Re: New dma-noncoherent code, looking for comment and people to test

[Remi Machet <rmachet@slac.stanford.edu>]

On Tue, 2008-09-30 at 17:21 +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2008-09-29 at 10:26 -0700, Remi Machet wrote:
> > 
> > I also removed the HIGHMEM support in dma_sync since memory allocated for
> > DMA transfer should always be in ZONE_DMA (ie not in ZONE_HIGHMEM).
> 
> While I like the idea of simplifying that stuff, the above sentence is
> incorrect unfortunately.
> 
> ZONE_DMA is an artifact of x86 ISA DMA limitations. You -will- get
> request for mapping pages for DMA that have been allocated within
> different zones (notably highmem).
> 
> The problem with highmem is that whether you can or not DMA to/from
> highmem is somewhat unclear, drivers set flags individually in various
> layers to allow it, which is definitely not the right place to do so. So
> while it would be nice to think we never will, in practice, we do.
> 

Yes, I realized that looking at the changes Becky's made. I will put
back the highmem support when merging my changes with those.

Thanks!

Remi