iio: max31856: add support for runtime-configuring the thermocouple type

The sensor support various thermocouple types (e.g. J, K, N, ...). The
driver allows to configure this parameter using a DT property.

This is useful when i.e. the thermocouple is physically tied to the sensor
and it is usually not removed, or when it is at least known in advance
which sensor will be connected to the circuit.

However, if the user can randomly connect any kind of thermocouples (i.e.
the device exposes a connector, and the user is free to connect its own
sensors), it would be more appropriate to provide a mechanism to
dynamically switch from one thermocouple type to another. This can be i.e.
handled in userspace by a GUI, a configuration file or a program that
detects the thermocouple type by reading a GPIO, or a eeprom on the probe,
or whatever.

This patch adds a IIO attribute that can be used to override, at run-time,
the DT-provided setting (which serves as default).

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>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/temperature/max31856.c

@@ -6,6 +6,7 @@
  * Copyright (C) 2018-2019 Rockwell Collins
  */
 
+#include <linux/ctype.h>
 #include <linux/module.h>
 #include <linux/init.h>
 #include <linux/err.h>
@@ -53,7 +54,8 @@ static const struct iio_chan_spec max31856_channels[] = {
 	{	/* Thermocouple Temperature */
 		.type = IIO_TEMP,
 		.info_mask_separate =
-			BIT(IIO_CHAN_INFO_RAW) | BIT(IIO_CHAN_INFO_SCALE),
+			BIT(IIO_CHAN_INFO_RAW) | BIT(IIO_CHAN_INFO_SCALE) |
+			BIT(IIO_CHAN_INFO_THERMOCOUPLE_TYPE),
 		.info_mask_shared_by_type =
 			BIT(IIO_CHAN_INFO_OVERSAMPLING_RATIO)
 	},
@@ -75,6 +77,10 @@ struct max31856_data {
 	int averaging;
 };
 
+static const char max31856_tc_types[] = {
+	'B', 'E', 'J', 'K', 'N', 'R', 'S', 'T'
+};
+
 static int max31856_read(struct max31856_data *data, u8 reg,
 			 u8 val[], unsigned int read_size)
 {
@@ -232,6 +238,9 @@ static int max31856_read_raw(struct iio_dev *indio_dev,
 	case IIO_CHAN_INFO_OVERSAMPLING_RATIO:
 		*val = 1 << data->averaging;
 		return IIO_VAL_INT;
+	case IIO_CHAN_INFO_THERMOCOUPLE_TYPE:
+		*val = max31856_tc_types[data->thermocouple_type];
+		return IIO_VAL_CHAR;
 	default:
 		ret = -EINVAL;
 		break;
@@ -240,6 +249,18 @@ static int max31856_read_raw(struct iio_dev *indio_dev,
 	return ret;
 }
 
+static int max31856_write_raw_get_fmt(struct iio_dev *indio_dev,
+				      struct iio_chan_spec const *chan,
+				      long mask)
+{
+	switch (mask) {
+	case IIO_CHAN_INFO_THERMOCOUPLE_TYPE:
+		return IIO_VAL_CHAR;
+	default:
+		return IIO_VAL_INT;
+	}
+}
+
 static int max31856_write_raw(struct iio_dev *indio_dev,
 			      struct iio_chan_spec const *chan,
 			      int val, int val2, long mask)
@@ -259,7 +280,24 @@ static int max31856_write_raw(struct iio_dev *indio_dev,
 		data->averaging = msb;
 		max31856_init(data);
 		break;
+	case IIO_CHAN_INFO_THERMOCOUPLE_TYPE:
+	{
+		int tc_type = -1;
+		int i;
+
+		for (i = 0; i < ARRAY_SIZE(max31856_tc_types); i++) {
+			if (max31856_tc_types[i] == toupper(val)) {
+				tc_type = i;
+				break;
+			}
+		}
+		if (tc_type < 0)
+			return -EINVAL;
 
+		data->thermocouple_type = tc_type;
+		max31856_init(data);
+		break;
+	}
 	default:
 		return -EINVAL;
 	}
@@ -356,6 +394,7 @@ static const struct attribute_group max31856_group = {
 static const struct iio_info max31856_info = {
 	.read_raw = max31856_read_raw,
 	.write_raw = max31856_write_raw,
+	.write_raw_get_fmt = max31856_write_raw_get_fmt,
 	.attrs = &max31856_group,
 };