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

[Jonathan Cameron]

On Sat, 29 May 2021 20:59:13 -0400
Liam Beguin <liambeguin@xxxxxxxxx> wrote:

> From: Liam Beguin <lvb@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
Hi Liam,

Comments in here follow through from the bindings.

Jonathan

> ---
>  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);

As below. Generic offset, not this specific one please ;)

> +
> +	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"))

As mentioned later, stick to celcius + an explicit offset.

There will be devices that have their own offset which doesn't happen to
be -273.15

> +		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);