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

From: Guenter Roeck
Date: Wed Nov 18 2020 - 10:46:47 EST

On Wed, Nov 18, 2020 at 04:43:07PM +0200, Alexandru Ardelean wrote:
> On Wed, Nov 11, 2020 at 11:08 AM Alexandru Ardelean
> <alexandru.ardelean@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> > 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.
>
Good catch.

Guenter

> > } 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
> >