+ dma-debug-dont-report-false-positives-with-dma_bounce_unaligned_kmalloc.patch added to mm-hotfixes-unstable branch

+ dma-debug-dont-report-false-positives-with-dma_bounce_unaligned_kmalloc.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: dma-debug: don't report false positives with DMA_BOUNCE_UNALIGNED_KMALLOC
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     dma-debug-dont-report-false-positives-with-dma_bounce_unaligned_kmalloc.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/dma-debug-dont-report-false-positives-with-dma_bounce_unaligned_kmalloc.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Marek Szyprowski <m.szyprowski@samsung.com>
Subject: dma-debug: don't report false positives with DMA_BOUNCE_UNALIGNED_KMALLOC
Date: Thu, 9 Oct 2025 16:15:08 +0200

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.

Link: https://lkml.kernel.org/r/20251009141508.2342138-1-m.szyprowski@samsung.com
Fixes: 370645f41e6e ("dma-mapping: force bouncing if the kmalloc() size is not cache-line-aligned")
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Inki Dae <m.szyprowski@samsung.com>
Cc: Robin Murohy <robin.murphy@arm.com>
Cc: "Isaac J. Manjarres" <isaacmanjarres@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 kernel/dma/debug.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/kernel/dma/debug.c~dma-debug-dont-report-false-positives-with-dma_bounce_unaligned_kmalloc
+++ a/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_deb
 	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");
 	}
_

Patches currently in -mm which might be from m.szyprowski@samsung.com are

dma-debug-dont-report-false-positives-with-dma_bounce_unaligned_kmalloc.patch