hwmon: (ina2xx) Make calibration register value fixed

Calibration register is used for calculating current register in
hardware according to datasheet:
current = shunt_volt * calib_register / 2048 (ina 226)
current = shunt_volt * calib_register / 4096 (ina 219)

Fix calib_register value to 2048 for ina226 and 4096 for ina 219 in
order to avoid truncation error and provide best precision allowed
by shunt_voltage measurement. Make current scale value follow changes
of shunt_resistor from sysfs as calib_register value is now fixed.

Power_lsb value should also follow shunt_resistor changes as stated in
datasheet:
power_lsb = 25 * current_lsb (ina 226)
power_lsb = 20 * current_lsb (ina 219)
Signed-off-by: Maciej Purski <m.purski@samsung.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/ina2xx.c

@@ -95,18 +95,20 @@ enum ina2xx_ids { ina219, ina226 };
 
 struct ina2xx_config {
 	u16 config_default;
-	int calibration_factor;
+	int calibration_value;
 	int registers;
 	int shunt_div;
 	int bus_voltage_shift;
 	int bus_voltage_lsb;	/* uV */
-	int power_lsb;		/* uW */
+	int power_lsb_factor;
 };
 
 struct ina2xx_data {
 	const struct ina2xx_config *config;
 
 	long rshunt;
+	long current_lsb_uA;
+	long power_lsb_uW;
 	struct mutex config_lock;
 	struct regmap *regmap;
 
@@ -116,21 +118,21 @@ struct ina2xx_data {
 static const struct ina2xx_config ina2xx_config[] = {
 	[ina219] = {
 		.config_default = INA219_CONFIG_DEFAULT,
-		.calibration_factor = 40960000,
+		.calibration_value = 4096,
 		.registers = INA219_REGISTERS,
 		.shunt_div = 100,
 		.bus_voltage_shift = 3,
 		.bus_voltage_lsb = 4000,
-		.power_lsb = 20000,
+		.power_lsb_factor = 20,
 	},
 	[ina226] = {
 		.config_default = INA226_CONFIG_DEFAULT,
-		.calibration_factor = 5120000,
+		.calibration_value = 2048,
 		.registers = INA226_REGISTERS,
 		.shunt_div = 400,
 		.bus_voltage_shift = 0,
 		.bus_voltage_lsb = 1250,
-		.power_lsb = 25000,
+		.power_lsb_factor = 25,
 	},
 };
 
@@ -169,12 +171,16 @@ static u16 ina226_interval_to_reg(int interval)
 	return INA226_SHIFT_AVG(avg_bits);
 }
 
+/*
+ * Calibration register is set to the best value, which eliminates
+ * truncation errors on calculating current register in hardware.
+ * According to datasheet (eq. 3) the best values are 2048 for
+ * ina226 and 4096 for ina219. They are hardcoded as calibration_value.
+ */
 static int ina2xx_calibrate(struct ina2xx_data *data)
 {
-	u16 val = DIV_ROUND_CLOSEST(data->config->calibration_factor,
-				    data->rshunt);
-
-	return regmap_write(data->regmap, INA2XX_CALIBRATION, val);
+	return regmap_write(data->regmap, INA2XX_CALIBRATION,
+			    data->config->calibration_value);
 }
 
 /*
@@ -187,10 +193,6 @@ static int ina2xx_init(struct ina2xx_data *data)
 	if (ret < 0)
 		return ret;
 
-	/*
-	 * Set current LSB to 1mA, shunt is in uOhms
-	 * (equation 13 in datasheet).
-	 */
 	return ina2xx_calibrate(data);
 }
 
@@ -268,15 +270,15 @@ static int ina2xx_get_value(struct ina2xx_data *data, u8 reg,
 		val = DIV_ROUND_CLOSEST(val, 1000);
 		break;
 	case INA2XX_POWER:
-		val = regval * data->config->power_lsb;
+		val = regval * data->power_lsb_uW;
 		break;
 	case INA2XX_CURRENT:
-		/* signed register, LSB=1mA (selected), in mA */
-		val = (s16)regval;
+		/* signed register, result in mA */
+		val = regval * data->current_lsb_uA;
+		val = DIV_ROUND_CLOSEST(val, 1000);
 		break;
 	case INA2XX_CALIBRATION:
-		val = DIV_ROUND_CLOSEST(data->config->calibration_factor,
-					regval);
+		val = regval;
 		break;
 	default:
 		/* programmer goofed */
@@ -304,9 +306,32 @@ static ssize_t ina2xx_show_value(struct device *dev,
 			ina2xx_get_value(data, attr->index, regval));
 }
 
-static ssize_t ina2xx_set_shunt(struct device *dev,
-				struct device_attribute *da,
-				const char *buf, size_t count)
+/*
+ * In order to keep calibration register value fixed, the product
+ * of current_lsb and shunt_resistor should also be fixed and equal
+ * to shunt_voltage_lsb = 1 / shunt_div multiplied by 10^9 in order
+ * to keep the scale.
+ */
+static int ina2xx_set_shunt(struct ina2xx_data *data, long val)
+{
+	unsigned int dividend = DIV_ROUND_CLOSEST(1000000000,
+						  data->config->shunt_div);
+	if (val <= 0 || val > dividend)
+		return -EINVAL;
+
+	mutex_lock(&data->config_lock);
+	data->rshunt = val;
+	data->current_lsb_uA = DIV_ROUND_CLOSEST(dividend, val);
+	data->power_lsb_uW = data->config->power_lsb_factor *
+			     data->current_lsb_uA;
+	mutex_unlock(&data->config_lock);
+
+	return 0;
+}
+
+static ssize_t ina2xx_store_shunt(struct device *dev,
+				  struct device_attribute *da,
+				  const char *buf, size_t count)
 {
 	unsigned long val;
 	int status;
@@ -316,18 +341,9 @@ static ssize_t ina2xx_set_shunt(struct device *dev,
 	if (status < 0)
 		return status;
 
-	if (val == 0 ||
-	    /* Values greater than the calibration factor make no sense. */
-	    val > data->config->calibration_factor)
-		return -EINVAL;
-
-	mutex_lock(&data->config_lock);
-	data->rshunt = val;
-	status = ina2xx_calibrate(data);
-	mutex_unlock(&data->config_lock);
+	status = ina2xx_set_shunt(data, val);
 	if (status < 0)
 		return status;
-
 	return count;
 }
 
@@ -387,7 +403,7 @@ static SENSOR_DEVICE_ATTR(power1_input, S_IRUGO, ina2xx_show_value, NULL,
 
 /* shunt resistance */
 static SENSOR_DEVICE_ATTR(shunt_resistor, S_IRUGO | S_IWUSR,
-			  ina2xx_show_value, ina2xx_set_shunt,
+			  ina2xx_show_value, ina2xx_store_shunt,
 			  INA2XX_CALIBRATION);
 
 /* update interval (ina226 only) */
@@ -448,10 +464,7 @@ static int ina2xx_probe(struct i2c_client *client,
 			val = INA2XX_RSHUNT_DEFAULT;
 	}
 
-	if (val <= 0 || val > data->config->calibration_factor)
-		return -ENODEV;
-
-	data->rshunt = val;
+	ina2xx_set_shunt(data, val);
 
 	ina2xx_regmap_config.max_register = data->config->registers;