[PATCH v1 5/9] iio: afe: rescale: add support for temperature sensors

[Liam Beguin <liambeguin@gmail.com>]

From: Liam Beguin <lvb@xiphos.com>

Add support for various linear temperature sensors.

temperature-sense-rtd is used when the measured temperature is a
function of the sensor's resistance (like RTD sensors).

temperature-sense-current is used when the measured temperature is a
function of the sensor's output current (like the AD590)

temperature-sense-amplifier is used when the measured temperature is a
function of the sensor's voltage (like the LTC2997)

Signed-off-by: Liam Beguin <lvb@xiphos.com>
---
 drivers/iio/afe/iio-rescale.c | 141 ++++++++++++++++++++++++++++++++++
 1 file changed, 141 insertions(+)

diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
index 3bd1f11f21db..eb53d833bf7c 100644
--- a/drivers/iio/afe/iio-rescale.c
+++ b/drivers/iio/afe/iio-rescale.c
@@ -222,10 +222,133 @@ static int rescale_voltage_divider_props(struct device *dev,
 	return 0;
 }
 
+static int rescale_temp_sense_rtd_props(struct device *dev,
+					struct rescale *rescale)
+{
+	u32 factor;
+	u32 alpha;
+	u32 iexc;
+	u32 tmp;
+	int ret;
+	u32 r0;
+
+	ret = device_property_read_u32(dev, "excitation-current-microamp",
+				       &iexc);
+	if (ret) {
+		dev_err(dev, "failed to read excitation-current-microamp: %d\n",
+			ret);
+		return ret;
+	}
+
+	ret = device_property_read_u32(dev, "alpha-micro-ohms-per-ohm-celsius",
+				       &alpha);
+	if (ret) {
+		dev_err(dev, "failed to read alpha-micro-ohms-per-celsius: %d\n",
+			ret);
+		return ret;
+	}
+
+	ret = device_property_read_u32(dev, "r-naught-ohms", &r0);
+	if (ret) {
+		dev_err(dev, "failed to read r-naught-ohms: %d\n", ret);
+		return ret;
+	}
+
+	/*
+	 * The transfer function:
+	 *
+	 *	- V(T) = R(T) * iexc
+	 *	- R(T) = r0 * (1 + alpha * T)
+	 *
+	 *	T = 1 / (alpha * r0 * iexc) * (V - r0 * iexc)
+	 */
+	tmp = r0 * iexc * alpha / 1000000;
+	factor = gcd(tmp, 1000000);
+	rescale->numerator = 1000000 / factor;
+	rescale->denominator = tmp / factor;
+
+	rescale->offset = -1 * ((r0 * iexc) / 1000);
+
+	return 0;
+}
+
+static int rescale_temp_sense_current_props(struct device *dev,
+					    struct rescale *rescale)
+{
+	u32 alpha;
+	u32 sense;
+	int ret;
+
+	ret = device_property_read_u32(dev, "sense-resistor-ohms", &sense);
+	if (ret) {
+		dev_err(dev, "failed to read the sense resistance: %d\n", ret);
+		return ret;
+	}
+
+	ret = device_property_read_u32(dev, "alpha-micro-amps-per-degree",
+				       &alpha);
+	if (ret) {
+		dev_err(dev, "failed to read alpha-micro-amps-per-degree: %d\n",
+			ret);
+		return ret;
+	}
+
+	/*
+	 * The transfer function:
+	 *
+	 *	- V(K) = Rsense * Isense(K)
+	 *	- K = Isense(K) / alpha
+	 *	- C = K - 273.15
+	 *
+	 *	C = 1 / (Rsense * alpha) * (V - 273.15 * Rsense * alpha)
+	 */
+	rescale->numerator = 1000000;
+	rescale->denominator = alpha * sense;
+
+	if (device_property_read_bool(dev, "use-kelvin-scale"))
+		rescale->offset = -1 * ((27315 * alpha * sense) / 100000);
+
+	return 0;
+}
+
+static int rescale_temp_sense_amplifier_props(struct device *dev,
+					      struct rescale *rescale)
+{
+	u32 alpha;
+	int ret;
+
+	ret = device_property_read_u32(dev, "alpha-micro-volts-per-degree",
+				       &alpha);
+	if (ret) {
+		dev_err(dev, "failed to read alpha-micro-volts-per-degree: %d\n",
+			ret);
+		return ret;
+	}
+
+	/*
+	 * The transfer function:
+	 *
+	 *	- K = V(K) / alpha
+	 *	- C = K - 273.15
+	 *
+	 *	C = 1 / (alpha) * (V - 273.15 * alpha)
+	 */
+	rescale->numerator = 1000000;
+	rescale->denominator = alpha;
+
+	if (device_property_read_bool(dev, "use-kelvin-scale"))
+		rescale->offset = -1 * ((27315 * alpha) / 100000);
+
+	return 0;
+}
+
 enum rescale_variant {
 	CURRENT_SENSE_AMPLIFIER,
 	CURRENT_SENSE_SHUNT,
 	VOLTAGE_DIVIDER,
+	TEMP_SENSE_RTD,
+	TEMP_SENSE_CURRENT,
+	TEMP_SENSE_AMPLIFIER,
 };
 
 static const struct rescale_cfg rescale_cfg[] = {
@@ -241,6 +364,18 @@ static const struct rescale_cfg rescale_cfg[] = {
 		.type = IIO_VOLTAGE,
 		.props = rescale_voltage_divider_props,
 	},
+	[TEMP_SENSE_RTD] = {
+		.type = IIO_TEMP,
+		.props = rescale_temp_sense_rtd_props,
+	},
+	[TEMP_SENSE_CURRENT] = {
+		.type = IIO_TEMP,
+		.props = rescale_temp_sense_current_props,
+	},
+	[TEMP_SENSE_AMPLIFIER] = {
+		.type = IIO_TEMP,
+		.props = rescale_temp_sense_amplifier_props,
+	},
 };
 
 static const struct of_device_id rescale_match[] = {
@@ -250,6 +385,12 @@ static const struct of_device_id rescale_match[] = {
 	  .data = &rescale_cfg[CURRENT_SENSE_SHUNT], },
 	{ .compatible = "voltage-divider",
 	  .data = &rescale_cfg[VOLTAGE_DIVIDER], },
+	{ .compatible = "temperature-sense-rtd",
+	  .data = &rescale_cfg[TEMP_SENSE_RTD], },
+	{ .compatible = "temperature-sense-current",
+	  .data = &rescale_cfg[TEMP_SENSE_CURRENT], },
+	{ .compatible = "temperature-sense-amplifier",
+	  .data = &rescale_cfg[TEMP_SENSE_AMPLIFIER], },
 	{ /* sentinel */ }
 };
 MODULE_DEVICE_TABLE(of, rescale_match);
-- 
2.30.1.489.g328c10930387