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From: Michael Kelley <mhklinux@outlook.com>
To: Robin Murphy <robin.murphy@arm.com>, "joro@8bytes.org" <joro@8bytes.org>
CC: "iommu@lists.linux.dev" <iommu@lists.linux.dev>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"will@kernel.org" <will@kernel.org>, "petr@tesarici.cz" <petr@tesarici.cz>,
	"hch@lst.de" <hch@lst.de>
Subject: RE: [PATCH] iommu/dma: Document min_align_mask assumption
Thread-Topic: [PATCH] iommu/dma: Document min_align_mask assumption
Thread-Index: AQHabixNyEI53EwZYE21bqklpqvnybEnuUvg
Date: Mon, 4 Mar 2024 15:47:24 +0000
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 <dbb4d2d8e5d1691ac9a6c67e9758904e6c447ba5.1709553942.git.robin.murphy@arm.com>
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From: Robin Murphy <robin.murphy@arm.com> Sent: Monday, March 4, 2024 4:06 =
AM
>=20
> iommu-dma does not explicitly reference min_align_mask since we already
> assume that that will be less than or equal to any typical IOVA granule.
> We wouldn't realistically expect to see the case where it is larger, and
> that would be non-trivial to support, however for the sake of reasoning
> (particularly around the interaction with SWIOTLB), let's clearly
> enforce the assumption.
>=20
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> ---
>  drivers/iommu/dma-iommu.c | 5 +++++
>  1 file changed, 5 insertions(+)
>=20
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 50ccc4f1ef81..b58f5a3311c3 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -859,6 +859,11 @@ static dma_addr_t __iommu_dma_map(struct device *dev=
, phys_addr_t phys,
>  	    iommu_deferred_attach(dev, domain))
>  		return DMA_MAPPING_ERROR;
>=20
> +	/* If anyone ever wants this we'd need support in the IOVA allocator */
> +	if (dev_WARN_ONCE(dev, dma_get_min_align_mask(dev) > iova_mask(iovad),
> +	    "Unsupported alignment constraint\n"))
> +		return DMA_MAPPING_ERROR;
> +
>  	size =3D iova_align(iovad, size + iova_off);
>=20
>  	iova =3D iommu_dma_alloc_iova(domain, size, dma_mask, dev);
> --
> 2.39.2.101.g768bb238c484.dirty

Reviewed-by: Michael Kelley <mhklinux@outlook.com>