On Wed, 6 Jul 2011, Russell King - ARM Linux wrote:

quoted

quoted

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.