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

Jonathan Cameron
2021-06-01T16:34:38Z

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

> 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>
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);