linux-kernel - Re: [PATCH 1/3] crypto: marvell/cesa - replace dma_to_phys with dma_map

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Message-ID: <56EBE5BC.6000000@arm.com>
Date:	Fri, 18 Mar 2016 11:25:48 +0000
From:	Robin Murphy <robin.murphy@....com>
To:	Boris Brezillon <boris.brezillon@...e-electrons.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Herbert Xu <herbert@...dor.apana.org.au>, timur@...eaurora.org,
	Arnaud Ebalard <arno@...isbad.org>,
	linux-kernel@...r.kernel.org, okaya@...eaurora.org,
	cov@...eaurora.org, "David S. Miller" <davem@...emloft.net>,
	nwatters@...eaurora.org, linux-crypto@...r.kernel.org
Subject: Re: [PATCH 1/3] crypto: marvell/cesa - replace dma_to_phys with
 dma_map_single

On 18/03/16 09:30, Boris Brezillon wrote:
> On Thu, 17 Mar 2016 23:50:20 +0000
> Russell King - ARM Linux <linux@....linux.org.uk> wrote:
>
>> On Thu, Mar 17, 2016 at 07:17:24PM -0400, okaya@...eaurora.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