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

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Date:	Tue, 16 Oct 2012 09:59:28 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Hiroshi Doyu <hdoyu@...dia.com>
Cc:	Marek Szyprowski <m.szyprowski@...sung.com>,
	Inki Dae <inki.dae@...sung.com>, Arnd Bergmann <arnd@...db.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, linux-tegra@...r.kernel.org
Subject: Re: [Linaro-mm-sig] [RFC 0/2] DMA-mapping & IOMMU - physically
	contiguous allocations

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.

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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