Re: [Linaro-mm-sig] [RFC 0/2] DMA-mapping & IOMMU - physicallycontiguous allocations

From: Russell King - ARM Linux
Date: Tue Oct 16 2012 - 04:59:37 EST

On Tue, Oct 16, 2012 at 09:04:34AM +0300, Hiroshi Doyu wrote:
> In addition to those contiguous/discontiguous page allocation, is
> there any way to _import_ anonymous pages allocated by a process to be
> used in dma-mapping API later?
> I'm considering the following scenario, an user process allocates a
> buffer by malloc() in advance, and then it asks some driver to convert
> that buffer into IOMMU'able/DMA'able ones later. In this case, pages
> are discouguous and even they may not be yet allocated at
> malloc()/mmap().

That situation is covered. It's the streaming API you're wanting for that.
dma_map_sg() - but you may need additional cache handling via
flush_dcache_page() to ensure that your code is safe for all CPU cache

Remember that pages allocated into userspace will be cacheable, so a cache
flush is required before they can be DMA'd. Hence the streaming API.
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