linux-kernel - [PATCH] dma-direct: Skip cache prep for HighMem coherent allocations

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Message-ID: <20260102155118.2551804-1-aneesh.kumar@kernel.org>
Date: Fri,  2 Jan 2026 21:21:18 +0530
From: "Aneesh Kumar K.V (Arm)" <aneesh.kumar@...nel.org>
To: iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Cc: Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Arnd Bergmann <arnd@...nel.org>,
	Linus Walleij <linusw@...nel.org>,
	Matthew Wilcox <willy@...radead.org>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	"Aneesh Kumar K.V (Arm)" <aneesh.kumar@...nel.org>
Subject: [PATCH] dma-direct: Skip cache prep for HighMem coherent allocations

dma_direct_alloc() calls arch_dma_prep_coherent() to clean any dirty
cache lines from the kernel linear alias before creating a coherent
remapping.

HighMem pages have no kernel alias mapping, so there are no alias cache
lines to clean. Skip arch_dma_prep_coherent() for HighMem allocations.

Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@...nel.org>
---
 kernel/dma/direct.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 50c3fe2a1d55..ffa267020a1e 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -272,7 +272,8 @@ void *dma_direct_alloc(struct device *dev, size_t size,
 			prot = pgprot_decrypted(prot);
 
 		/* remove any dirty cache lines on the kernel alias */
-		arch_dma_prep_coherent(page, size);
+		if (!PageHighMem(page))
+			arch_dma_prep_coherent(page, size);
 
 		/* create a coherent mapping */
 		ret = dma_common_contiguous_remap(page, size, prot,
-- 
2.43.0