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

[Guenter Roeck]

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.

> 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.

Guenter

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