29 Apr
2011
29 Apr
'11
10:51 p.m.
On Fri, 2011-04-29 at 12:56 +0100, Alan Cox wrote:
I believe that the PC graphics cards that have noncoherent DMA mappings are all of the unified memory (integrated into the northbridge) kind, so they are not on the same host bridge as all regular PCI devices, even if they appear as a PCI device.
The AGP GART is not coherent on a lot of systems - not necessarily unified memory though, it can be a plug in AGP card too. The GART is basically an IOMMU (and indeed in the later AMD case used exactly as that)
Right. Actually there's also the ability for PCIe devices to set a "no snoop" bit on transactions and thus behave in a non-coherent manner. Hopefully most sane PHBs ignore that bit ...
Cheers, Ben.