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

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