Re: [PATCH v2 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*

[Rob Clark]

On Wed, Nov 28, 2018 at 2:39 AM Christoph Hellwig <hch@xxxxxxxxxxxxx> 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.

BR,
-R