staging:iio:Documentation Simple dummy driver to explain the basics

The documenation explaining how to go about writing a driver is lagging
horribly, so here is another approach; an actual driver with
lots of explanatory comments.

Note it is currently minimal in that there are no events and no
buffer. With care they can probably be added in additional files
without messing up the clarity of what we have here.

V2: Addressed some of Manuel Stahl's feedback.

Fixed up kernel doc.
Added more general description.

Signed-off-by: Jonathan Cameron <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/iio/Kconfig

···
 source "drivers/staging/iio/resolver/Kconfig"
 source "drivers/staging/iio/trigger/Kconfig"
 
+config IIO_SIMPLE_DUMMY
+       tristate "An example driver with no hardware requirements"
+       help
+	Driver intended mainly as documentation for how to write
+	a driver. May also be useful for testing userspace code
+	without hardward.
+
+
 endif # IIO

drivers/staging/iio/Makefile

···
 obj-$(CONFIG_IIO_SW_RING) += ring_sw.o
 obj-$(CONFIG_IIO_KFIFO_BUF) += kfifo_buf.o
 
+obj-$(CONFIG_IIO_SIMPLE_DUMMY) += iio_simple_dummy.o
+
 obj-y += accel/
 obj-y += adc/
 obj-y += addac/

drivers/staging/iio/iio_simple_dummy.c

···
+/**
+ * Copyright (c) 2011 Jonathan Cameron
+ *
+ * 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.
+ *
+ * A reference industrial I/O driver to illustrate the functionality available.
+ *
+ * There are numerous real drivers to illustrate the finer points.
+ * The purpose of this driver is to provide a driver with far more comments
+ * and explanatory notes than any 'real' driver would have.
+ * Anyone starting out writing an IIO driver should first make sure they
+ * understand all of this driver except those bits specifically marked
+ * as being present to allow us to 'fake' the presence of hardware.
+ */
+#include <linux/kernel.h>
+#include <linux/slab.h>
+#include <linux/module.h>
+#include <linux/moduleparam.h>
+
+#include "iio.h"
+
+/*
+ * A few elements needed to fake a bus for this driver
+ * Note instances parmeter controls how many of these
+ * dummy devices are registered.
+ */
+static unsigned instances = 1;
+module_param(instances, int, 0);
+
+/* Pointer array used to fake bus elements */
+static struct iio_dev **iio_dummy_devs;
+
+/* Fake a name for the part number, usually obtained from the id table */
+static const char *iio_dummy_part_number = "iio_dummy_part_no";
+
+/**
+ * struct iio_dummy_accel_calibscale - realworld to register mapping
+ * @val: first value in read_raw - here integer part.
+ * @val2: second value in read_raw etc - here micro part.
+ * @regval: register value - magic device specific numbers.
+ */
+struct iio_dummy_accel_calibscale {
+	int val;
+	int val2;
+	int regval; /* what would be written to hardware */
+};
+
+static const struct iio_dummy_accel_calibscale dummy_scales[] = {
+	{ 0, 100, 0x8 }, /* 0.000100 */
+	{ 0, 133, 0x7 }, /* 0.000133 */
+	{ 733, 13, 0x9 }, /* 733.00013 */
+};
+
+/**
+ * struct iio_dummy_state - device instance specific state.
+ * @dac_val:			cache for dac value
+ * @single_ended_adc_val:	cache for single ended adc value
+ * @differential_adc_val:	cache for differential adc value
+ * @accel_val:			cache for acceleration value
+ * @accel_calibbias:		cache for acceleration calibbias
+ * @accel_calibscale:		cache for acceleration calibscale
+ * @lock:			lock to ensure state is consistent
+ */
+struct iio_dummy_state {
+	int dac_val;
+	int single_ended_adc_val;
+	int differential_adc_val[2];
+	int accel_val;
+	int accel_calibbias;
+	const struct iio_dummy_accel_calibscale *accel_calibscale;
+	struct mutex lock;
+};
+
+/*
+ * iio_dummy_channels - Description of available channels
+ *
+ * This array of structures tells the IIO core about what the device
+ * actually provides for a given channel.
+ */
+static struct iio_chan_spec iio_dummy_channels[] = {
+	/* indexed ADC channel in_voltage0_raw etc */
+	{
+		.type = IIO_VOLTAGE,
+		/* Channel has a numeric index of 0 */
+		.indexed = 1,
+		.channel = 0,
+		/* What other information is available? */
+		.info_mask =
+		/*
+		 * in_voltage0_offset
+		 * Offset for userspace to apply prior to scale
+		 * when converting to standard units (microvolts)
+		 */
+		(1 << IIO_CHAN_INFO_OFFSET_SEPARATE) |
+		/*
+		 * in_voltage0_scale
+		 * Multipler for userspace to apply post offset
+		 * when converting to standard units (microvolts)
+		 */
+		(1 << IIO_CHAN_INFO_SCALE_SEPARATE),
+
+	},
+	/* Differential ADC channel in_voltage1-voltage2_raw etc*/
+	{
+		.type = IIO_VOLTAGE,
+		.differential = 1,
+		/*
+		 * Indexing for differential channels uses channel
+		 * for the positive part, channel2 for the negative.
+		 */
+		.indexed = 1,
+		.channel = 1,
+		.channel2 = 2,
+		.info_mask =
+		/*
+		 * in_voltage-voltage_scale
+		 * Shared version of scale - shared by differential
+		 * input channels of type IIO_VOLTAGE.
+		 */
+		(1 << IIO_CHAN_INFO_SCALE_SHARED),
+	},
+	/* Differential ADC channel in_voltage3-voltage4_raw etc*/
+	{
+		.type = IIO_VOLTAGE,
+		.differential = 1,
+		.indexed = 1,
+		.channel = 3,
+		.channel2 = 4,
+		.info_mask =
+		(1 << IIO_CHAN_INFO_SCALE_SHARED),
+	},
+	/* DAC channel out_voltage0_raw */
+	{
+		.type = IIO_VOLTAGE,
+		.output = 1,
+		.indexed = 1,
+		.channel = 0,
+	},
+	/*
+	 * 'modified' (i.e. axis specified) acceleration channel
+	 * in_accel_z_raw
+	 */
+	{
+		.type = IIO_ACCEL,
+		.modified = 1,
+		/* Channel 2 is use for modifiers */
+		.channel2 = IIO_MOD_X,
+		.info_mask =
+		/*
+		 * Internal bias correction value. Applied
+		 * by the hardware or driver prior to userspace
+		 * seeing the readings. Typically part of hardware
+		 * calibration.
+		 */
+		(1 << IIO_CHAN_INFO_CALIBBIAS_SEPARATE),
+	},
+};
+
+/**
+ * iio_dummy_read_raw() - data read function.
+ * @indio_dev:	the struct iio_dev associated with this device instance
+ * @chan:	the channel whose data is to be read
+ * @val:	first element of returned value (typically INT)
+ * @val2:	second element of returned value (typically MICRO)
+ * @mask:	what we actually want to read. 0 is the channel, everything else
+ *		is as per the info_mask in iio_chan_spec.
+ */
+static int iio_dummy_read_raw(struct iio_dev *indio_dev,
+			      struct iio_chan_spec const *chan,
+			      int *val,
+			      int *val2,
+			      long mask)
+{
+	struct iio_dummy_state *st = iio_priv(indio_dev);
+	int ret = -EINVAL;
+
+	mutex_lock(&st->lock);
+	switch (mask) {
+	case 0: /* magic value - channel value read */
+		switch (chan->type) {
+		case IIO_VOLTAGE:
+			if (chan->output) {
+				/* Set integer part to cached value */
+				*val = st->dac_val;
+				ret = IIO_VAL_INT;
+			} else if (chan->differential) {
+				if (chan->channel == 1)
+					*val = st->differential_adc_val[0];
+				else
+					*val = st->differential_adc_val[1];
+				ret = IIO_VAL_INT;
+			} else {
+				*val = st->single_ended_adc_val;
+				ret = IIO_VAL_INT;
+			}
+			break;
+		case IIO_ACCEL:
+			*val = st->accel_val;
+			ret = IIO_VAL_INT;
+			break;
+		default:
+			break;
+		}
+		break;
+	case (1 << IIO_CHAN_INFO_OFFSET_SEPARATE):
+		/* only single ended adc -> 7 */
+		*val = 7;
+		ret = IIO_VAL_INT;
+		break;
+	case (1 << IIO_CHAN_INFO_SCALE_SEPARATE):
+		/* only single ended adc -> 0.001333 */
+		*val = 0;
+		*val2 = 1333;
+		ret = IIO_VAL_INT_PLUS_MICRO;
+		break;
+	case (1 << IIO_CHAN_INFO_SCALE_SHARED):
+		/* all differential adc channels -> 0.000001344 */
+		*val = 0;
+		*val2 = 1344;
+		ret = IIO_VAL_INT_PLUS_NANO;
+		break;
+	case (1 << IIO_CHAN_INFO_CALIBBIAS_SEPARATE):
+		/* only the acceleration axis - read from cache */
+		*val = st->accel_calibbias;
+		ret = IIO_VAL_INT;
+		break;
+	case (1 << IIO_CHAN_INFO_CALIBSCALE_SEPARATE):
+		*val = st->accel_calibscale->val;
+		*val2 = st->accel_calibscale->val2;
+		ret = IIO_VAL_INT_PLUS_MICRO;
+		break;
+	default:
+		break;
+	}
+	mutex_unlock(&st->lock);
+	return ret;
+}
+
+/**
+ * iio_dummy_write_raw() - data write function.
+ * @indio_dev:	the struct iio_dev associated with this device instance
+ * @chan:	the channel whose data is to be read
+ * @val:	first element of returned value (typically INT)
+ * @val2:	second element of returned value (typically MICRO)
+ * @mask:	what we actually want to read. 0 is the channel, everything else
+ *		is as per the info_mask in iio_chan_spec.
+ *
+ * Note that all raw writes are assumed IIO_VAL_INT and info mask elements
+ * are assumed to be IIO_INT_PLUS_MICRO unless the callback write_raw_get_fmt
+ * in struct iio_info is provided by the driver.
+ */
+static int iio_dummy_write_raw(struct iio_dev *indio_dev,
+			       struct iio_chan_spec const *chan,
+			       int val,
+			       int val2,
+			       long mask)
+{
+	int i;
+	int ret = 0;
+	struct iio_dummy_state *st = iio_priv(indio_dev);
+
+	switch (mask) {
+	case 0:
+		if (chan->output == 0)
+			return -EINVAL;
+
+		/* Locking not required as writing single value */
+		mutex_lock(&st->lock);
+		st->dac_val = val;
+		mutex_unlock(&st->lock);
+		return 0;
+	case (1 << IIO_CHAN_INFO_CALIBBIAS_SEPARATE):
+		mutex_lock(&st->lock);
+		/* Compare against table - hard matching here */
+		for (i = 0; i < ARRAY_SIZE(dummy_scales); i++)
+			if (val == dummy_scales[i].val &&
+			    val2 == dummy_scales[i].val2)
+				break;
+		if (i == ARRAY_SIZE(dummy_scales))
+			ret = -EINVAL;
+		else
+			st->accel_calibscale = &dummy_scales[i];
+		mutex_unlock(&st->lock);
+		return ret;
+	default:
+		return -EINVAL;
+	}
+}
+
+/*
+ * Device type specific information.
+ */
+static const struct iio_info iio_dummy_info = {
+	.driver_module = THIS_MODULE,
+	.read_raw = &iio_dummy_read_raw,
+	.write_raw = &iio_dummy_write_raw,
+};
+
+/**
+ * iio_dummy_init_device() - device instance specific init
+ * @indio_dev: the iio device structure
+ *
+ * Most drivers have one of these to set up default values,
+ * reset the device to known state etc.
+ */
+static int iio_dummy_init_device(struct iio_dev *indio_dev)
+{
+	struct iio_dummy_state *st = iio_priv(indio_dev);
+
+	st->dac_val = 0;
+	st->single_ended_adc_val = 73;
+	st->differential_adc_val[0] = 33;
+	st->differential_adc_val[1] = -34;
+	st->accel_val = 34;
+	st->accel_calibbias = -7;
+	st->accel_calibscale = &dummy_scales[0];
+
+	return 0;
+}
+
+/**
+ * iio_dummy_probe() - device instance probe
+ * @index: an id number for this instance.
+ *
+ * Arguments are bus type specific.
+ * I2C: iio_dummy_probe(struct i2c_client *client,
+ *                      const struct i2c_device_id *id)
+ * SPI: iio_dummy_probe(struct spi_device *spi)
+ */
+static int __devinit iio_dummy_probe(int index)
+{
+	int ret;
+	struct iio_dev *indio_dev;
+	struct iio_dummy_state *st;
+
+	/*
+	 * Allocate an IIO device.
+	 *
+	 * This structure contains all generic state
+	 * information about the device instance.
+	 * It also has a region (accessed by iio_priv()
+	 * for chip specific state information.
+	 */
+	indio_dev = iio_allocate_device(sizeof(*st));
+	if (indio_dev == NULL) {
+		ret = -ENOMEM;
+		goto error_ret;
+	}
+
+	st = iio_priv(indio_dev);
+	mutex_init(&st->lock);
+
+	iio_dummy_init_device(indio_dev);
+	/*
+	 * With hardware: Set the parent device.
+	 * indio_dev->dev.parent = &spi->dev;
+	 * indio_dev->dev.parent = &client->dev;
+	 */
+
+	 /*
+	 * Make the iio_dev struct available to remove function.
+	 * Bus equivalents
+	 * i2c_set_clientdata(client, indio_dev);
+	 * spi_set_drvdata(spi, indio_dev);
+	 */
+	iio_dummy_devs[index] = indio_dev;
+
+
+	/*
+	 * Set the device name.
+	 *
+	 * This is typically a part number and obtained from the module
+	 * id table.
+	 * e.g. for i2c and spi:
+	 *    indio_dev->name = id->name;
+	 *    indio_dev->name = spi_get_device_id(spi)->name;
+	 */
+	indio_dev->name = iio_dummy_part_number;
+
+	/* Provide description of available channels */
+	indio_dev->channels = iio_dummy_channels;
+	indio_dev->num_channels = ARRAY_SIZE(iio_dummy_channels);
+
+	/*
+	 * Provide device type specific interface functions and
+	 * constant data.
+	 */
+	indio_dev->info = &iio_dummy_info;
+
+	/* Specify that device provides sysfs type interfaces */
+	indio_dev->modes = INDIO_DIRECT_MODE;
+
+	ret = iio_device_register(indio_dev);
+	if (ret < 0)
+		goto error_free_device;
+
+	return 0;
+error_free_device:
+	/* Note free device should only be called, before registration
+	 * has succeeded. */
+	iio_free_device(indio_dev);
+error_ret:
+	return ret;
+}
+
+/**
+ * iio_dummy_remove() - device instance removal function
+ * @index: device index.
+ *
+ * Parameters follow those of iio_dummy_probe for buses.
+ */
+static int iio_dummy_remove(int index)
+{
+	/*
+	 * Get a pointer to the device instance iio_dev structure
+	 * from the bus subsystem. E.g.
+	 * struct iio_dev *indio_dev = i2c_get_clientdata(client);
+	 * struct iio_dev *indio_dev = spi_get_drvdata(spi);
+	 */
+	struct iio_dev *indio_dev = iio_dummy_devs[index];
+
+	/* Unregister the device */
+	iio_device_unregister(indio_dev);
+
+	/* Device specific code to power down etc */
+
+	/* Free all structures */
+	iio_free_device(indio_dev);
+
+	return 0;
+}
+
+/**
+ * iio_dummy_init() -  device driver registration
+ *
+ * Varies depending on bus type of the device. As there is no device
+ * here, call probe directly. For information on device registration
+ * i2c:
+ * Documentation/i2c/writing-clients
+ * spi:
+ * Documentation/spi/spi-summary
+ */
+static __init int iio_dummy_init(void)
+{
+	int i, ret;
+	if (instances > 10) {
+		instances = 1;
+		return -EINVAL;
+	}
+	/* Fake a bus */
+	iio_dummy_devs = kzalloc(sizeof(*iio_dummy_devs)*instances, GFP_KERNEL);
+	/* Here we have no actual device so call probe */
+	for (i = 0; i < instances; i++) {
+		ret = iio_dummy_probe(i);
+		if (ret < 0)
+			return ret;
+	}
+	return 0;
+}
+module_init(iio_dummy_init);
+
+/**
+ * iio_dummy_exit() - device driver removal
+ *
+ * Varies depending on bus type of the device.
+ * As there is no device here, call remove directly.
+ */
+static __exit void iio_dummy_exit(void)
+{
+	int i;
+	for (i = 0; i < instances; i++)
+		iio_dummy_remove(i);
+	kfree(iio_dummy_devs);
+}
+module_exit(iio_dummy_exit);
+
+MODULE_AUTHOR("Jonathan Cameron <[e-mail]>");
+MODULE_DESCRIPTION("IIO dummy driver");
+MODULE_LICENSE("GPL v2");