Radeon trouble Jessie 8.3. [drm:radeon_pci_probe] *ERROR* with firmware-linux-nonfree installed

[Trond Arild Ydersbond <t_a_ydersbond@yahoo.no>]

I have got this working on the same machine with Wheezy 7.6 and Knoppixes up to and including 7.6.1, but it seems that radeon_pci_probe refuses to recognize the installed firmware. Have tried different settings, but dmesg stille gives the same error.
*ERROR* radeon kernel modesetting for R600 or later requires firmware-linux-nonfree

What can I do about this?

Trond