Date: Fri, 18 Mar 2016 10:30:28 +0100
From: Boris Brezillon <boris.brezillon@free-electrons.com>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: okaya@codeaurora.org, linux-arm-kernel@lists.infradead.org,
        timur@codeaurora.org, cov@codeaurora.org, nwatters@codeaurora.org,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Arnaud Ebalard <arno@natisbad.org>, linux-kernel@vger.kernel.org,
        linux-crypto@vger.kernel.org, "David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH 1/3] crypto: marvell/cesa - replace dma_to_phys with
 dma_map_single
Message-ID: <20160318103028.4be15b78@bbrezillon>
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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.


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