mount-matrix: Support IIO sysfs in_accel_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 "by type" option which is used
by drivers that use individual matrices for each of the sensors. Tested
on Nexus 7 and Acer A500 tablet devices using MPU6050 and KXTF9 IIO drivers
respectively, now monitor-sensor reports display's orientation correctly
on these devices.

src/accel-mount-matrix.c

@@ -50,6 +50,23 @@ setup_mount_matrix (GUdevDevice *device)
 		g_clear_pointer (&ret, g_free);
 	}
 
+	/* Some IIO drivers provide multiple sensors via the same sysfs path
+	 * and thus they may have different matrices like in a case of
+	 * accelerometer and angular velocity for example. The accelerometer
+	 * mount matrix is named as in_accel_mount_matrix in that case.
+	 *
+	 * 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_accel_mount_matrix");
+	if (mount_matrix) {
+		if (parse_mount_matrix (mount_matrix, &ret))
+			return ret;
+
+		g_warning ("Failed to parse in_accel_mount_matrix ('%s') from sysfs",
+			   mount_matrix);
+		g_clear_pointer (&ret, g_free);
+	}
+
 	/* Linux kernel IIO accelerometer drivers provide mount matrix
 	 * via standardized sysfs interface.
 	 *