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From: Will Deacon <will@kernel.org>
To: David Stevens <stevensd@chromium.org>
Cc: Robin Murphy <robin.murphy@arm.com>,
	Joerg Roedel <joro@8bytes.org>,
	Lu Baolu <baolu.lu@linux.intel.com>, Tom Murphy <murphyt7@tcd.ie>,
	iommu@lists.linux-foundation.org,
	open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 3/4] dma-iommu: pass SKIP_CPU_SYNC to swiotlb unmap
Date: Fri, 6 Aug 2021 12:38:02 +0100	[thread overview]
Message-ID: <20210806113802.GB2531@willie-the-truck> (raw)
In-Reply-To: <CAD=HUj4+62dYZTWfbPjh8eLRY6FQak8nBS8OD85t0xk_+JvDpA@mail.gmail.com>

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@kernel.org> wrote:
> >
> > On Fri, Jul 09, 2021 at 12:35:01PM +0900, David Stevens wrote:
> > > From: David Stevens <stevensd@chromium.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@chromium.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

  reply	other threads:[~2021-08-06 11:38 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-09  3:34 [PATCH v2 0/4] Fixes for dma-iommu swiotlb bounce buffers David Stevens
2021-07-09  3:34 ` [PATCH v2 1/4] dma-iommu: fix sync_sg with swiotlb David Stevens
2021-08-02 13:30   ` Will Deacon
2021-08-05  3:40     ` David Stevens
2021-07-09  3:35 ` [PATCH v2 2/4] dma-iommu: fix arch_sync_dma for map " David Stevens
2021-08-02 13:40   ` Will Deacon
2021-08-02 13:50     ` Will Deacon
2021-07-09  3:35 ` [PATCH v2 3/4] dma-iommu: pass SKIP_CPU_SYNC to swiotlb unmap David Stevens
2021-08-02 13:54   ` Will Deacon
2021-08-05  5:26     ` David Stevens
2021-08-06 11:38       ` Will Deacon [this message]
2021-07-09  3:35 ` [PATCH v2 4/4] dma-iommu: Check CONFIG_SWIOTLB more broadly David Stevens

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