linux-kernel - [PATCH] dma-debug: don't report false positives with DMA_BOUNCE_UNALIGNED

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Message-Id: <20251009141508.2342138-1-m.szyprowski@samsung.com>
Date: Thu,  9 Oct 2025 16:15:08 +0200
From: Marek Szyprowski <m.szyprowski@...sung.com>
To: iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Cc: Marek Szyprowski <m.szyprowski@...sung.com>, Robin Murphy
	<robin.murphy@....com>, Christoph Hellwig <hch@....de>, Andrew Morton
	<akpm@...ux-foundation.org>, Catalin Marinas <catalin.marinas@....com>
Subject: [PATCH] dma-debug: don't report false positives with
 DMA_BOUNCE_UNALIGNED_KMALLOC

Commit 370645f41e6e ("dma-mapping: force bouncing if the kmalloc() size
is not cache-line-aligned") introduced DMA_BOUNCE_UNALIGNED_KMALLOC
feature and lets architecture specific code to configure kmalloc slabs
with sizes smaller than the value of dma_get_cache_alignment().

When that feature is enabled, the physical address of some small
kmalloc()-ed buffers might be not aligned to the CPU cachelines, thus not
really suitable for typical DMA. To properly handle that case a SWIOTLB
buffer bouncing is used, so no CPU cache corruption occurs. When that
happens, there is no point reporting a false-positive DMA-API warning
that the buffer is not properly aligned, as this is not a client driver
fault.

Fixes: 370645f41e6e ("dma-mapping: force bouncing if the kmalloc() size is not cache-line-aligned")
Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
---
 kernel/dma/debug.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
index b82399437db0..266598d837c5 100644
--- a/kernel/dma/debug.c
+++ b/kernel/dma/debug.c
@@ -23,6 +23,7 @@
 #include <linux/ctype.h>
 #include <linux/list.h>
 #include <linux/slab.h>
+#include <linux/swiotlb.h>
 #include <asm/sections.h>
 #include "debug.h"

@@ -594,7 +595,9 @@ static void add_dma_entry(struct dma_debug_entry *entry, unsigned long attrs)
 	if (rc == -ENOMEM) {
 		pr_err_once("cacheline tracking ENOMEM, dma-debug disabled\n");
 		global_disable = true;
-	} else if (rc == -EEXIST && !(attrs & DMA_ATTR_SKIP_CPU_SYNC)) {
+	} else if (rc == -EEXIST && !(attrs & DMA_ATTR_SKIP_CPU_SYNC) &&
+		   !(IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) &&
+		     is_swiotlb_allocated())) {
 		err_printk(entry->dev, entry,
 			"cacheline tracking EEXIST, overlapping mappings aren't supported\n");
 	}
-- 
2.34.1