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Date:   Thu, 20 Aug 2020 18:52:13 +0200
From:   Christoph Hellwig <hch@....de>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        alsa-devel@...a-project.org, linux-ia64@...r.kernel.org,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        nouveau@...ts.freedesktop.org, linux-nvme@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        linux-mm@...ck.org, Marek Szyprowski <m.szyprowski@...sung.com>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        Joonyoung Shim <jy0922.shim@...sung.com>,
        linux-scsi@...r.kernel.org,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Matt Porter <mporter@...nel.crashing.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Pawel Osciak <pawel@...iak.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Joerg Roedel <joro@...tes.org>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <linux-arm-kernel@...ts.infradead.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-parisc@...r.kernel.org, netdev@...r.kernel.org,
        Seung-Woo Kim <sw0312.kim@...sung.com>,
        linux-mips@...r.kernel.org
Subject: Re: [PATCH 05/28] media/v4l2: remove
 V4L2-FLAG-MEMORY-NON-CONSISTENT

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.

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