hwmon: (adm1275) Allow setting shunt reg value

The ADM series of hotswap controllers support extending
the current measurement range by using a sensing resistor
value other than the typical 1 mOhm. For example, using a 0.5 mOhm
sensing resistor doubles the maximal current can be measured.

Current driver assumes a shunt resistor value of 1 mOhm in calculation,
meaning for other resistor values, hwmon will report scaled
current/power measurements. This patch parses device tree parameter
"shunt-resistor-micro-ohms", if there is one.
Signed-off-by: Kun Yi <kunyi@google.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/adm1275

@@ -58,6 +58,9 @@ The ADM1075, unlike many other PMBus devices, does not support internal voltage
 or current scaling. Reported voltages, currents, and power are raw measurements,
 and will typically have to be scaled.
 
+The shunt value in micro-ohms can be set via device tree at compile-time. Please
+refer to the Documentation/devicetree/bindings/hwmon/adm1275.txt for bindings
+if the device tree is used.
 
 Platform data support
 ---------------------

drivers/hwmon/pmbus/adm1275.c

@@ -373,6 +373,7 @@ static int adm1275_probe(struct i2c_client *client,
 	const struct coefficients *coefficients;
 	int vindex = -1, voindex = -1, cindex = -1, pindex = -1;
 	int tindex = -1;
+	u32 shunt;
 
 	if (!i2c_check_functionality(client->adapter,
 				     I2C_FUNC_SMBUS_READ_BYTE_DATA
@@ -421,6 +422,13 @@ static int adm1275_probe(struct i2c_client *client,
 	if (!data)
 		return -ENOMEM;
 
+	if (of_property_read_u32(client->dev.of_node,
+				 "shunt-resistor-micro-ohms", &shunt))
+		shunt = 1000; /* 1 mOhm if not set via DT */
+
+	if (shunt == 0)
+		return -EINVAL;
+
 	data->id = mid->driver_data;
 
 	info = &data->info;
@@ -654,12 +662,15 @@ static int adm1275_probe(struct i2c_client *client,
 		info->R[PSC_VOLTAGE_OUT] = coefficients[voindex].R;
 	}
 	if (cindex >= 0) {
-		info->m[PSC_CURRENT_OUT] = coefficients[cindex].m;
+		/* Scale current with sense resistor value */
+		info->m[PSC_CURRENT_OUT] =
+			coefficients[cindex].m * shunt / 1000;
 		info->b[PSC_CURRENT_OUT] = coefficients[cindex].b;
 		info->R[PSC_CURRENT_OUT] = coefficients[cindex].R;
 	}
 	if (pindex >= 0) {
-		info->m[PSC_POWER] = coefficients[pindex].m;
+		info->m[PSC_POWER] =
+			coefficients[pindex].m * shunt / 1000;
 		info->b[PSC_POWER] = coefficients[pindex].b;
 		info->R[PSC_POWER] = coefficients[pindex].R;
 	}