[PATCH v3] iio: adc: ina2xx: Make calibration register value fixed

[Maciej Purski <m.purski@samsung.com>]

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)

This is a part of the patchset: https://lkml.org/lkml/2017/11/22/394

Signed-off-by: Maciej Purski <m.purski@samsung.com>

---
Changes in v3:
- remove variable current_lsb and calculate it on each read of current
  and power scale value
- update comments

Changes in v2:
- rebase on top of the latest next
- remove a redundant variable - power_lsb_uW
- fix comments
---
 drivers/iio/adc/ina2xx-adc.c | 56 +++++++++++++++++++++-----------------------
 1 file changed, 27 insertions(+), 29 deletions(-)

diff --git a/drivers/iio/adc/ina2xx-adc.c b/drivers/iio/adc/ina2xx-adc.c
index ddf8781..3488100 100644
--- a/drivers/iio/adc/ina2xx-adc.c
+++ b/drivers/iio/adc/ina2xx-adc.c
@@ -124,11 +124,11 @@ enum ina2xx_ids { ina219, ina226 };
 
 struct ina2xx_config {
 	u16 config_default;
-	int calibration_factor;
+	int calibration_value;
 	int shunt_voltage_lsb;	/* nV */
 	int bus_voltage_shift;	/* position of lsb */
 	int bus_voltage_lsb;	/* uV */
-	int power_lsb;		/* uW */
+	int power_lsb_factor;
 	enum ina2xx_ids chip_id;
 };
 
@@ -149,20 +149,20 @@ struct ina2xx_chip_info {
 static const struct ina2xx_config ina2xx_config[] = {
 	[ina219] = {
 		.config_default = INA219_CONFIG_DEFAULT,
-		.calibration_factor = 40960000,
+		.calibration_value = 4096,
 		.shunt_voltage_lsb = 10000,
 		.bus_voltage_shift = INA219_BUS_VOLTAGE_SHIFT,
 		.bus_voltage_lsb = 4000,
-		.power_lsb = 20000,
+		.power_lsb_factor = 20,
 		.chip_id = ina219,
 	},
 	[ina226] = {
 		.config_default = INA226_CONFIG_DEFAULT,
-		.calibration_factor = 5120000,
+		.calibration_value = 2048,
 		.shunt_voltage_lsb = 2500,
 		.bus_voltage_shift = 0,
 		.bus_voltage_lsb = 1250,
-		.power_lsb = 25000,
+		.power_lsb_factor = 25,
 		.chip_id = ina226,
 	},
 };
@@ -228,15 +228,17 @@ static int ina2xx_read_raw(struct iio_dev *indio_dev,
 			return IIO_VAL_FRACTIONAL;
 
 		case INA2XX_POWER:
-			/* processed (mW) = raw*lsb (uW) / 1000 */
-			*val = chip->config->power_lsb;
-			*val2 = 1000;
+			/* processed (mW) = raw * factor * current_lsb (mW)*/
+			*val = chip->config->power_lsb_factor *
+			       chip->config->shunt_voltage_lsb;
+			*val2 = chip->shunt_resistor_uohm;
 			return IIO_VAL_FRACTIONAL;
 
 		case INA2XX_CURRENT:
-			/* processed (mA) = raw (mA) */
-			*val = 1;
-			return IIO_VAL_INT;
+			/* processed (mA) = raw * lsb ([nV] / [uOhm] = [mA]) */
+			*val = chip->config->shunt_voltage_lsb;
+			*val2 = chip->shunt_resistor_uohm;
+			return IIO_VAL_FRACTIONAL;
 		}
 
 	case IIO_CHAN_INFO_HARDWAREGAIN:
@@ -541,25 +543,26 @@ static ssize_t ina2xx_allow_async_readout_store(struct device *dev,
 }
 
 /*
- * Set current LSB to 1mA, shunt is in uOhms
- * (equation 13 in datasheet). We hardcode a Current_LSB
- * of 1.0 x10-3. The only remaining parameter is RShunt.
- * There is no need to expose the CALIBRATION register
- * to the user for now. But we need to reset this register
- * if the user updates RShunt after driver init, e.g upon
- * reading an EEPROM/Probe-type value.
+ * Calibration register is set to the best value, which eliminates
+ * truncation errors on calculating current register in hardware.
+ * According to datasheet (INA 226: eq. 3, INA219: eq. 4) the best values
+ * are 2048 for ina226 and 4096 for ina219. They are hardcoded as
+ * calibration_value.
  */
 static int ina2xx_set_calibration(struct ina2xx_chip_info *chip)
 {
-	u16 regval = DIV_ROUND_CLOSEST(chip->config->calibration_factor,
-				   chip->shunt_resistor_uohm);
-
-	return regmap_write(chip->regmap, INA2XX_CALIBRATION, regval);
+	return regmap_write(chip->regmap, INA2XX_CALIBRATION,
+			    chip->config->calibration_value);
 }
 
+/*
+ * As the calibration register is fixed, the product of current_lsb
+ * and shunt_resistor should also be fixed and equal
+ * to shunt_voltage_lsb. Current_lsb will be calculated in read_raw()
+ */
 static int set_shunt_resistor(struct ina2xx_chip_info *chip, unsigned int val)
 {
-	if (val <= 0 || val > chip->config->calibration_factor)
+	if (val == 0 || val > INT_MAX)
 		return -EINVAL;
 
 	chip->shunt_resistor_uohm = val;
@@ -592,11 +595,6 @@ static ssize_t ina2xx_shunt_resistor_store(struct device *dev,
 	if (ret)
 		return ret;
 
-	/* Update the Calibration register */
-	ret = ina2xx_set_calibration(chip);
-	if (ret)
-		return ret;
-
 	return len;
 }
 
-- 
2.7.4