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Message-ID: <CA+U=DsoQNfYQ2o-wKpcqUJ9Xj9u9U4Nas8xKYhcYusU4HyHx1g@mail.gmail.com>
Date:   Mon, 9 Nov 2020 08:41:55 +0200
From:   Alexandru Ardelean <ardeleanalex@...il.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Alexandru Ardelean <alexandru.ardelean@...log.com>,
        linux-hwmon@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        jdelvare@...e.com, Mark.Thoren@...log.com
Subject: Re: [PATCH 3/3] hwmon: (ltc2945): add support for sense resistor

On Fri, Nov 6, 2020 at 3:17 PM Guenter Roeck <linux@...ck-us.net> wrote:
>
> On Fri, Nov 06, 2020 at 12:18:25PM +0200, Alexandru Ardelean 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.
> >
>
> This will require a matching deevicetree document.

Ack
Will create a dt binding schema doc.
Are you fine with being added as maintainer in the DT doc?
Seeing as you are the original author of the driver.

>
> > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@...log.com>
> > ---
> >  drivers/hwmon/ltc2945.c | 48 ++++++++++++++++++-----------------------
> >  1 file changed, 21 insertions(+), 27 deletions(-)
> >
> > diff --git a/drivers/hwmon/ltc2945.c b/drivers/hwmon/ltc2945.c
> > index 75d997d31e01..500401a82c49 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);
> >               } 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 +465,10 @@ 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;
> > +
>
> Devicetree could set shunt-resistor-micro-ohms to 0, which would result
> in divide by 0 errors.

Ack
Will do a check for this.

>
> Guenter
>
> >       st->regmap = regmap;
> >
> >       /* Clear faults */
> > --
> > 2.27.0
> >

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