As soon as it loads the radeon module it displays only artifacts, it works with UMS (radeon.nomodeset=0). With R500 it works flawlessly, even with KMS. 2.6.38 does work with R600, I didn't test 2.6.39. Card is an HD3870 (RV670).
Can you bisect?
Unfortunately I can't, I can access this machine only few hours at week at the moment and I'm currently using an X1950 XTX because I need working graphics.
Anyway if you want me to test a specific snapshot or two I can do it.
Make sure your kernel has this commit:
Yes, it has. I'm using latest snapshot from torvalds branch as of now.
I noticed I was previously embedding the additional firmwares in the kernel image (the radeon module was statically compiled), while now I compiled it as a module and I probably don't have the firmwares in a known path. I will check it.
The problem was the missing ucode, I'm sorry.