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

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Message-ID: <CAFp+6iHDh3-5+RTikaPz1SMqxCaVMJ86WvPJjpSVp9GjzOfLfQ@mail.gmail.com>
Date:   Wed, 28 Nov 2018 14:32:34 +0530
From:   Vivek Gautam <vivek.gautam@...eaurora.org>
To:     hch@...radead.org
Cc:     David Airlie <airlied@...ux.ie>, Rob Clark <robdclark@...il.com>,
        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>, seanpaul@...omium.org
Subject: Re: [PATCH v2 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*

Hi Christoph ,

On Wed, Nov 28, 2018 at 1:10 PM 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.

Thanks for reviewing this change.
>From what I understand the things in drm, we don't use the default
iommu-dma domain
for drm devices. Rather we allocate a new iommu domain, and therefore we can't
use the default dma ops. Hence we need separate dma_sync_sg*() to
flush/invalidate
the cache. For this we need to initialize the sg's addresses.

Please correct me if my understanding is incomplete.

Best regards
Vivek



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