linux-kernel - Re: [PATCHv4 3/5] common: dma-mapping: Introduce common remapping functions

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Message-ID: <20140718141335.GC4608@arm.com>
Date:	Fri, 18 Jul 2014 15:13:35 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Olof Johansson <olof@...om.net>
Cc:	Laura Abbott <lauraa@...eaurora.org>,
	Will Deacon <Will.Deacon@....com>,
	David Riley <davidriley@...omium.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Ritesh Harjain <ritesh.harjani@...il.com>,
	linux-mm <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv4 3/5] common: dma-mapping: Introduce common remapping
 functions

On Wed, Jul 09, 2014 at 11:46:56PM +0100, Olof Johansson wrote:
> On Wed, Jul 2, 2014 at 11:03 AM, Laura Abbott <lauraa@...eaurora.org> wrote:
> > For architectures without coherent DMA, memory for DMA may
> > need to be remapped with coherent attributes. Factor out
> > the the remapping code from arm and put it in a
> > common location to reduced code duplication.
> >
> > Signed-off-by: Laura Abbott <lauraa@...eaurora.org>
> 
> Hm. The switch from ioremap to map_vm_area() here seems to imply that
> lib/ioremap can/should be reworked to use just wrap the vmalloc
> functions too?

ioremap_page_range() does not require the allocation of a map page array
and assumes that the mapped memory is physically contiguous. This would
be more efficient than the vmap() implementation which is generic enough
to allow non-contiguous memory allocations.

At some point, we'll have to support SMMU on arm64 and we can have 4
combinations of coherency and IOMMU. When an IOMMU is present, we don't
require physically contiguous memory but we may require non-cacheable
mappings, in which case vmap comes in handy.

-- 
Catalin
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