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References: <20201111091259.46773-1-alexandru.ardelean@analog.com> <20201111091259.46773-4-alexandru.ardelean@analog.com>
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From:   Alexandru Ardelean <ardeleanalex@gmail.com>
Date:   Wed, 18 Nov 2020 16:43:07 +0200
Message-ID: <CA+U=Dsqh1MDGjWfULdnmw5+2tTD4dutiodqr+uCdPhsFMbs=iQ@mail.gmail.com>
Subject: Re: [PATCH v2 3/4] hwmon: (ltc2945): add support for sense resistor
To:     Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc:     linux-hwmon@vger.kernel.org,
        devicetree <devicetree@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Guenter Roeck <linux@roeck-us.net>, jdelvare@suse.com,
        "Thoren, Mark" <mark.thoren@analog.com>
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Precedence: bulk

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
>