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

From: Vivek Gautam
Date: Wed Nov 28 2018 - 04:02:53 EST

Hi Christoph ,

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

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
the cache. For this we need to initialize the sg's addresses.

Please correct me if my understanding is incomplete.

Best regards

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