linux-kernel - Re: [PATCH v2 3/4] dma-iommu: pass SKIP_CPU

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Date:   Thu, 5 Aug 2021 14:26:10 +0900
From:   David Stevens <stevensd@...omium.org>
To:     Will Deacon <will@...nel.org>
Cc:     Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Tom Murphy <murphyt7@....ie>, iommu@...ts.linux-foundation.org,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/4] dma-iommu: pass SKIP_CPU_SYNC to swiotlb unmap

On Mon, Aug 2, 2021 at 10:54 PM Will Deacon <will@...nel.org> wrote:
>
> On Fri, Jul 09, 2021 at 12:35:01PM +0900, David Stevens wrote:
> > From: David Stevens <stevensd@...omium.org>
> >
> > If SKIP_CPU_SYNC isn't already set, then iommu_dma_unmap_(page|sg) has
> > already called iommu_dma_sync_(single|sg)_for_cpu, so there is no need
> > to copy from the bounce buffer again.
> >
> > Signed-off-by: David Stevens <stevensd@...omium.org>
> > ---
> >  drivers/iommu/dma-iommu.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> > index e79e274d2dc5..0a9a9a343e64 100644
> > --- a/drivers/iommu/dma-iommu.c
> > +++ b/drivers/iommu/dma-iommu.c
> > @@ -505,7 +505,8 @@ static void __iommu_dma_unmap_swiotlb(struct device *dev, dma_addr_t dma_addr,
> >       __iommu_dma_unmap(dev, dma_addr, size);
> >
> >       if (unlikely(is_swiotlb_buffer(phys)))
> > -             swiotlb_tbl_unmap_single(dev, phys, size, dir, attrs);
> > +             swiotlb_tbl_unmap_single(dev, phys, size, dir,
> > +                                      attrs | DMA_ATTR_SKIP_CPU_SYNC);
> >  }
>
> I think it would be cleaner to drop DMA_ATTR_SKIP_CPU_SYNC in the callers
> once they've called iommu_dma_sync_*_for_cpu().

Dropping that flag in iommu_dma_unmap_* would result in always copying
from the swiotlb here, which is the opposite direction of what this
patch is trying to do.

This change is aiming to address the case where DMA_ATTR_SKIP_CPU_SYNC
isn't passed to dma_unmap_*. In that case, there are calls to
swiotlb_sync_single_for_cpu from iommu_dma_sync_*_for_cpu, and calls
to swiotlb_tlb_unmap_single here. That means we copy from the swiotlb
twice. Adding the DMA_ATTR_SKIP_CPU_SYNC flag here skips the second
copy.

-David

> Will