[Linux-fbdev-devel] Re: radeon, apertures & memory mapping

[Benjamin Herrenschmidt]

On Sun, 2005-03-13 at 17:17 -0500, Jon Smirl wrote:
> On Mon, 14 Mar 2005 08:52:46 +1100, Benjamin Herrenschmidt
> <benh at kernel.crashing.org> wrote:
> > 
> > > The best model would be to chuck the AGP/PCI Express interface on the
> > > board and have a hyperchannel instead. Hyperchannel provides full
> > > cache consistency without all of these flushing problems. The GPU
> > > really is another specialized CPU, give it a CPU class memory
> > > interface.
> > 
> > You mean HyperTransport ? Well, PCI Express isn't far from that
> > neither...
> 
> Yes, HyperTransport. That's what I get for writing email while the
> babies are crying.
> 
> Does PCI Express support a full cache coherency protocol like
> HyperTransport?

Or just like PCI :)

AGP is just a special case, and I'm not even sure wether the cache
incoherency is a spec thing or just that all implementations are
broken :) I'll check when I find my AGP spec.

>  So if the GPU touches a page of system memory, the
> system memory will get flushed out of the CPU cache? In that case we
> don't need to mark shared GPU/CPU memory as non-cachable.

Well, that's what happen with a normal cache coherent setup, but I'm
fairly sure that won't happen with AGP on a lot of machines.

Ben.