linux-kernel - [PATCH] iommu/io-pgtable-arm: Drop DMA API usage for CMOs

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Message-ID: <20260108113846.56179-1-smostafa@google.com>
Date: Thu,  8 Jan 2026 11:38:46 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: iommu@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org
Cc: robin.murphy@....com, will@...nel.org, joro@...tes.org, 
	Mostafa Saleh <smostafa@...gle.com>, Jason Gunthorpe <jgg@...pe.ca>
Subject: [PATCH] iommu/io-pgtable-arm: Drop DMA API usage for CMOs

As part of the KVM SMMUv3 series[1], we are trying to factor out
the kernel specific code from io-pgtable-arm so it can also compile
for the hypervisor.

Jason pointed out that the DMA-API calls are not really needed [2].

Looking more into this. Initially, the io-pgtable API let drivers
do the CMOs using tlb::flush_pgtable() where drivers were using the
DMA API (map/unmap_single) only to do CMOs as the low-level cache
functions won’t be available for modules.

This was later moved to the core code [3], with possibility to
convert it to full blown DMA-API code if there was a use case.

However, no such use case appeared, and we can simplify the code
slightly by doing CMO directly instead of going through the DMA-API
functions just for CMOs.

Although HTTU is not used at the moment, leave the
arch_sync_dma_for_cpu() in __arm_lpae_free_pages() as this is not a
hot path.

Removing the DMA-API will also remove some extra checks ensuring that
the IOMMU can deal with physical addrs. we can add those check at
page table creation time (Something as use_dma_iommu/is_swiotlb_active
However, that seemed a little too much as Linux doesn't support such
topologies anyways.

[1] https://lore.kernel.org/linux-iommu/20251117184815.1027271-1-smostafa@google.com/
[2] https://lore.kernel.org/linux-iommu/20251216005834.GC31492@ziepe.ca/
[3] https://lore.kernel.org/linux-iommu/7c5584d3efa61ee6b0b87efb72f24f32852aafb7.1438195011.git.robin.murphy@arm.com/

Suggested-by: Jason Gunthorpe <jgg@...pe.ca>
Signed-off-by: Mostafa Saleh <smostafa@...gle.com>
---
 drivers/iommu/io-pgtable-arm.c | 39 +++++++---------------------------
 1 file changed, 8 insertions(+), 31 deletions(-)

diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
index e6626004b323..0da9195155ec 100644
--- a/drivers/iommu/io-pgtable-arm.c
+++ b/drivers/iommu/io-pgtable-arm.c
@@ -15,7 +15,7 @@
 #include <linux/sizes.h>
 #include <linux/slab.h>
 #include <linux/types.h>
-#include <linux/dma-mapping.h>
+#include <linux/dma-map-ops.h>
 
 #include <asm/barrier.h>
 
@@ -254,7 +254,6 @@ static void *__arm_lpae_alloc_pages(size_t size, gfp_t gfp,
 {
 	struct device *dev = cfg->iommu_dev;
 	size_t alloc_size;
-	dma_addr_t dma;
 	void *pages;
 
 	/*
@@ -271,32 +270,10 @@ static void *__arm_lpae_alloc_pages(size_t size, gfp_t gfp,
 	if (!pages)
 		return NULL;
 
-	if (!cfg->coherent_walk) {
-		dma = dma_map_single(dev, pages, size, DMA_TO_DEVICE);
-		if (dma_mapping_error(dev, dma))
-			goto out_free;
-		/*
-		 * We depend on the IOMMU being able to work with any physical
-		 * address directly, so if the DMA layer suggests otherwise by
-		 * translating or truncating them, that bodes very badly...
-		 */
-		if (dma != virt_to_phys(pages))
-			goto out_unmap;
-	}
-
+	if (!cfg->coherent_walk)
+		arch_sync_dma_for_device(__arm_lpae_dma_addr(pages), size,
+					 DMA_TO_DEVICE);
 	return pages;
-
-out_unmap:
-	dev_err(dev, "Cannot accommodate DMA translation for IOMMU page tables\n");
-	dma_unmap_single(dev, dma, size, DMA_TO_DEVICE);
-
-out_free:
-	if (cfg->free)
-		cfg->free(cookie, pages, size);
-	else
-		iommu_free_pages(pages);
-
-	return NULL;
 }
 
 static void __arm_lpae_free_pages(void *pages, size_t size,
@@ -304,8 +281,8 @@ static void __arm_lpae_free_pages(void *pages, size_t size,
 				  void *cookie)
 {
 	if (!cfg->coherent_walk)
-		dma_unmap_single(cfg->iommu_dev, __arm_lpae_dma_addr(pages),
-				 size, DMA_TO_DEVICE);
+		arch_sync_dma_for_cpu(__arm_lpae_dma_addr(pages),
+				      size, DMA_TO_DEVICE);
 
 	if (cfg->free)
 		cfg->free(cookie, pages, size);
@@ -316,8 +293,8 @@ static void __arm_lpae_free_pages(void *pages, size_t size,
 static void __arm_lpae_sync_pte(arm_lpae_iopte *ptep, int num_entries,
 				struct io_pgtable_cfg *cfg)
 {
-	dma_sync_single_for_device(cfg->iommu_dev, __arm_lpae_dma_addr(ptep),
-				   sizeof(*ptep) * num_entries, DMA_TO_DEVICE);
+	arch_sync_dma_for_device(__arm_lpae_dma_addr(ptep),
+				 sizeof(*ptep) * num_entries, DMA_TO_DEVICE);
 }
 
 static void __arm_lpae_clear_pte(arm_lpae_iopte *ptep, struct io_pgtable_cfg *cfg, int num_entries)
-- 
2.52.0.351.gbe84eed79e-goog