Industrial I/O Subsystem in kernel consumers.

The IIO subsystem can act as a layer under other elements of the kernel
providing a means of obtaining ADC type readings or of driving DAC type
signals.  The functionality supported will grow as use cases arise.

Describing the channel mapping (iio/machine.h)

Channel associations are described using:

struct iio_map {
	const char *adc_channel_label;
	const char *consumer_dev_name;
	const char *consumer_channel;
};

adc_channel_label identifies the channel on the IIO device by being
matched against the datasheet_name field of the iio_chan_spec.

consumer_dev_name allows identification of the consumer device.
This are then used to find the channel mapping from the consumer device (see
below).

Finally consumer_channel is a string identifying the channel to the consumer.
(Perhaps 'battery_voltage' or similar).

An array of these structures is then passed to the IIO driver.

Supporting in kernel interfaces in the driver (driver.h)

The driver must provide datasheet_name values for its channels and
must pass the iio_map structures and a pointer to its own iio_dev structure
 on to the core via a call to iio_map_array_register.  On removal,
iio_map_array_unregister reverses this process.

The result of this is that the IIO core now has all the information needed
to associate a given channel with the consumer requesting it.

Acting as an IIO consumer (consumer.h)

The consumer first has to obtain an iio_channel structure from the core
by calling iio_channel_get().  The correct channel is identified by:

* matching dev or dev_name against consumer_dev and consumer_dev_name
* matching consumer_channel against consumer_channel in the map

There are then a number of functions that can be used to get information
about this channel such as it's current reading.

e.g.
iio_st_read_channel_raw() - get a reading
iio_st_read_channel_type() - get the type of channel

There is also provision for retrieving all of the channels associated
with a given consumer.  This is useful for generic drivers such as
iio_hwmon where the number and naming of channels is not known by the
consumer driver.  To do this, use iio_st_channel_get_all.

