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

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Date:   Fri, 6 Aug 2021 12:38:02 +0100
From:   Will Deacon <will@...nel.org>
To:     David Stevens <stevensd@...omium.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 Thu, Aug 05, 2021 at 02:26:10PM +0900, David Stevens wrote:
> 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.

Sorry, probably poor wording on my part. What I mean is, rather than add
DMA_ATTR_SKIP_CPU_SYNC here, how about having the callers include it
in attrs instead, since they're the ones doing the initial sync?

Will