would confuse the users when the machine works in low temperature
environment. This shows the sub-zero temperature in an signed value and
users can get it properly from sensors.
Signed-off-by: Riwen Lu <luriwen@kylinos.cn>
Tested-by: Xin Chen <chenxin@kylinos.cn>
Link: https://lore.kernel.org/r/20210604030959.736379-1-luriwen@kylinos.cnSigned-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/scpi-hwmon.c

@@ -99,6 +99,15 @@ scpi_show_sensor(struct device *dev, struct device_attribute *attr, char *buf)
 
 	scpi_scale_reading(&value, sensor);
 
+	/*
+	 * Temperature sensor values are treated as signed values based on
+	 * observation even though that is not explicitly specified, and
+	 * because an unsigned u64 temperature does not really make practical
+	 * sense especially when the temperature is below zero degrees Celsius.
+	 */
+	if (sensor->info.class == TEMPERATURE)
+		return sprintf(buf, "%lld\n", (s64)value);
+
 	return sprintf(buf, "%llu\n", value);
 }