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From:   Tomasz Figa <tfiga@chromium.org>
Date:   Thu, 20 Aug 2020 19:41:03 +0200
Message-ID: <CAAFQd5BcH-_S=WDvqYvSPxMvQuN5atO8q=xktbMaPS-DOCAYbw@mail.gmail.com>
Subject: Re: [PATCH 05/28] media/v4l2: remove V4L2-FLAG-MEMORY-NON-CONSISTENT
To:     Christoph Hellwig <hch@lst.de>
Cc:     Robin Murphy <robin.murphy@arm.com>, alsa-devel@alsa-project.org,
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On Thu, Aug 20, 2020 at 6:52 PM Christoph Hellwig <hch@lst.de> wrote:
>
> On Thu, Aug 20, 2020 at 12:24:31PM +0200, Tomasz Figa wrote:
> > > Of course this still uses the scatterlist structure with its annoying
> > > mix of input and output parametes, so I'd rather not expose it as
> > > an official API at the DMA layer.
> >
> > The problem with the above open coded approach is that it requires
> > explicit handling of the non-IOMMU and IOMMU cases and this is exactly
> > what we don't want to have in vb2 and what was actually the job of the
> > DMA API to hide. Is the plan to actually move the IOMMU handling out
> > of the DMA API?
> >
> > Do you think we could instead turn it into a dma_alloc_noncoherent()
> > helper, which has similar semantics as dma_alloc_attrs() and handles
> > the various corner cases (e.g. invalidate_kernel_vmap_range and
> > flush_kernel_vmap_range) to achieve the desired functionality without
> > delegating the "hell", as you called it, to the users?
>
> Yes, I guess I could do something in that direction.  At least for
> dma-iommu, which thanks to Robin should be all you'll need in the
> foreseeable future.

That would be really great. Let me know if we can help by testing with
V4L2/vb2 or in any other way.

Best regards,
Tomasz