Regarding the issue with the nouveau drivers in vanilla kernels > 3.18.16 not being able to read the BIOS / DCB block out of the OpenFirmware nVidia GPU FCODE ROM.
It seems that there has been some progress on this regression which affected powermac's with nvidia gpu's.
A patch has been created and i have applied it to vanilla 4.3.0-rc4 kernel and it seems to fix this particular issue on my config (g5 quad with quadro fx4500).
Please note for me the patch didnt apply cleanly, it needed some patching by hand for hunks that were rejected for shadowof.c.
Please note that the fix hasn't hit mainline yet.
Link to original bug report:
link to patch:
If interested, please look at the bug report for more info.