On Fri, 2011-04-29 at 09:34 -0400, Jerome Glisse wrote:
Radeon memory controller can do non snooped pci transaction, as far as i have tested most of the x86 pci bridge don't try to be coherent then ie they don't analyze pci dma and ask for cpu flush they just perform the request (and i guess it's what all bridge will do), so it endup being noncoherent. I haven't done any benchmark of how faster it's for the GPU when it's not snooping but i guess it can give 50% boost as it likely drastictly reduce pci transaction overhead.
I am talking here about device that you plug into any pci or pcie slot, so it's not igp integrated into northbridge or into the cpu.
Right, the card has nothing to do with the snooping process, it's purely a feature of the bridge, based on a flag optionally set by the card. As I said earlier, bridges have the freedom to ignore it, which we do on ppc, so that's a non issue.
Cheers, Ben.