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

[Marek Szyprowski]

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