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

[Christoph Lameter]

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.

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