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

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Message-ID: <56EC0EF6.7060009@codeaurora.org>
Date:	Fri, 18 Mar 2016 10:21:42 -0400
From:	Sinan Kaya <okaya@...eaurora.org>
To:	Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:	Robin Murphy <robin.murphy@....com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	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, 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 3/18/2016 10:20 AM, Boris Brezillon wrote:
> On Fri, 18 Mar 2016 09:51:37 -0400
> Sinan Kaya <okaya@...eaurora.org> wrote:
> 
>> On 3/18/2016 7:25 AM, Robin Murphy wrote:
>>> 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
>>>
>>
>> Thanks for the link. 
>>
>> dma_map_resource looks like to be the correct way of doing things. Just from
>> the purist point of view, a driver is not supposed to know the physical address
>> of a DMA address. That kills the intent of using DMA API. When programming descriptors,
>> the DMA addresses should be programmed not physical addresses so that the same 
>> driver can be used in a system with IOMMU. The IOMMU DMA ops will remap the DMA 
>> address to a bus address that is not physical address. All of this operation needs
>> to be isolated from the device driver.
>>
>>
>> I don't know the architecture or the driver enough to write this. This is not ideally
>> right but I can do this if Boris you are OK with this. 
>>
>>      engine->sram_dma = res->start;
> 
> I don't know.
> 
> How about waiting for the 'dma_{map,unmap}_resource' discussion to
> settle down before removing phy_to_dma()/dma_to_phys() APIs (as
> suggested by Robin and Russell)?
> 
> 

Sure, that's fine for me.

-- 
Sinan Kaya
Qualcomm Technologies, Inc. on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project