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

[Guenter Roeck]

On 11/8/20 10:41 PM, Alexandru Ardelean wrote:
> On Fri, Nov 6, 2020 at 3:17 PM Guenter Roeck <linux@xxxxxxxxxxxx> 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.
> 

Sure.

Guenter

>>
>>> Signed-off-by: Alexandru Ardelean <alexandru.ardelean@xxxxxxxxxx>
>>> ---
>>>  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
>>>