[PATCH 6/8] drivers: add Contiguous Memory Allocator

[Russell King - ARM Linux]

On Wed, Jul 06, 2011 at 11:05:00AM -0500, Christoph Lameter wrote:
> On Wed, 6 Jul 2011, Russell King - ARM Linux wrote:
> 
> > > > they typically don't fall into the highmem zone.  As the dmabounce
> > > > code allocates from the DMA coherent allocator to provide it with
> > > > guaranteed DMA-able memory, that would be rather inconvenient.
> > >
> > > True. The dmabounce code would consequently have to allocate
> > > the memory through an internal function that avoids the
> > > contiguous allocation area and goes straight to ZONE_DMA memory
> > > as it does today.
> >
> > CMA's whole purpose for existing is to provide _dma-able_ contiguous
> > memory for things like cameras and such like found on crippled non-
> > scatter-gather hardware.  If that memory is not DMA-able what's the
> > point?
> 
> ZONE_DMA is a zone for memory of legacy (crippled) devices that cannot DMA
> into all of memory (and so is ZONE_DMA32). Memory from ZONE_NORMAL can be
> used for DMA as well and a fully capable device would be expected to
> handle any memory in the system for DMA transfers.
> 
> "guaranteed" dmaable memory? DMA abilities are device specific. Well maybe
> you can call ZONE_DMA memory to be guaranteed if you guarantee that any
> device must at mininum be able to perform DMA into ZONE_DMA memory. But
> there may not be much of that memory around so you would want to limit
> the use of that scarce resource.

Precisely, which is what ZONE_DMA is all about.  I *have* been a Linux
kernel hacker for the last 18 years and do know these things, especially
as ARM has had various issues with DMA memory limitations over those
years - and have successfully had platforms working reliably given that
and ZONE_DMA.