Date: Fri, 18 Mar 2016 12:32:09 +0100
From: Boris Brezillon <boris.brezillon@free-electrons.com>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Russell King - ARM Linux <linux@arm.linux.org.uk>,
        linux-arm-kernel@lists.infradead.org,
        Herbert Xu <herbert@gondor.apana.org.au>, timur@codeaurora.org,
        Arnaud Ebalard <arno@natisbad.org>, linux-kernel@vger.kernel.org,
        okaya@codeaurora.org, cov@codeaurora.org,
        "David S. Miller" <davem@davemloft.net>, nwatters@codeaurora.org,
        linux-crypto@vger.kernel.org
Subject: Re: [PATCH 1/3] crypto: marvell/cesa - replace dma_to_phys with
 dma_map_single
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On Fri, 18 Mar 2016 11:25:48 +0000
Robin Murphy <robin.murphy@arm.com> wrote:

> On 18/03/16 09:30, Boris Brezillon wrote:
> > On Thu, 17 Mar 2016 23:50:20 +0000
> > Russell King - ARM Linux <linux@arm.linux.org.uk> wrote:
> >
> >> On Thu, Mar 17, 2016 at 07:17:24PM -0400, okaya@codeaurora.org wrote:
> >>> What is the correct way? I don't want to write engine->sram_dma = sram
> >>
> >> Well, what the driver _is_ wanting to do is to go from a CPU physical
> >> address to a device DMA address.  phys_to_dma() looks like the correct
> >> thing there to me, but I guess that's just an offset and doesn't take
> >> account of any IOMMU that may be in the way.
> >>
> >> If you have an IOMMU, then the whole phys_to_dma() thing is a total
> >> failure as it only does a linear translation, and there are no
> >> interfaces in the kernel to take account of an IOMMU in the way.  So,
> >> it needs something designed for the job, implemented and discussed by
> >> the normal methods of proposing a new cross-arch interface for drivers
> >> to use.
> >>
> >> What I'm certain of, though, is that the change proposed in this patch
> >> will break current users of this driver: virt_to_page() on an address
> >> returned by ioremap() is completely undefined, and will result in
> >> either a kernel oops, or if not poking at memory which isn't a struct
> >> page, ultimately resulting in something that isn't SRAM being pointed
> >> to by "engine->sram_dma".
> >>
> >
> > Or we could just do
> >
> > 	engine->sram_dma = res->start;
> >
> > which is pretty much what the SRAM/genalloc code is doing already.
> 
> As Russell points out this is yet another type of "set up a DMA master 
> to access something other than kernel RAM" - there's already discussion 
> in progress over how to handle this for dmaengine slaves[1], so 
> gathering more use-cases might help distil exactly what the design of 
> not-strictly-DMA-but-so-closely-coupled-it-can't-really-live-anywhere-else 
> needs to be.
> 
> Robin.
> 
> [1]:http://lists.infradead.org/pipermail/linux-arm-kernel/2016-March/414422.html
> 

Hm, interesting, thanks for the pointer.

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