linux-kernel - Re: [PATCH v2 1/1] drm: msm: Replace dma_map_sg with dma_sync

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Message-ID: <CAFp+6iFDanGLoWf95CR2Etq76tC8+dpciOcJzou4QfAUO3WRQA@mail.gmail.com>
Date:   Thu, 29 Nov 2018 15:14:21 +0530
From:   Vivek Gautam <vivek.gautam@...eaurora.org>
To:     Rob Clark <robdclark@...il.com>
Cc:     hch@...radead.org, David Airlie <airlied@...ux.ie>,
        Tomasz Figa <tfiga@...omium.org>,
        open list <linux-kernel@...r.kernel.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Jordan Crouse <jcrouse@...eaurora.org>,
        Sean Paul <seanpaul@...omium.org>
Subject: Re: [PATCH v2 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*

On Wed, Nov 28, 2018 at 6:09 PM Rob Clark <robdclark@...il.com> wrote:
>
> On Wed, Nov 28, 2018 at 2:39 AM Christoph Hellwig <hch@...radead.org> wrote:
> >
> > > +             /*
> > > +              * dma_sync_sg_*() flush the physical pages, so point
> > > +              * sg->dma_address to the physical ones for the right behavior.
> > > +              */
> > > +             for_each_sg(msm_obj->sgt->sgl, s, msm_obj->sgt->nents, i)
> > > +                     sg_dma_address(s) = sg_phys(s);
> > > +
> >
> > I'm sorry, but this is completely bogus and not acceptable.
> >
> > The only place that is allowed to initialize sg_dma_address is
> > dma_map_sg.  If the default dma ops don't work for your setup we have
> > major a problem and need to fix the dma api / iommu integration instead
> > of hacking around it.
>
> I agree that the dma/iommu integration is very problematic for drm (in
> particular, gpu drivers that use the iommu as an gpu mmu)..  Really we
> need a way that a driver can opt-out of this, and access the cpu cache
> APIs directly, skipping the dma API entirely.  But as it is, we've had
> to hack around the dma API.  I'm not really sure this hack is any
> worse than abusing dma_(un)map_sg() for doing cache operations.
>
> I probably should have paid more attention and nak'd the dma/iommu
> integration before it landed.  But given that now we are stuck in this
> situation, while I'm certainly interested if anyone has some ideas
> about how to let drivers opt out of the dma/iommu integration and
> bypass the dma API layer, I'm ok with replacing a hack with a less-bad
> hack.

May I take it as a positive nod to respin the next version?

Regards
Vivek

>
> BR,
> -R



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