linux-kernel - Re: [PATCH] iommu/dma: add missing support for DMA_ATTR_MMIO for dma_iova

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Message-ID: <0b214028-299b-46aa-8d98-0f789153c612@arm.com>
Date: Mon, 24 Nov 2025 17:15:48 +0000
From: Robin Murphy <robin.murphy@....com>
To: Marek Szyprowski <m.szyprowski@...sung.com>, iommu@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Cc: Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
 Leon Romanovsky <leon@...nel.org>, Jason Gunthorpe <jgg@...pe.ca>
Subject: Re: [PATCH] iommu/dma: add missing support for DMA_ATTR_MMIO for
 dma_iova_unlink()

On 2025-11-24 5:09 pm, Marek Szyprowski wrote:
> Commit c288d657dd51 added support for DMA_ATTR_MMIO attribute in the
> dma_iova_link() code path, but missed that the CPU cache is being also
> touched in the dma_iova_unlink() path. Fix this.
> 
> Fixes: c288d657dd51 ("iommu/dma: implement DMA_ATTR_MMIO for dma_iova_link().")
> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
> ---
>   drivers/iommu/dma-iommu.c | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index e52d19d2e833..c92088855450 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -2008,7 +2008,7 @@ static void iommu_dma_iova_unlink_range_slow(struct device *dev,
>   			end - addr, iovad->granule - iova_start_pad);
>   
>   		if (!dev_is_dma_coherent(dev) &&
> -		    !(attrs & DMA_ATTR_SKIP_CPU_SYNC))
> +		    !(attrs & (DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_MMIO)))
>   			arch_sync_dma_for_cpu(phys, len, dir);
>   
>   		swiotlb_tbl_unmap_single(dev, phys, len, dir, attrs);
> @@ -2032,7 +2032,8 @@ static void __iommu_dma_iova_unlink(struct device *dev,
>   	size_t unmapped;
>   
>   	if ((state->__size & DMA_IOVA_USE_SWIOTLB) ||
> -	    (!dev_is_dma_coherent(dev) && !(attrs & DMA_ATTR_SKIP_CPU_SYNC)))
> +	    (!dev_is_dma_coherent(dev) &&
> +	     !(attrs & (DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_MMIO))))

This isn't needed, since there's no way an MMIO mapping could have used 
SWIOTLB.

Thanks,
Robin.

>   		iommu_dma_iova_unlink_range_slow(dev, addr, size, dir, attrs);
>   
>   	iommu_iotlb_gather_init(&iotlb_gather);