PowerPC agpmode issues

[Mike]

Hi.
Managed to get the Radeon R300 running on mesa 11.1.1 with an old 2013
patch from Michel Dànzer, next problem is of course enabling agpmode,
running with pci-mode with radeon.agpmode=-1 works, but is of course slow,
and seems to load the cpu a lot.

Upon initial investigation i could not initially believe agp could be this
this broken for this long, until i found this.
 "committed with Ben Skeggs on Feb 26, 2013"
https://github.com/DespairFactor/bullhead/commit/650e1203c11354ba84d69ba445abc0efcfe3890a
http://lxr.free-electrons.com/source/drivers/gpu/drm/nouveau/nouveau_agp.c?v=4.2
#ifdef __powerpc__
/* Disable AGP by default on all PowerPC machines for
* now -- At least some UniNorth-2 AGP bridges are
* known to be broken: DMA from the host to the card
* works just fine, but writeback from the card to the
* host goes straight to memory untranslated bypassing
* the GATT somehow, making them quite painful to deal
* with...
*/
if (nouveau_agpmode == -1)
return false;
#endif

 and now later this:
https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/nouveau/nvkm/subdev/pci/agp.c
#ifdef __powerpc__
/* Disable AGP by default on all PowerPC machines for now -- At
* least some UniNorth-2 AGP bridges are known to be broken:
* DMA from the host to the card works just fine, but writeback
* from the card to the host goes straight to memory
* untranslated bypassing that GATT somehow, making them quite
* painful to deal with...
*/
mode = 0;
#endif

All seems to point to serious issues had around the time of change to ums
to kms and a serious regression hitting the linux kernel? No?

Cheers
-Mike
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