iio: Documentation: Add IIO configfs documentation

Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Acked-by: Crt Mori <cmo@melexis.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

diff --git a/Documentation/ABI/testing/configfs-iio b/Documentation/ABI/testing/configfs-iio
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+What:          /config/iio
+Date:          October 2015
+KernelVersion: 4.4
+Contact:       linux-iio@vger.kernel.org
+Description:
+               This represents Industrial IO configuration entry point
+               directory. It contains sub-groups corresponding to IIO
+               objects.
+
+What:          /config/iio/triggers
+Date:          October 2015
+KernelVersion: 4.4
+Description:
+               Industrial IO software triggers directory.
+
+What:          /config/iio/triggers/hrtimers
+Date:          October 2015
+KernelVersion: 4.4
+Description:
+               High resolution timers directory. Creating a directory here
+               will result in creating a hrtimer trigger in the IIO subsystem.

diff --git a/Documentation/iio/iio_configfs.txt b/Documentation/iio/iio_configfs.txt
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+Industrial IIO configfs support
+
+1. Overview
+
+Configfs is a filesystem-based manager of kernel objects. IIO uses some
+objects that could be easily configured using configfs (e.g.: devices,
+triggers).
+
+See Documentation/filesystems/configfs/configfs.txt for more information
+about how configfs works.
+
+2. Usage
+
+In order to use configfs support in IIO we need to select it at compile
+time via CONFIG_IIO_CONFIGFS config option.
+
+Then, mount the configfs filesystem (usually under /config directory):
+
+$ mkdir /config
+$ mount -t configfs none /config
+
+At this point, all default IIO groups will be created and can be accessed
+under /config/iio. Next chapters will describe available IIO configuration
+objects.
+
+3. Software triggers
+
+One of the IIO default configfs groups is the "triggers" group. It is
+automagically accessible when the configfs is mounted and can be found
+under /config/iio/triggers.
+
+IIO software triggers implementation offers support for creating multiple
+trigger types. A new trigger type is usually implemented as a separate
+kernel module following the interface in include/linux/iio/sw_trigger.h:
+
+/*
+ * drivers/iio/trigger/iio-trig-sample.c
+ * sample kernel module implementing a new trigger type
+ */
+#include <linux/iio/sw_trigger.h>
+
+
+static struct iio_sw_trigger *iio_trig_sample_probe(const char *name)
+{
+       /*
+        * This allocates and registers an IIO trigger plus other
+        * trigger type specific initialization.
+        */
+}
+
+static int iio_trig_hrtimer_remove(struct iio_sw_trigger *swt)
+{
+       /*
+        * This undoes the actions in iio_trig_sample_probe
+        */
+}
+
+static const struct iio_sw_trigger_ops iio_trig_sample_ops = {
+       .probe          = iio_trig_sample_probe,
+       .remove         = iio_trig_sample_remove,
+};
+
+static struct iio_sw_trigger_type iio_trig_sample = {
+       .name = "trig-sample",
+       .owner = THIS_MODULE,
+       .ops = &iio_trig_sample_ops,
+};
+
+module_iio_sw_trigger_driver(iio_trig_sample);
+
+Each trigger type has its own directory under /config/iio/triggers. Loading
+iio-trig-sample module will create 'trig-sample' trigger type directory
+/config/iio/triggers/trig-sample.
+
+We support the following interrupt sources (trigger types):
+       * hrtimer, uses high resolution timers as interrupt source
+
+3.1 Hrtimer triggers creation and destruction
+
+Loading iio-trig-hrtimer module will register hrtimer trigger types allowing
+users to create hrtimer triggers under /config/iio/triggers/hrtimer.
+
+e.g:
+
+$ mkdir /config/triggers/hrtimer/instance1
+$ rmdir /config/triggers/hrtimer/instance1
+
+Each trigger can have one or more attributes specific to the trigger type.
+
+3.2 "hrtimer" trigger types attributes
+
+"hrtimer" trigger type doesn't have any configurable attribute from /config dir.
+It does introduce the sampling_frequency attribute to trigger directory.