[Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1

[Benjamin Herrenschmidt]

On Fri, 2011-04-29 at 14:06 +0200, Thomas Hellstrom wrote:
> I think Jerome has mentioned at one point that the Radeon graphics
> cards support non-coherent mappings.

If the card is PCI/PCI-X/PCIe then coherency is not its business, it's
the business of the host bridge. However, on PCIe at least, the card can
indeed set a "no snoop" attribute on DMA transactions to request "no
coherency". At least the systems have the latitude to just ignore that
bit (like we do on all ppc afaik) :-)

> Fwiw, the PowerVR SGX MMU also supports this mode of operation,
> although  it being functional I guess depends on the system
> implementation.

Right, it's not a GPU thing, it's really a system design thing.

Cheers,
Ben.