iio: max31856: add option for setting mains filter rejection frequency

This sensor has an embedded notch filter for reducing interferences caused
by the power mains. This filter can be tuned to reject either 50Hz or 60Hz
(and harmonics).

Currently the said setting is left alone (the sensor defaults to 60Hz).
This patch introduces a IIO attribute that allows the user to set the said
filter to the desired frequency.

NOTE: this has been intentionally not tied to any DT property to allow
the configuration of this setting from userspace, e.g. with a GUI or by
reading a configuration file, or maybe reading a GPIO tied to a physical
switch or accessing some device that can autodetect the line frequency.

Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: Patrick Havelange <patrick.havelange@essensium.com>
Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Cc: Matt Ranostay <matt.ranostay@konsulko.com>
Cc: Chuhong Yuan <hslester96@gmail.com>
Cc: Daniel Gomez <dagmcr@gmail.com>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Reviewed-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/temperature/max31856.c

@@ -23,6 +23,7 @@
 #define MAX31856_CR0_1SHOT         BIT(6)
 #define MAX31856_CR0_OCFAULT       BIT(4)
 #define MAX31856_CR0_OCFAULT_MASK  GENMASK(5, 4)
+#define MAX31856_CR0_FILTER_50HZ   BIT(0)
 #define MAX31856_TC_TYPE_MASK      GENMASK(3, 0)
 #define MAX31856_FAULT_OVUV        BIT(1)
 #define MAX31856_FAULT_OPEN        BIT(0)
@@ -63,6 +64,7 @@ static const struct iio_chan_spec max31856_channels[] = {
 struct max31856_data {
 	struct spi_device *spi;
 	u32 thermocouple_type;
+	bool filter_50hz;
 };
 
 static int max31856_read(struct max31856_data *data, u8 reg,
@@ -123,6 +125,11 @@ static int max31856_init(struct max31856_data *data)
 	reg_cr0_val &= ~MAX31856_CR0_1SHOT;
 	reg_cr0_val |= MAX31856_CR0_AUTOCONVERT;
 
+	if (data->filter_50hz)
+		reg_cr0_val |= MAX31856_CR0_FILTER_50HZ;
+	else
+		reg_cr0_val &= ~MAX31856_CR0_FILTER_50HZ;
+
 	return max31856_write(data, MAX31856_CR0_REG, reg_cr0_val);
 }
 
@@ -249,12 +256,54 @@ static ssize_t show_fault_oc(struct device *dev,
 	return show_fault(dev, MAX31856_FAULT_OPEN, buf);
 }
 
+static ssize_t show_filter(struct device *dev,
+			   struct device_attribute *attr,
+			   char *buf)
+{
+	struct iio_dev *indio_dev = dev_to_iio_dev(dev);
+	struct max31856_data *data = iio_priv(indio_dev);
+
+	return sprintf(buf, "%d\n", data->filter_50hz ? 50 : 60);
+}
+
+static ssize_t set_filter(struct device *dev,
+			  struct device_attribute *attr,
+			  const char *buf,
+			  size_t len)
+{
+	struct iio_dev *indio_dev = dev_to_iio_dev(dev);
+	struct max31856_data *data = iio_priv(indio_dev);
+	unsigned int freq;
+	int ret;
+
+	ret = kstrtouint(buf, 10, &freq);
+	if (ret)
+		return ret;
+
+	switch (freq) {
+	case 50:
+		data->filter_50hz = true;
+		break;
+	case 60:
+		data->filter_50hz = false;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	max31856_init(data);
+	return len;
+}
+
 static IIO_DEVICE_ATTR(fault_ovuv, 0444, show_fault_ovuv, NULL, 0);
 static IIO_DEVICE_ATTR(fault_oc, 0444, show_fault_oc, NULL, 0);
+static IIO_DEVICE_ATTR(in_temp_filter_notch_center_frequency, 0644,
+		       show_filter, set_filter, 0);
 
 static struct attribute *max31856_attributes[] = {
 	&iio_dev_attr_fault_ovuv.dev_attr.attr,
 	&iio_dev_attr_fault_oc.dev_attr.attr,
+	&iio_dev_attr_in_temp_filter_notch_center_frequency.dev_attr.attr,
 	NULL,
 };
 
@@ -280,6 +329,7 @@ static int max31856_probe(struct spi_device *spi)
 
 	data = iio_priv(indio_dev);
 	data->spi = spi;
+	data->filter_50hz = false;
 
 	spi_set_drvdata(spi, indio_dev);