netdev - Re: [PATCH 05/28] media/v4l2: remove V4L2-FLAG-MEMORY-NON-CONSISTENT

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Message-ID: <20200820045201.GB4570@lst.de>
Date:   Thu, 20 Aug 2020 06:52:01 +0200
From:   Christoph Hellwig <hch@....de>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>, 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 Wed, Aug 19, 2020 at 04:22:29PM +0200, Tomasz Figa wrote:
> > > FWIW, I asked back in time what the plan is for non-coherent
> > > allocations and it seemed like DMA_ATTR_NON_CONSISTENT and
> > > dma_sync_*() was supposed to be the right thing to go with. [2] The
> > > same thread also explains why dma_alloc_pages() isn't suitable for the
> > > users of dma_alloc_attrs() and DMA_ATTR_NON_CONSISTENT.
> >
> > AFAICS even back then Christoph was implying getting rid of
> > NON_CONSISTENT and *replacing* it with something streaming-API-based -
> 
> That's not how I read his reply from the thread I pointed to, but that
> might of course be my misunderstanding.

Yes.  Without changes like in this series just calling dma_sync_single_*
will break in various cases, e.g. because dma_alloc_attrs returns
memory remapped in the vmalloc space, and the dma_sync_single_*
implementation implementation can't cope with vmalloc addresses.