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

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Message-Id: <dbb4d2d8e5d1691ac9a6c67e9758904e6c447ba5.1709553942.git.robin.murphy@arm.com>
Date: Mon,  4 Mar 2024 12:05:42 +0000
From: Robin Murphy <robin.murphy@....com>
To: joro@...tes.org
Cc: iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	will@...nel.org,
	mhklinux@...look.com,
	petr@...arici.cz,
	hch@....de
Subject: [PATCH] iommu/dma: Document min_align_mask assumption

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>
---
 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);
-- 
2.39.2.101.g768bb238c484.dirty