UniNorth AGP: Some success

[Michel Lanners]

Hi all,

On  14 Aug, this message from Benjamin Herrenschmidt echoed through cyberspace:
>  - Machine is a PowerBook "Pismo" with a rev. of UniNorth that can do x2
> AGP only and no fast write (it does SBA fortunately). I use the alpha
> nopage() code for userland mappings and a home made ioremap_agp() for in-
> kernel mappings. Everything is mapped uncached, which is a performance
> hit on PPC, I'll have to see how I can make things cachable by adding the
> proper flush routines all over the place.

Depending on cache mode and data direction, you might not need cache
flushing. I mapped the 'control' framebuffer write-through; since only
the cpu writes there (it's not accelerated), that's ok without cache
flushing, but it does enable store gathering in the processor (and maybe
also in the PCI bridge; not sure...) Not sure whether that applies to
AGP as well.

>  - It's a bit slower than PCI GART (about 430 fps in gears, but it's not
> stable, while PCI GART gives me a stable 474 fps). This may be partially
> do to the fact that we have all the AGP memory uncachable, I think that
> will prevent the PPC from doing write-combining.

Yes, that's how I understand all the docs I read so far. I'd suggest
tackling cacheing issues first; that will probably provide the biggest
benefit.

Cheers, and happy hacking ;-)

Michel

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