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

[Alexandru Ardelean <alexandru.ardelean@analog.com>]

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);
 		} 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