Date: Thu, 31 Mar 2016 16:45:57 +0200
From: Boris Brezillon <boris.brezillon@free-electrons.com>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: David Woodhouse <dwmw2@infradead.org>,
        Brian Norris <computersforpeace@gmail.com>,
        linux-mtd@lists.infradead.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Gordon <david.s.gordon@intel.com>, linux-crypto@vger.kernel.org,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Vinod Koul <vinod.koul@intel.com>, Richard Weinberger <richard@nod.at>,
        Joerg Roedel <joro@8bytes.org>, linux-kernel@vger.kernel.org,
        linux-spi@vger.kernel.org, Vignesh R <vigneshr@ti.com>,
        linux-mm@kvack.org, iommu@lists.linux-foundation.org,
        Mark Brown <broonie@kernel.org>, Hans Verkuil <hans.verkuil@cisco.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        dmaengine@vger.kernel.org, Dan Williams <dan.j.williams@intel.com>,
        linux-media@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
        linux-arm-kernel@lists.infradead.org,
        Mauro Carvalho Chehab <m.chehab@samsung.com>
Subject: Re: [PATCH 2/4] scatterlist: add sg_alloc_table_from_buf() helper
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Hi Russell,

On Thu, 31 Mar 2016 15:14:13 +0100
Russell King - ARM Linux <linux@arm.linux.org.uk> wrote:

> 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.

Ok, you said 'non-high/lowmem', this means vmalloced and kmapped buffers
already fall in this case, right?

Could you tell me more about those specific extensions?

> 
> 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.

Which means some implementations already get this wrong (see
spi_map_buf(), and I'm pretty sure it's not the only one).

> 
> 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.
> 

Ok, you lost me. I'll have to do my homework and try to understand what
this means :).

Thanks for your valuable inputs.

Best Regards,

Boris

-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com