iio: Implement counter channel type and info constants

Quadrature encoders, such as rotary encoders and linear encoders, are
devices which are capable of encoding the relative position and
direction of motion of a shaft. This patch introduces several IIO
constants for supporting quadrature encoder counter devices.

  IIO_COUNT: Current count (main data provided by the counter device)
  IIO_INDEX: Counter device index value
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio

@@ -329,6 +329,7 @@ What:		/sys/bus/iio/devices/iio:deviceX/in_pressure_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_humidityrelative_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_velocity_sqrt(x^2+y^2+z^2)_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_illuminance_scale
+What:		/sys/bus/iio/devices/iio:deviceX/in_countY_scale
 KernelVersion:	2.6.35
 Contact:	linux-iio@vger.kernel.org
 Description:
@@ -1579,3 +1580,20 @@ Contact:	linux-iio@vger.kernel.org
 Description:
 		Raw (unscaled no offset etc.) electric conductivity reading that
 		can be processed to siemens per meter.
+
+What:		/sys/bus/iio/devices/iio:deviceX/in_countY_raw
+KernelVersion:	4.9
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Raw counter device counts from channel Y. For quadrature
+		counters, multiplication by an available [Y]_scale results in
+		the counts of a single quadrature signal phase from channel Y.
+
+What:		/sys/bus/iio/devices/iio:deviceX/in_indexY_raw
+KernelVersion:	4.9
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Raw counter device index value from channel Y. This attribute
+		provides an absolute positional reference (e.g. a pulse once per
+		revolution) which may be used to home positional systems as
+		required.

drivers/iio/industrialio-core.c

@@ -81,6 +81,8 @@ static const char * const iio_chan_type_name_spec[] = {
 	[IIO_PH] = "ph",
 	[IIO_UVINDEX] = "uvindex",
 	[IIO_ELECTRICALCONDUCTIVITY] = "electricalconductivity",
+	[IIO_COUNT] = "count",
+	[IIO_INDEX] = "index",
 };
 
 static const char * const iio_modifier_names[] = {

include/uapi/linux/iio/types.h

@@ -40,6 +40,8 @@ enum iio_chan_type {
 	IIO_PH,
 	IIO_UVINDEX,
 	IIO_ELECTRICALCONDUCTIVITY,
+	IIO_COUNT,
+	IIO_INDEX,
 };
 
 enum iio_modifier {