On Sun, 2005-03-13 at 17:17 -0500, Jon Smirl wrote:

On Mon, 14 Mar 2005 08:52:46 +1100, Benjamin Herrenschmidt
[off-list ref] wrote:

quoted

quoted

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.




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