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 <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/ABI/testing/sysfs-bus-iio

···
 Description:
 		Raw (unscaled no offset etc.) pH reading of a substance as a negative
 		base-10 logarithm of hydrodium ions in a litre of water.
+
+What:           /sys/bus/iio/devices/iio:deviceX/mount_matrix
+What:           /sys/bus/iio/devices/iio:deviceX/in_mount_matrix
+What:           /sys/bus/iio/devices/iio:deviceX/out_mount_matrix
+KernelVersion:  4.6
+Contact:        [e-mail]
+Description:
+		Mounting matrix for IIO sensors. This is a rotation matrix which
+		informs userspace about sensor chip's placement relative to the
+		main hardware it is mounted on.
+		Main hardware placement is defined according to the local
+		reference frame related to the physical quantity the sensor
+		measures.
+		Given that the rotation matrix is defined in a board specific
+		way (platform data and / or device-tree), the main hardware
+		reference frame definition is left to the implementor's choice
+		(see below for a magnetometer example).
+		Applications should apply this rotation matrix to samples so
+		that when main hardware reference frame is aligned onto local
+		reference frame, then sensor chip reference frame is also
+		perfectly aligned with it.
+		Matrix is a 3x3 unitary matrix and typically looks like
+		[0, 1, 0; 1, 0, 0; 0, 0, -1]. Identity matrix
+		[1, 0, 0; 0, 1, 0; 0, 0, 1] means sensor chip and main hardware
+		are perfectly aligned with each other.
+
+		For example, a mounting matrix for a magnetometer sensor informs
+		userspace about sensor chip's ORIENTATION relative to the main
+		hardware.
+		More specifically, main hardware orientation is defined with
+		respect to the LOCAL EARTH GEOMAGNETIC REFERENCE FRAME where :
+		* Y is in the ground plane and positive towards magnetic North ;
+		* X is in the ground plane, perpendicular to the North axis and
+		  positive towards the East ;
+		* Z is perpendicular to the ground plane and positive upwards.
+
+		An implementor might consider that for a hand-held device, a
+		'natural' orientation would be 'front facing camera at the top'.
+		The main hardware reference frame could then be described as :
+		* Y is in the plane of the screen and is positive towards the
+		  top of the screen ;
+		* X is in the plane of the screen, perpendicular to Y axis, and
+		  positive towards the right hand side of the screen ;
+		* Z is perpendicular to the screen plane and positive out of the
+		  screen.
+		Another example for a quadrotor UAV might be :
+		* Y is in the plane of the propellers and positive towards the
+		  front-view camera;
+		* X is in the plane of the propellers, perpendicular to Y axis,
+		  and positive towards the starboard side of the UAV ;
+		* Z is perpendicular to propellers plane and positive upwards.

drivers/iio/industrialio-core.c

···
 }
 EXPORT_SYMBOL_GPL(iio_enum_write);
 
+static const struct iio_mount_matrix iio_mount_idmatrix = {
+	.rotation = {
+		"1", "0", "0",
+		"0", "1", "0",
+		"0", "0", "1"
+	}
+};
+
+static int iio_setup_mount_idmatrix(const struct device *dev,
+				    struct iio_mount_matrix *matrix)
+{
+	*matrix = iio_mount_idmatrix;
+	dev_info(dev, "mounting matrix not found: using identity...\n");
+	return 0;
+}
+
+ssize_t iio_show_mount_matrix(struct iio_dev *indio_dev, uintptr_t priv,
+			      const struct iio_chan_spec *chan, char *buf)
+{
+	const struct iio_mount_matrix *mtx = ((iio_get_mount_matrix_t *)
+					      priv)(indio_dev, chan);
+
+	if (IS_ERR(mtx))
+		return PTR_ERR(mtx);
+
+	if (!mtx)
+		mtx = &iio_mount_idmatrix;
+
+	return snprintf(buf, PAGE_SIZE, "%s, %s, %s; %s, %s, %s; %s, %s, %s\n",
+			mtx->rotation[0], mtx->rotation[1], mtx->rotation[2],
+			mtx->rotation[3], mtx->rotation[4], mtx->rotation[5],
+			mtx->rotation[6], mtx->rotation[7], mtx->rotation[8]);
+}
+EXPORT_SYMBOL_GPL(iio_show_mount_matrix);
+
+/**
+ * of_iio_read_mount_matrix() - retrieve iio device mounting matrix from
+ *                              device-tree "mount-matrix" property
+ * @dev:	device the mounting matrix property is assigned to
+ * @propname:	device specific mounting matrix property name
+ * @matrix:	where to store retrieved matrix
+ *
+ * If device is assigned no mounting matrix property, a default 3x3 identity
+ * matrix will be filled in.
+ *
+ * Return: 0 if success, or a negative error code on failure.
+ */
+#ifdef CONFIG_OF
+int of_iio_read_mount_matrix(const struct device *dev,
+			     const char *propname,
+			     struct iio_mount_matrix *matrix)
+{
+	if (dev->of_node) {
+		int err = of_property_read_string_array(dev->of_node,
+				propname, matrix->rotation,
+				ARRAY_SIZE(iio_mount_idmatrix.rotation));
+
+		if (err == ARRAY_SIZE(iio_mount_idmatrix.rotation))
+			return 0;
+
+		if (err >= 0)
+			/* Invalid number of matrix entries. */
+			return -EINVAL;
+
+		if (err != -EINVAL)
+			/* Invalid matrix declaration format. */
+			return err;
+	}
+
+	/* Matrix was not declared at all: fallback to identity. */
+	return iio_setup_mount_idmatrix(dev, matrix);
+}
+#else
+int of_iio_read_mount_matrix(const struct device *dev,
+			     const char *propname,
+			     struct iio_mount_matrix *matrix)
+{
+	return iio_setup_mount_idmatrix(dev, matrix);
+}
+#endif
+EXPORT_SYMBOL(of_iio_read_mount_matrix);
+
 /**
  * iio_format_value() - Formats a IIO value into its string representation
  * @buf:	The buffer to which the formatted value gets written

include/linux/iio/iio.h

···
 }
 
 /**
+ * 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
  * @dir:		    Direction of the event