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Message-ID: <3656355.n9BamPbOAV@sbruens-linux>
Date: Thu, 7 Dec 2017 16:10:40 +0000
From: Brüns, Stefan <Stefan.Bruens@...h-aachen.de>
To: Maciej Purski <m.purski@...sung.com>
CC: Jonathan Cameron <jic23@...nel.org>,
"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>,
Peter Meerwald-Stadler <pmeerw@...erw.net>,
Lars-Peter Clausen <lars@...afoo.de>,
Hartmut Knaack <knaack.h@....de>,
"Jean Delvare" <jdelvare@...e.com>,
Guenter Roeck <linux@...ck-us.net>,
"Marek Szyprowski" <m.szyprowski@...sung.com>,
Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH v2] iio: adc: ina2xx: Make calibration register value
fixed
On Donnerstag, 7. Dezember 2017 13:02:47 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@...sung.com>
>
> ---
> 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 | 58
> +++++++++++++++++++++++--------------------- 1 file changed, 31
> insertions(+), 27 deletions(-)
>
> diff --git a/drivers/iio/adc/ina2xx-adc.c b/drivers/iio/adc/ina2xx-adc.c
> index ddf8781..3e4972f 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;
> };
>
> @@ -138,6 +138,7 @@ struct ina2xx_chip_info {
> const struct ina2xx_config *config;
> struct mutex state_lock;
> unsigned int shunt_resistor_uohm;
> + unsigned int current_lsb_uA;
I don't think you need current_lsb_uA, see below ...
> int avg;
> int int_time_vbus; /* Bus voltage integration time uS */
> int int_time_vshunt; /* Shunt voltage integration time uS */
> @@ -149,20 +150,20 @@ 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,
> },
> };
> @@ -229,14 +230,16 @@ static int ina2xx_read_raw(struct iio_dev *indio_dev,
>
> case INA2XX_POWER:
> /* processed (mW) = raw*lsb (uW) / 1000 */
> - *val = chip->config->power_lsb;
> + *val = chip->config->power_lsb_factor *
> + chip->current_lsb_uA;
> *val2 = 1000;
> return IIO_VAL_FRACTIONAL;
>
> case INA2XX_CURRENT:
> - /* processed (mA) = raw (mA) */
> - *val = 1;
> - return IIO_VAL_INT;
> + /* processed (mA) = raw*lsb (uA) / 1000 */
> + *val = chip->current_lsb_uA;
> + *val2 = 1000;
.. this is the only place where current_lsb_uA is used ...
> + return IIO_VAL_FRACTIONAL;
> }
>
> case IIO_CHAN_INFO_HARDWAREGAIN:
> @@ -541,28 +544,34 @@ 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);
> }
>
> +/*
> + * 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 set_shunt_resistor(struct ina2xx_chip_info *chip, unsigned int
> val) {
> - if (val <= 0 || val > chip->config->calibration_factor)
> + unsigned int dividend = DIV_ROUND_CLOSEST(1000000000,
> + chip->config->shunt_div);
> +
> + if (val <= 0 || val > dividend)
> return -EINVAL;
>
> chip->shunt_resistor_uohm = val;
> + chip->current_lsb_uA = DIV_ROUND_CLOSEST(dividend, val);
1st: shunt_div no longer exists, but shunt_voltage_lsb[nV], so this code does
not even compile ...
This function does two things - range check and calculation of the
curren_lsb_uA auxiliary variable.
Lets start with the second part:
read_raw returns the IIO_VAL_FRACTIONAL
val / val2 = current_lsb_uA / 1000 (1)
from set_shunt_resistor(...):
dividend = 1e9 / shunt_div (2)
current_lsb_uA = dividend / shunt_resistor_uohm (3)
shunt_div no longer exists, but can be substituted:
shunt_div = 1e6 / shunt_voltage_lsb (4)
substituting (2), (3) and (4) in (1), we end up with
val / val2 = 1e9 / (shunt_div * shunt_resistor_uohm * 1000)
=> val / val2 = 1e6 / ((1e6 / shunt_voltage_lsb) * shunt_resistor_uohm)
=> val / val2 = shunt_voltage_lsb / shunt_resistor_uohm
=> [mA] = [nV] / [uOhm]
The auxiliary variable is gone, as are the two DIV_ROUND_CLOSEST calls.
Part 2, range check:
lower bound is obviously 1 uOhm. The upper bound is only limited by
typeof(val2) and typeof(shunt_resistor_uohm), both are ints.
so the range check can be simplified to
if (val == 0 || val > INT_MAX)
return -EINVAL
Kind regards,
Stefan
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