Problems with of_find_compatible_node

Hello out there, I got a problem with the kernel function "of_find_compatible_node". I use it with: np = of_find_compatible_node(NULL,NULL, "xilinx,cpld-led"); or with np = of_find_compatible_node(NULL,NULL, "cpld-led"); but none of this works, because I get a null-pointer back. But it suppose to work? The target is a PPC-walnut-board. Which is the mechanism behind this function, which namespace is parsed in order to detect devices on the board? Can I parse the whole namespace in order to list up all devices? What else can I do to solve this problem? Thank you in advance... Regards /chris