Re: [PATCH v2 3/4] hwmon: (ltc2945): add support for sense resistor

Alexandru Ardelean
2020-11-18T14:44:15Z

On Wed, Nov 11, 2020 at 11:08 AM Alexandru Ardelean
<alexandru.ardelean@analog.com> wrote:
>
> The sense resistor is a parameter of the board. It should be configured in
> the driver via a device-tree / ACPI property, so that the proper current
> measurements can be done in the driver.
>
> It shouldn't be necessary that userspace need to know about the value of
> the resistor. It makes things a bit harder to make the application code
> portable from one board to another.
>
> This change implements support for the sense resistor to be configured from
> DT/ACPI and used in current calculations.
>
> Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
> ---
> drivers/hwmon/ltc2945.c | 53 ++++++++++++++++++++---------------------
> 1 file changed, 26 insertions(+), 27 deletions(-)
>
> diff --git a/drivers/hwmon/ltc2945.c b/drivers/hwmon/ltc2945.c
> index 6d4569a25471..909dd92a7a20 100644
> --- a/drivers/hwmon/ltc2945.c
> +++ b/drivers/hwmon/ltc2945.c
> @@ -61,9 +61,11 @@
> /**
> * struct ltc2945_state - driver instance specific data
> * @regmap regmap object to access device registers
> + * @r_sense_uohm current sense resistor value
> */
> struct ltc2945_state {
> struct regmap *regmap;
> + u32 r_sense_uohm;
> };
>
> static inline bool is_power_reg(u8 reg)
> @@ -101,9 +103,8 @@ static long long ltc2945_reg_to_val(struct device *dev, u8 reg)
> case LTC2945_MAX_POWER_THRES_H:
> case LTC2945_MIN_POWER_THRES_H:
> /*
> - * Convert to uW by assuming current is measured with
> - * an 1mOhm sense resistor, similar to current
> - * measurements.
> + * Convert to uW by and scale it with the configured
> + * sense resistor, similar to current measurements.
> * Control register bit 0 selects if voltage at SENSE+/VDD
> * or voltage at ADIN is used to measure power.
> */
> @@ -112,10 +113,10 @@ static long long ltc2945_reg_to_val(struct device *dev, u8 reg)
> return ret;
> if (control & CONTROL_MULT_SELECT) {
> /* 25 mV * 25 uV = 0.625 uV resolution. */
> - val *= 625LL;
> + val = DIV_ROUND_CLOSEST_ULL(val * 625LL * 1000, st->r_sense_uohm);
> } else {
> /* 0.5 mV * 25 uV = 0.0125 uV resolution. */
> - val = (val * 25LL) >> 1;
> + val = DIV_ROUND_CLOSEST_ULL(val * 25LL * 1000, st->r_sense_uohm) >> 1;
> }
> break;
> case LTC2945_VIN_H:
> @@ -140,13 +141,10 @@ static long long ltc2945_reg_to_val(struct device *dev, u8 reg)
> case LTC2945_MAX_SENSE_THRES_H:
> case LTC2945_MIN_SENSE_THRES_H:
> /*
> - * 25 uV resolution. Convert to current as measured with
> - * an 1 mOhm sense resistor, in mA. If a different sense
> - * resistor is installed, calculate the actual current by
> - * dividing the reported current by the sense resistor value
> - * in mOhm.
> + * 25 uV resolution. Convert to current and scale it
> + * with the value of the sense resistor.
> */
> - val *= 25;
> + val = DIV_ROUND_CLOSEST_ULL(val * 25 * 1000, st->r_sense_uohm);
> break;
> default:
> return -EINVAL;
> @@ -169,9 +167,8 @@ static int ltc2945_val_to_reg(struct device *dev, u8 reg,
> case LTC2945_MAX_POWER_THRES_H:
> case LTC2945_MIN_POWER_THRES_H:
> /*
> - * Convert to register value by assuming current is measured
> - * with an 1mOhm sense resistor, similar to current
> - * measurements.
> + * Convert to register value, scale it with the configured sense
> + * resistor value, similar to current measurements.
> * Control register bit 0 selects if voltage at SENSE+/VDD
> * or voltage at ADIN is used to measure power, which in turn
> * determines register calculations.
> @@ -181,14 +178,10 @@ static int ltc2945_val_to_reg(struct device *dev, u8 reg,
> return ret;
> if (control & CONTROL_MULT_SELECT) {
> /* 25 mV * 25 uV = 0.625 uV resolution. */
> - val = DIV_ROUND_CLOSEST_ULL(val, 625);
> + val = DIV_ROUND_CLOSEST_ULL(val * 1000, 625 * st->r_sense_uohm);

I think that in this ltc2945_val_to_reg(), I should do the math the
other way around.
i.e. val = DIV_ROUND_CLOSEST_ULL(val * st->r_sense_uohm, 625000);

I got confused initially and did it wrong.

> } else {
> - /*
> - * 0.5 mV * 25 uV = 0.0125 uV resolution.
> - * Divide first to avoid overflow;
> - * accept loss of accuracy.
> - */
> - val = DIV_ROUND_CLOSEST_ULL(val, 25) * 2;
> + /* 0.5 mV * 25 uV = 0.0125 uV resolution. */
> + val = DIV_ROUND_CLOSEST_ULL(val * 2 * 1000, 25 * st->r_sense_uohm);
> }
> break;
> case LTC2945_VIN_H:
> @@ -213,13 +206,10 @@ static int ltc2945_val_to_reg(struct device *dev, u8 reg,
> case LTC2945_MAX_SENSE_THRES_H:
> case LTC2945_MIN_SENSE_THRES_H:
> /*
> - * 25 uV resolution. Convert to current as measured with
> - * an 1 mOhm sense resistor, in mA. If a different sense
> - * resistor is installed, calculate the actual current by
> - * dividing the reported current by the sense resistor value
> - * in mOhm.
> + * 25 uV resolution. Convert to current and scale it
> + * with the value of the sense resistor, in mA.
> */
> - val = DIV_ROUND_CLOSEST_ULL(val, 25);
> + val = DIV_ROUND_CLOSEST_ULL(val * 1000, 25 * st->r_sense_uohm);
> break;
> default:
> return -EINVAL;
> @@ -475,6 +465,15 @@ static int ltc2945_probe(struct i2c_client *client)
> return PTR_ERR(regmap);
> }
>
> + if (device_property_read_u32(dev, "shunt-resistor-micro-ohms",
> + &st->r_sense_uohm))
> + st->r_sense_uohm = 1000;
> +
> + if (st->r_sense_uohm == 0) {
> + dev_err(dev, "Zero value provided for sense resistor in DT");
> + return -EINVAL;
> + }
> +
> st->regmap = regmap;
>
> /* Clear faults */
> --
> 2.17.1
>