mount-matrix: Support IIO sysfs in_mount_matrix

Linux kernel IIO drivers provide mount matrix via standardized sysfs
interface. The IIO mount matrix could specified by "directory" and by
"type", this patch adds support for the "directory" type which may be
used by drivers that use the same shared matrix for all of the sensors
or if driver provides only one accelerometer sensor at all.

src/accel-mount-matrix.c

@@ -50,6 +50,21 @@ setup_mount_matrix (GUdevDevice *device)
 		g_clear_pointer (&ret, g_free);
 	}
 
+	/* Linux kernel IIO accelerometer drivers provide mount matrix
+	 * via standardized sysfs interface.
+	 *
+	 * See https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-bus-iio
+	 * for more details. */
+	mount_matrix = g_udev_device_get_sysfs_attr (device, "in_mount_matrix");
+	if (mount_matrix) {
+		if (parse_mount_matrix (mount_matrix, &ret))
+			return ret;
+
+		g_warning ("Failed to parse in_mount_matrix ('%s') from sysfs",
+			   mount_matrix);
+		g_clear_pointer (&ret, g_free);
+	}
+
 	g_debug ("Failed to auto-detect mount matrix, falling back to identity");
 	parse_mount_matrix (NULL, &ret);
 	return ret;