Comment 20 for bug 1571416

(In reply to Mathieu Malaterre from comment #8)
> (In reply to Ilia Mirkin from comment #7)
> > FWIW on nouveau we just default agpmode=0 in a #ifdef __powerpc__. Perhaps
> > the same thing can work for radeon.
>
> Well radeon_agpmode is already set to 0 already[*]. My patch force to
> PCI(-1) when user did not specify any value.
>
> [*]https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/radeon/
> radeon_drv.c#L173

Oh, the logic is flipped from nouveau... on nouveau, -1 == autodetect and 0 == no agp. So in your case, just set it to -1 instead of 0 for powerpc. i.e.

#ifdef __powerpc__
int radeon_agpmode = -1;
#else
int radeon_agpmode = 0;
#endif

or something similar.