[Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1

[arnd@arndb.de (Arnd Bergmann)]

On Thursday 28 April 2011, Russell King - ARM Linux wrote:
> What I'd suggest is that dma_alloc_noncoherent() should be architecture
> independent, and should call into whatever iommu support the device has
> to setup an approprite iommu mapping.  IOW, I don't see any need for
> every architecture to provide its own dma_alloc_noncoherent() allocation
> function - or indeed every iommu implementation to deal with the
> allocation issues either.

Almost all architectures today define dma_alloc_noncoherent to
dma_alloc_coherent, which is totally fine on architectures
where cacheable coherent mappings are the default or where
we don't need to flush individual cache lines for dma_sync_*.

The problem with backing either of the two with alloc_pages or
alloc_pages_exact is that you cannot do large allocation when
physical memory is fragmented, even if you have an IOMMU.

IMHO the allocation for both dma_alloc_coherent and
dma_alloc_noncoherent should therefore depend on whether you
have an IOMMU. If you do, you can easily allocate megabytes,
e.g. for use as a frame buffer.

	Arnd