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SubjectRe: [PATCH 2/2] hwmon: (ina2xx) Make calibration register value fixed
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On 11/22/2017 07:32 AM, 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)
>
> Signed-off-by: Maciej Purski <m.purski@samsung.com>

Setting the calibration register to a specific value may optimize precision,
but limits the supported value range, which is the whole point of providing
a calibration register. What am I missing here ?

Guenter

> ---
> drivers/hwmon/ina2xx.c | 87 +++++++++++++++++++++++++++++---------------------
> 1 file changed, 50 insertions(+), 37 deletions(-)
>
> diff --git a/drivers/hwmon/ina2xx.c b/drivers/hwmon/ina2xx.c
> index 62e38fa..e362a93 100644
> --- a/drivers/hwmon/ina2xx.c
> +++ b/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;
>
>

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