hwmon: scmi: Scale values to target desired HWMON units

If the SCMI firmware implementation is reporting values in a scale that
is different from the HWMON units, we need to scale up or down the value
according to how far apart they are.
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
[sudeep.holla: added check of scale = 0 for early exit in scmi_hwmon_scale]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/hwmon/scmi-hwmon.c

@@ -18,6 +18,50 @@ struct scmi_sensors {
 	const struct scmi_sensor_info **info[hwmon_max];
 };
 
+static inline u64 __pow10(u8 x)
+{
+	u64 r = 1;
+
+	while (x--)
+		r *= 10;
+
+	return r;
+}
+
+static int scmi_hwmon_scale(const struct scmi_sensor_info *sensor, u64 *value)
+{
+	s8 scale = sensor->scale;
+	u64 f;
+
+	switch (sensor->type) {
+	case TEMPERATURE_C:
+	case VOLTAGE:
+	case CURRENT:
+		scale += 3;
+		break;
+	case POWER:
+	case ENERGY:
+		scale += 6;
+		break;
+	default:
+		break;
+	}
+
+	if (scale == 0)
+		return 0;
+
+	if (abs(scale) > 19)
+		return -E2BIG;
+
+	f = __pow10(abs(scale));
+	if (scale > 0)
+		*value *= f;
+	else
+		*value = div64_u64(*value, f);
+
+	return 0;
+}
+
 static int scmi_hwmon_read(struct device *dev, enum hwmon_sensor_types type,
 			   u32 attr, int channel, long *val)
 {
@@ -29,6 +73,10 @@ static int scmi_hwmon_read(struct device *dev, enum hwmon_sensor_types type,
 
 	sensor = *(scmi_sensors->info[type] + channel);
 	ret = h->sensor_ops->reading_get(h, sensor->id, false, &value);
+	if (ret)
+		return ret;
+
+	ret = scmi_hwmon_scale(sensor, &value);
 	if (!ret)
 		*val = value;