linux-kernel - Re: [PATCH v3 5/7] ARM: NOMMU: Introduce dma operations for noMMU

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Date:   Wed, 19 Apr 2017 15:12:01 +0100
From:   Vladimir Murzin <vladimir.murzin@....com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kbuild-all@...org,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Robin Murphy <robin.murphy@....com>, sza@....hu
Subject: Re: [PATCH v3 5/7] ARM: NOMMU: Introduce dma operations for noMMU

On 19/04/17 11:10, Arnd Bergmann wrote:
> On Fri, Mar 10, 2017 at 10:23 AM, Vladimir Murzin
> <vladimir.murzin@....com> wrote:
>> R/M classes of cpus can have memory covered by MPU which in turn might
>> configure RAM as Normal i.e. bufferable and cacheable. It breaks
>> dma_alloc_coherent() and friends, since data can stuck in caches now
>> or be buffered.
>>
>> This patch factors out DMA support for NOMMU configuration into
>> separate entity which provides dedicated dma_ops. We have to handle
>> there several cases:
>> - configurations with MMU/MPU setup
>> - configurations without MMU/MPU setup
>> - special case for M-class, since caches and MPU there are optional
>>
>> In general we rely on default DMA area for coherent allocations or/and
>> per-device memory reserves suitable for coherent DMA, so if such
>> regions are set coherent allocations go from there.
>>
>> In case MPU/MPU was not setup we fallback to normal page allocator for
>> DMA memory allocation.
>>
>> In case we run M-class cpus, for configuration without cache support
>> (like Cortex-M3/M4) dma operations are forced to be coherent and wired
>> with dma-noop (such decision is made based on cacheid global
>> variable); however, if caches are detected there and no DMA coherent
>> region is given (either default or per-device), dma is disallowed even
>> MPU is not set - it is because M-class implement system memory map
>> which defines part of address space as Normal memory.
>>
>> Reported-by: Alexandre Torgue <alexandre.torgue@...com>
>> Reported-by: Andras Szemzo <sza@....hu>
>> Tested-by: Benjamin Gaignard <benjamin.gaignard@...aro.org>
>> Tested-by: Andras Szemzo <sza@....hu>
>> Tested-by: Alexandre TORGUE <alexandre.torgue@...com>
>> Reviewed-by: Robin Murphy <robin.murphy@....com>
>> Signed-off-by: Vladimir Murzin <vladimir.murzin@....com>
> 
> I thought I found a couple of problems with this, but after reading more
> carefully I was wrong about all of them and I think the code is doing
> exactly the right thing. I also like the way this is split out into a separate
> set of dma_map_ops to simplify the normal MMU case.
> 
> Acked-by: Arnd Bergmann <arnd@...db.de>
> 

Much appreciated!

Vladimir