iio: max31856: add support for configuring the HW averaging

This sensor can perform samples averaging in hardware, but currently the
driver leaves this setting alone (default is no averaging).

This patch binds this HW setting to the "oversampling_ratio" IIO attribute
and allows the user to set the averaging as desired (the HW supports
averaging of 2, 5, 8 or 16 samples; in-between values are rounded up).

Cc: Hartmut Knaack <[e-mail]>
Cc: Lars-Peter Clausen <[e-mail]>
Cc: Peter Meerwald-Stadler <[e-mail]>
Cc: Colin Ian King <[e-mail]>
Cc: Patrick Havelange <[e-mail]>
Cc: Matt Weber <[e-mail]>
Cc: Matt Ranostay <[e-mail]>
Cc: Chuhong Yuan <[e-mail]>
Cc: Daniel Gomez <[e-mail]>
Cc: [e-mail]
Signed-off-by: Andrea Merello <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/temperature/max31856.c

···
 #include <linux/spi/spi.h>
 #include <linux/iio/iio.h>
 #include <linux/iio/sysfs.h>
+#include <linux/util_macros.h>
 #include <dt-bindings/iio/temperature/thermocouple.h>
 /*
  * The MSB of the register value determines whether the following byte will
···
 #define MAX31856_CR0_OCFAULT       BIT(4)
 #define MAX31856_CR0_OCFAULT_MASK  GENMASK(5, 4)
 #define MAX31856_CR0_FILTER_50HZ   BIT(0)
+#define MAX31856_AVERAGING_MASK    GENMASK(6, 4)
+#define MAX31856_AVERAGING_SHIFT   4
 #define MAX31856_TC_TYPE_MASK      GENMASK(3, 0)
 #define MAX31856_FAULT_OVUV        BIT(1)
 #define MAX31856_FAULT_OPEN        BIT(0)
···
 		.type = IIO_TEMP,
 		.info_mask_separate =
 			BIT(IIO_CHAN_INFO_RAW) | BIT(IIO_CHAN_INFO_SCALE),
+		.info_mask_shared_by_type =
+			BIT(IIO_CHAN_INFO_OVERSAMPLING_RATIO)
 	},
 	{	/* Cold Junction Temperature */
 		.type = IIO_TEMP,
···
 		.modified = 1,
 		.info_mask_separate =
 			BIT(IIO_CHAN_INFO_RAW) | BIT(IIO_CHAN_INFO_SCALE),
+		.info_mask_shared_by_type =
+			BIT(IIO_CHAN_INFO_OVERSAMPLING_RATIO)
 	},
 };
 
···
 	struct spi_device *spi;
 	u32 thermocouple_type;
 	bool filter_50hz;
+	int averaging;
 };
 
 static int max31856_read(struct max31856_data *data, u8 reg,
···
 
 	reg_cr1_val &= ~MAX31856_TC_TYPE_MASK;
 	reg_cr1_val |= data->thermocouple_type;
+
+	reg_cr1_val &= ~MAX31856_AVERAGING_MASK;
+	reg_cr1_val |= data->averaging << MAX31856_AVERAGING_SHIFT;
+
 	ret = max31856_write(data, MAX31856_CR1_REG, reg_cr1_val);
 	if (ret)
 		return ret;
···
 			return IIO_VAL_INT_PLUS_MICRO;
 		}
 		break;
+	case IIO_CHAN_INFO_OVERSAMPLING_RATIO:
+		*val = 1 << data->averaging;
+		return IIO_VAL_INT;
 	default:
 		ret = -EINVAL;
 		break;
 	}
 
 	return ret;
+}
+
+static int max31856_write_raw(struct iio_dev *indio_dev,
+			      struct iio_chan_spec const *chan,
+			      int val, int val2, long mask)
+{
+	struct max31856_data *data = iio_priv(indio_dev);
+	int msb;
+
+	switch (mask) {
+	case IIO_CHAN_INFO_OVERSAMPLING_RATIO:
+		if (val > 16 || val < 1)
+			return -EINVAL;
+		msb = fls(val) - 1;
+		/* Round up to next 2pow if needed */
+		if (BIT(msb) < val)
+			msb++;
+
+		data->averaging = msb;
+		max31856_init(data);
+		break;
+
+	default:
+		return -EINVAL;
+	}
+
+	return 0;
 }
 
 static ssize_t show_fault(struct device *dev, u8 faultbit, char *buf)
···
 
 static const struct iio_info max31856_info = {
 	.read_raw = max31856_read_raw,
+	.write_raw = max31856_write_raw,
 	.attrs = &max31856_group,
 };