linux-kernel - Re: [PATCH 2/4] scatterlist: add sg_alloc_table_from

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Message-ID: <20160331141412.GK19428@n2100.arm.linux.org.uk>
Date:	Thu, 31 Mar 2016 15:14:13 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:	David Woodhouse <dwmw2@...radead.org>,
	Brian Norris <computersforpeace@...il.com>,
	linux-mtd@...ts.infradead.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Gordon <david.s.gordon@...el.com>,
	linux-crypto@...r.kernel.org,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Vinod Koul <vinod.koul@...el.com>,
	Richard Weinberger <richard@....at>,
	Joerg Roedel <joro@...tes.org>, linux-kernel@...r.kernel.org,
	linux-spi@...r.kernel.org, Vignesh R <vigneshr@...com>,
	linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
	Mark Brown <broonie@...nel.org>,
	Hans Verkuil <hans.verkuil@...co.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	dmaengine@...r.kernel.org, Dan Williams <dan.j.williams@...el.com>,
	linux-media@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	linux-arm-kernel@...ts.infradead.org,
	Mauro Carvalho Chehab <m.chehab@...sung.com>
Subject: Re: [PATCH 2/4] scatterlist: add sg_alloc_table_from_buf() helper

On Thu, Mar 31, 2016 at 02:29:42PM +0200, Boris Brezillon wrote:
> sg_alloc_table_from_buf() provides an easy solution to create an sg_table
> from a virtual address pointer. This function takes care of dealing with
> vmallocated buffers, buffer alignment, or DMA engine limitations (maximum
> DMA transfer size).

Please note that the DMA API does not take account of coherency of memory
regions other than non-high/lowmem - there are specific extensions to
deal with this.

What this means is that having an API that takes any virtual address
pointer, converts it to a scatterlist which is then DMA mapped, is
unsafe.

It'll be okay for PIPT and non-aliasing VIPT cache architectures, but
for other cache architectures this will hide this problem and make
review harder.

-- 
RMK's Patch system: http://www.arm.linux.org.uk/developer/patches/
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