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Date:   Wed, 11 Aug 2021 07:07:25 +0100
From:   Christoph Hellwig <hch@infradead.org>
To:     David Stevens <stevensd@chromium.org>
Cc:     Robin Murphy <robin.murphy@arm.com>, Will Deacon <will@kernel.org>,
        linux-kernel@vger.kernel.org, Tom Murphy <murphyt7@tcd.ie>,
        iommu@lists.linux-foundation.org
Subject: Re: [PATCH v3 3/5] dma-iommu: add SKIP_CPU_SYNC after syncing
Message-ID: <YRNpHcskc0Vv1ofa@infradead.org>
References: <20210811024247.1144246-1-stevensd@google.com>
 <20210811024247.1144246-4-stevensd@google.com>
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> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 4f0cc4a0a61f..be0214b1455c 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -859,8 +859,11 @@ static dma_addr_t iommu_dma_map_page(struct device *dev, struct page *page,
>  static void iommu_dma_unmap_page(struct device *dev, dma_addr_t dma_handle,
>  		size_t size, enum dma_data_direction dir, unsigned long attrs)
>  {
> -	if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC))
> +	if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC)) {
>  		iommu_dma_sync_single_for_cpu(dev, dma_handle, size, dir);
> +		attrs |= DMA_ATTR_SKIP_CPU_SYNC;
> +	}
> +
>  	__iommu_dma_unmap_swiotlb(dev, dma_handle, size, dir, attrs);

I don't think this is the correct way to go.  Instead just call the
raw cache sync helper instead of iommu_dma_sync_single_for_cpu, similar
to what dma-direct does.  Same for the map side.