iio: ABI: Add quaternion axis modifier

This modifier applies to the IIO_ROT channel type, and indicates a data
representation that specifies the {x, y, z} components of the normalized
quaternion vector.

Signed-off-by: Francesco Lavra <flavra@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio

@@ -1755,6 +1755,21 @@ Description:
 		measurement from channel Y. Units after application of scale and
 		offset are milliamps.
 
+What:		/sys/bus/iio/devices/iio:deviceX/in_rot_quaternionaxis_raw
+KernelVersion:	7.1
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Raw value of {x, y, z} components of the quaternion vector. These
+		components represent the axis about which a rotation occurs, and are
+		subject to the following constraints:
+
+		- the quaternion vector is normalized, i.e. w^2 + x^2 + y^2 + z^2 = 1
+		- the rotation angle is within the [-pi, pi] range, i.e. the w
+		  component (which represents the amount of rotation) is non-negative
+
+		These constraints allow the w value to be calculated from the other
+		components: w = sqrt(1 - (x^2 + y^2 + z^2)).
+
 What:		/sys/.../iio:deviceX/in_energy_en
 What:		/sys/.../iio:deviceX/in_distance_en
 What:		/sys/.../iio:deviceX/in_velocity_sqrt(x^2+y^2+z^2)_en

drivers/iio/industrialio-core.c

@@ -157,6 +157,7 @@ static const char * const iio_modifier_names[] = {
 	[IIO_MOD_ACTIVE] = "active",
 	[IIO_MOD_REACTIVE] = "reactive",
 	[IIO_MOD_APPARENT] = "apparent",
+	[IIO_MOD_QUATERNION_AXIS] = "quaternionaxis",
 };
 
 /* relies on pairs of these shared then separate */

include/uapi/linux/iio/types.h

@@ -113,6 +113,7 @@ enum iio_modifier {
 	IIO_MOD_ACTIVE,
 	IIO_MOD_REACTIVE,
 	IIO_MOD_APPARENT,
+	IIO_MOD_QUATERNION_AXIS,
 };
 
 enum iio_event_type {

tools/iio/iio_event_monitor.c

@@ -145,6 +145,7 @@ static const char * const iio_modifier_names[] = {
 	[IIO_MOD_ACTIVE] = "active",
 	[IIO_MOD_REACTIVE] = "reactive",
 	[IIO_MOD_APPARENT] = "apparent",
+	[IIO_MOD_QUATERNION_AXIS] = "quaternionaxis",
 };
 
 static bool event_is_known(struct iio_event_data *event)