linux-kernel - Re: [PATCH] iommu/dma: Document min_align

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Message-ID: <20240304153938.1fa984b7@meshulam.tesarici.cz>
Date: Mon, 4 Mar 2024 15:39:38 +0100
From: Petr Tesařík <petr@...arici.cz>
To: Robin Murphy <robin.murphy@....com>
Cc: joro@...tes.org, iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
 will@...nel.org, mhklinux@...look.com, hch@....de
Subject: Re: [PATCH] iommu/dma: Document min_align_mask assumption

On Mon,  4 Mar 2024 12:05:42 +0000
Robin Murphy <robin.murphy@....com> wrote:

> 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.
> 
> Signed-off-by: Robin Murphy <robin.murphy@....com>

Looks good to me.

Reviewed-by: Petr Tesarik <petr.tesarik1@...wei-partners.com>

Thank you!

Petr T

> ---
>  drivers/iommu/dma-iommu.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> 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;
>  
> +	/* 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 = iova_align(iovad, size + iova_off);
>  
>  	iova = iommu_dma_alloc_iova(domain, size, dma_mask, dev);