30 bits DMA and ppc

[Benjamin Herrenschmidt]

> The non-iommu case might want to do that for other devices as well,
> i.e. 32-bit limited ones on 64-bit machines.

Yup, that would be useful with some 3rd party bridges that don't have an
iommu ... :)

> Yep, the table is already split once, and I'm not sure in retrospect how
> useful that split was anyway, it can maybe go away. Switching it around
> shouldn't be a big issue.

No real need to split permanently... the search could be restricted to a
given range easily.

Ben.