The offset variable is introduced to FrameBuffer::Plane. In order to detect that the plane is used while the offset is not set, this adds the assertion to FrameBuffer::planes(). It should be removed in the future. Signed-off-by: Hirokazu Honda <hiroh@chromium.org> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>