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

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Date:   Mon, 2 Aug 2021 14:54:46 +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,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/4] dma-iommu: pass SKIP_CPU_SYNC to swiotlb unmap

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().

Will