iio:core: mounting matrix support

Expose a rotation matrix to indicate userspace the chip placement with
respect to the overall hardware system. This is needed to adjust
coordinates sampled from a sensor chip when its position deviates from the
main hardware system.

Final coordinates computation is delegated to userspace since:
* computation may involve floating point arithmetics ;
* it allows an application to combine adjustments with arbitrary
  transformations.

This 3 dimentional space rotation matrix is expressed as 3x3 array of
strings to support floating point numbers. It may be retrieved from a
"[<dir>_][<type>_]mount_matrix" sysfs attribute file. It is declared into a
device / driver specific DTS property or platform data.

Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

include/linux/iio/iio.h

@@ -147,6 +147,37 @@ ssize_t iio_enum_write(struct iio_dev *indio_dev,
 	.private = (uintptr_t)(_e), \
 }
 
+/**
+ * struct iio_mount_matrix - iio mounting matrix
+ * @rotation: 3 dimensional space rotation matrix defining sensor alignment with
+ *            main hardware
+ */
+struct iio_mount_matrix {
+	const char *rotation[9];
+};
+
+ssize_t iio_show_mount_matrix(struct iio_dev *indio_dev, uintptr_t priv,
+			      const struct iio_chan_spec *chan, char *buf);
+int of_iio_read_mount_matrix(const struct device *dev, const char *propname,
+			     struct iio_mount_matrix *matrix);
+
+typedef const struct iio_mount_matrix *
+	(iio_get_mount_matrix_t)(const struct iio_dev *indio_dev,
+				 const struct iio_chan_spec *chan);
+
+/**
+ * IIO_MOUNT_MATRIX() - Initialize mount matrix extended channel attribute
+ * @_shared:	Whether the attribute is shared between all channels
+ * @_get:	Pointer to an iio_get_mount_matrix_t accessor
+ */
+#define IIO_MOUNT_MATRIX(_shared, _get) \
+{ \
+	.name = "mount_matrix", \
+	.shared = (_shared), \
+	.read = iio_show_mount_matrix, \
+	.private = (uintptr_t)(_get), \
+}
+
 /**
  * struct iio_event_spec - specification for a channel event
  * @type:		    Type of the event