linux-kernel - Re: [PATCH v3 0/2] [media] videobuf2-dc: Add support for cacheable MMAP

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Message-id: <7505cb31-6bd1-7f76-f975-aa5e61e567f0@samsung.com>
Date:   Thu, 13 Jul 2017 13:13:28 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Thierry Escande <thierry.escande@...labora.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Sakari Ailus <sakari.ailus@....fi>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        Pawel Osciak <pawel@...iak.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Hans Verkuil <hverkuil@...all.nl>,
        Shuah Khan <shuahkh@....samsung.com>,
        Russell King <rmk+kernel@....linux.org.uk>
Subject: Re: [PATCH v3 0/2] [media] videobuf2-dc: Add support for cacheable MMAP

Hi Christoph,

On 2017-07-05 19:33, Christoph Hellwig wrote:
> On Mon, Jul 03, 2017 at 11:27:32AM +0200, Marek Szyprowski wrote:
>> The main question here if we want to merge incomplete solution or not. As
>> for now, there is no support in ARM/ARM64 for NON_CONSISTENT attribute.
>> Also none of the v4l2 drivers use it. Sadly support for NON_CONSISTENT
>> attribute is not fully implemented nor even defined in mainline.
>>
> DMA_ATTR_NON_CONSISTENT is the way to get the dma_alloc_noncoherent
> semantics through the dma_alloc_attr API, and as such I think it is
> pretty well defined, although the documentation in
> Documentation/DMA-attributes.txt is really bad and we need to improve
> it, by merging it with the dma_alloc_noncoherent description in
> Documentation/DMA-API.txt. My series to remove dma_alloc_noncoherent
> updates the latter to mention DMA_ATTR_NON_CONSISTENT, but
> we should probably merge Documentation/DMA-API.txt,
> Documentation/DMA-attributes.txt and Documentation/DMA-API-HOWTO.txt
> into a single coherent document.

Right. I started conversion of dma_alloc_noncoherent to NON_CONSISTENT
DMA attribute, but later I got stuck at the details of cache 
synchronization.

>> I know that it works fine for some vendor kernel trees, but supporting it in
>> mainline was a bit controversial. There is no proper way to sync cache for
>> such
>> buffers. Calling dma_sync_sg worked so far, but it has to be first agreed as
>> a proper DMA API.
> As documented in Documentation/DMA-API.txt the proper way to sync
> noncoherent/nonconsistent regions is to call dma_cache_sync.  It seems
> like it generally is the same as dma_sync_range/sg so if we could
> eventually merge these APIs that should reduce the confusion further.

Original dma_alloc_noncoherent utilized dma_cache_sync() function, which had
some flaws, which prevented me to continue that task and introduce it to ARM
architecture. The dma_alloc_noncoherent() and dma_cache_sync() API lacks
buffer ownership and imprecisely defines how and when the caches has to be
synchronized. dma_cache_sync() also lacks DMA address argument, what also
complicates potential lightweight implementation.

IMHO it would make sense to change it to work similar to the other
dma_sync_*_for_{cpu,device} functions, but I didn't find enough time to
finally take a try.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland