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

From: Catalin Marinas
Date: Tue Oct 16 2012 - 06:04:40 EST

On 16 October 2012 09:59, Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxxx> wrote:
> 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
> architectures.

For user-allocated pages you first need get_user_pages() to make sure
they are in memory (and will stay there). This function also calls
flush_dcache_page(). Then you can build the sg list for dma_map_sg().

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