iio: dpot-dac: DAC driver based on a digital potentiometer

It is assumed that the dpot is used as a voltage divider between the
current dpot wiper setting and the maximum resistance of the dpot. The
divided voltage is provided by a vref regulator.

.------.
.-----------. | |
| vref |--' .---.
| regulator |--. | |
'-----------' | | d |
| | p |
| | o | wiper
| | t |<---------+
| | |
| '---' dac output voltage
| |
'------+------------+

Signed-off-by: Peter Rosin <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/ABI/testing/sysfs-bus-iio-dac-dpot-dac

···
+What:		/sys/bus/iio/devices/iio:deviceX/out_voltageY_raw_available
+Date:		October 2016
+KernelVersion:	4.9
+Contact:	Peter Rosin <[e-mail]>
+Description:
+		The range of available values represented as the minimum value,
+		the step and the maximum value, all enclosed in square brackets.
+		Example: [0 1 256]

MAINTAINERS

···
 M:	Peter Rosin <[e-mail]>
 L:	[e-mail]
 S:	Maintained
+F:	Documentation/ABI/testing/sysfs-bus-iio-dac-dpot-dac
 F:	Documentation/devicetree/bindings/iio/dac/dpot-dac.txt
+F:	drivers/iio/dac/dpot-dac.c
 
 IIO SUBSYSTEM AND DRIVERS
 M:	Jonathan Cameron <[e-mail]>

drivers/iio/dac/Kconfig

···
 	  To compile this driver as a module choose M here: the module will be called
 	  ad8801.
 
+config DPOT_DAC
+	tristate "DAC emulation using a DPOT"
+	depends on OF
+	help
+	  Say yes here to build support for DAC emulation using a digital
+	  potentiometer.
+
+	  To compile this driver as a module, choose M here: the module will be
+	  called dpot-dac.
+
 config LPC18XX_DAC
 	tristate "NXP LPC18xx DAC driver"
 	depends on ARCH_LPC18XX || COMPILE_TEST

drivers/iio/dac/Makefile

···
 obj-$(CONFIG_AD7303) += ad7303.o
 obj-$(CONFIG_AD8801) += ad8801.o
 obj-$(CONFIG_CIO_DAC) += cio-dac.o
+obj-$(CONFIG_DPOT_DAC) += dpot-dac.o
 obj-$(CONFIG_LPC18XX_DAC) += lpc18xx_dac.o
 obj-$(CONFIG_M62332) += m62332.o
 obj-$(CONFIG_MAX517) += max517.o

drivers/iio/dac/dpot-dac.c

···
+/*
+ * IIO DAC emulation driver using a digital potentiometer
+ *
+ * Copyright (C) 2016 Axentia Technologies AB
+ *
+ * Author: Peter Rosin <[e-mail]>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+/*
+ * It is assumed that the dpot is used as a voltage divider between the
+ * current dpot wiper setting and the maximum resistance of the dpot. The
+ * divided voltage is provided by a vref regulator.
+ *
+ *                   .------.
+ *    .-----------.  |      |
+ *    | vref      |--'    .---.
+ *    | regulator |--.    |   |
+ *    '-----------'  |    | d |
+ *                   |    | p |
+ *                   |    | o |  wiper
+ *                   |    | t |<---------+
+ *                   |    |   |
+ *                   |    '---'       dac output voltage
+ *                   |      |
+ *                   '------+------------+
+ */
+
+#include <linux/err.h>
+#include <linux/iio/consumer.h>
+#include <linux/iio/iio.h>
+#include <linux/module.h>
+#include <linux/of.h>
+#include <linux/platform_device.h>
+#include <linux/regulator/consumer.h>
+
+struct dpot_dac {
+	struct regulator *vref;
+	struct iio_channel *dpot;
+	u32 max_ohms;
+};
+
+static const struct iio_chan_spec dpot_dac_iio_channel = {
+	.type = IIO_VOLTAGE,
+	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW)
+			    | BIT(IIO_CHAN_INFO_SCALE),
+	.info_mask_separate_available = BIT(IIO_CHAN_INFO_RAW),
+	.output = 1,
+	.indexed = 1,
+};
+
+static int dpot_dac_read_raw(struct iio_dev *indio_dev,
+			     struct iio_chan_spec const *chan,
+			     int *val, int *val2, long mask)
+{
+	struct dpot_dac *dac = iio_priv(indio_dev);
+	int ret;
+	unsigned long long tmp;
+
+	switch (mask) {
+	case IIO_CHAN_INFO_RAW:
+		return iio_read_channel_raw(dac->dpot, val);
+
+	case IIO_CHAN_INFO_SCALE:
+		ret = iio_read_channel_scale(dac->dpot, val, val2);
+		switch (ret) {
+		case IIO_VAL_FRACTIONAL_LOG2:
+			tmp = *val * 1000000000LL;
+			do_div(tmp, dac->max_ohms);
+			tmp *= regulator_get_voltage(dac->vref) / 1000;
+			do_div(tmp, 1000000000LL);
+			*val = tmp;
+			return ret;
+		case IIO_VAL_INT:
+			/*
+			 * Convert integer scale to fractional scale by
+			 * setting the denominator (val2) to one...
+			 */
+			*val2 = 1;
+			ret = IIO_VAL_FRACTIONAL;
+			/* ...and fall through. */
+		case IIO_VAL_FRACTIONAL:
+			*val *= regulator_get_voltage(dac->vref) / 1000;
+			*val2 *= dac->max_ohms;
+			break;
+		}
+
+		return ret;
+	}
+
+	return -EINVAL;
+}
+
+static int dpot_dac_read_avail(struct iio_dev *indio_dev,
+			       struct iio_chan_spec const *chan,
+			       const int **vals, int *type, int *length,
+			       long mask)
+{
+	struct dpot_dac *dac = iio_priv(indio_dev);
+
+	switch (mask) {
+	case IIO_CHAN_INFO_RAW:
+		*type = IIO_VAL_INT;
+		return iio_read_avail_channel_raw(dac->dpot, vals, length);
+	}
+
+	return -EINVAL;
+}
+
+static int dpot_dac_write_raw(struct iio_dev *indio_dev,
+			      struct iio_chan_spec const *chan,
+			      int val, int val2, long mask)
+{
+	struct dpot_dac *dac = iio_priv(indio_dev);
+
+	switch (mask) {
+	case IIO_CHAN_INFO_RAW:
+		return iio_write_channel_raw(dac->dpot, val);
+	}
+
+	return -EINVAL;
+}
+
+static const struct iio_info dpot_dac_info = {
+	.read_raw = dpot_dac_read_raw,
+	.read_avail = dpot_dac_read_avail,
+	.write_raw = dpot_dac_write_raw,
+	.driver_module = THIS_MODULE,
+};
+
+static int dpot_dac_channel_max_ohms(struct iio_dev *indio_dev)
+{
+	struct device *dev = &indio_dev->dev;
+	struct dpot_dac *dac = iio_priv(indio_dev);
+	unsigned long long tmp;
+	int ret;
+	int val;
+	int val2;
+	int max;
+
+	ret = iio_read_max_channel_raw(dac->dpot, &max);
+	if (ret < 0) {
+		dev_err(dev, "dpot does not indicate its raw maximum value\n");
+		return ret;
+	}
+
+	switch (iio_read_channel_scale(dac->dpot, &val, &val2)) {
+	case IIO_VAL_INT:
+		return max * val;
+	case IIO_VAL_FRACTIONAL:
+		tmp = (unsigned long long)max * val;
+		do_div(tmp, val2);
+		return tmp;
+	case IIO_VAL_FRACTIONAL_LOG2:
+		tmp = val * 1000000000LL * max >> val2;
+		do_div(tmp, 1000000000LL);
+		return tmp;
+	default:
+		dev_err(dev, "dpot has a scale that is too weird\n");
+	}
+
+	return -EINVAL;
+}
+
+static int dpot_dac_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct iio_dev *indio_dev;
+	struct dpot_dac *dac;
+	enum iio_chan_type type;
+	int ret;
+
+	indio_dev = devm_iio_device_alloc(dev, sizeof(*dac));
+	if (!indio_dev)
+		return -ENOMEM;
+
+	platform_set_drvdata(pdev, indio_dev);
+	dac = iio_priv(indio_dev);
+
+	indio_dev->name = dev_name(dev);
+	indio_dev->dev.parent = dev;
+	indio_dev->info = &dpot_dac_info;
+	indio_dev->modes = INDIO_DIRECT_MODE;
+	indio_dev->channels = &dpot_dac_iio_channel;
+	indio_dev->num_channels = 1;
+
+	dac->vref = devm_regulator_get(dev, "vref");
+	if (IS_ERR(dac->vref)) {
+		if (PTR_ERR(dac->vref) != -EPROBE_DEFER)
+			dev_err(&pdev->dev, "failed to get vref regulator\n");
+		return PTR_ERR(dac->vref);
+	}
+
+	dac->dpot = devm_iio_channel_get(dev, "dpot");
+	if (IS_ERR(dac->dpot)) {
+		if (PTR_ERR(dac->dpot) != -EPROBE_DEFER)
+			dev_err(dev, "failed to get dpot input channel\n");
+		return PTR_ERR(dac->dpot);
+	}
+
+	ret = iio_get_channel_type(dac->dpot, &type);
+	if (ret < 0)
+		return ret;
+
+	if (type != IIO_RESISTANCE) {
+		dev_err(dev, "dpot is of the wrong type\n");
+		return -EINVAL;
+	}
+
+	ret = dpot_dac_channel_max_ohms(indio_dev);
+	if (ret < 0)
+		return ret;
+	dac->max_ohms = ret;
+
+	ret = regulator_enable(dac->vref);
+	if (ret) {
+		dev_err(dev, "failed to enable the vref regulator\n");
+		return ret;
+	}
+
+	ret = iio_device_register(indio_dev);
+	if (ret) {
+		dev_err(dev, "failed to register iio device\n");
+		goto disable_reg;
+	}
+
+	return 0;
+
+disable_reg:
+	regulator_disable(dac->vref);
+	return ret;
+}
+
+static int dpot_dac_remove(struct platform_device *pdev)
+{
+	struct iio_dev *indio_dev = platform_get_drvdata(pdev);
+	struct dpot_dac *dac = iio_priv(indio_dev);
+
+	iio_device_unregister(indio_dev);
+	regulator_disable(dac->vref);
+
+	return 0;
+}
+
+static const struct of_device_id dpot_dac_match[] = {
+	{ .compatible = "dpot-dac" },
+	{ /* sentinel */ }
+};
+MODULE_DEVICE_TABLE(of, dpot_dac_match);
+
+static struct platform_driver dpot_dac_driver = {
+	.probe = dpot_dac_probe,
+	.remove = dpot_dac_remove,
+	.driver = {
+		.name = "iio-dpot-dac",
+		.of_match_table = dpot_dac_match,
+	},
+};
+module_platform_driver(dpot_dac_driver);
+
+MODULE_DESCRIPTION("DAC emulation driver using a digital potentiometer");
+MODULE_AUTHOR("Peter Rosin <[e-mail]>");
+MODULE_LICENSE("GPL v2");