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SubjectRe: [PATCH v3] iio: adc: ina2xx: Make calibration register value fixed
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On Friday, December 15, 2017 8:59:23 AM CET Maciej Purski wrote:
> 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>

The code itself looks fine now, although I have some nitpicks regarding the
comments, see below.

Regards,

Stefan

> ---
> 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;

A comment for the power_lsb_factor would be nice, something like
/* fixed relation between current and power lsb, uW/uA */

> 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;
> }

It think this may be somewhat easier to follow if you place the CURRENT case
above the POWER case, and then extend the comments for both cases, e.g.:

/*
* processed (mA) = raw * current_lsb (mA)
* current_lsb (mA) = shunt_voltage_lsb (nV) / shunt_resistor (uOhm)
*/

and
/*
* processed (mW) = raw * power_lsb (mW)
* power_lsb (mW) = power_lsb_factor (mW/mA) * current_lsb (mA)
*/

> 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()
> + */

I think this comment is misplaced here. Neither current_lsb nor
shunt_voltage_lsb are used in the remaining code, the resistor value is just
stored.

> 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;
> }


--
Stefan Brüns / Bergstraße 21 / 52062 Aachen
home: +49 241 53809034 mobile: +49 151 50412019[unhandled content-type:application/pgp-signature]
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