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

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Thu, Jul 13, 2017 at 01:13:28PM +0200, Marek Szyprowski wrote:
> 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.

My conclusion of the dma_alloc_noncoherent() and dma_cache_sync() API
when it was introduced is that it's basically a completely broken
interface, and I've never seen any point to it.  Maybe some of that is
because it's badly documented - which in turn makes it badly designed
(because there's no specification detailing what it's supposed to be
doing.)

I'd like to see that thing die...

-- 
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