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

From: Christoph Hellwig
Date: Wed Jul 05 2017 - 13:33:36 EST

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.

> 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.