iio: core: Introduce DISTANCE channel type

Some devices export an estimation of the distance the user has covered
since the last reset.

One of this devices is Freescale's MMA9553L
(http://www.freescale.com/files/sensors/doc/ref_manual/MMA9553LSWRM.pdf)
that computes the distance based on the stride length and step rate.

Introduce a new channel type DISTANCE to export these values.

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio

@@ -283,6 +283,7 @@ What:		/sys/bus/iio/devices/iio:deviceX/in_accel_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_accel_peak_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_anglvel_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_energy_scale
+What:		/sys/bus/iio/devices/iio:deviceX/in_distance_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_magn_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_magn_x_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_magn_y_scale
@@ -1059,6 +1060,15 @@ Description:
 		device (e.g.: human activity sensors report energy burnt by the
 		user). Units after application of scale are Joules.
 
+What:		/sys/.../iio:deviceX/in_distance_input
+What:		/sys/.../iio:deviceX/in_distance_raw
+KernelVersion:	3.20
+Contact:	linux-iio@vger.kernel.org
+Description:
+		This attribute is used to read the distance covered by the user
+		since the last reboot while activated. Units after application
+		of scale are meters.
+
 What:		/sys/bus/iio/devices/iio:deviceX/store_eeprom
 KernelVersion:	3.4.0
 Contact:	linux-iio@vger.kernel.org

drivers/iio/industrialio-core.c

@@ -73,6 +73,7 @@ static const char * const iio_chan_type_name_spec[] = {
 	[IIO_ACTIVITY] = "activity",
 	[IIO_STEPS] = "steps",
 	[IIO_ENERGY] = "energy",
+	[IIO_DISTANCE] = "distance",
 };
 
 static const char * const iio_modifier_names[] = {

include/linux/iio/types.h

@@ -33,6 +33,7 @@ enum iio_chan_type {
 	IIO_ACTIVITY,
 	IIO_STEPS,
 	IIO_ENERGY,
+	IIO_DISTANCE,
 };
 
 enum iio_modifier {