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@samsung.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

On Thu, Oct 09, 2025 at 04:15:08PM +0200, Marek Szyprowski wrote:
> @@ -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())) {

Maybe use is_swiotlb_active(entry->dev) instead for completeness. Either
way:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

On 10.10.2025 12:40, Catalin Marinas wrote:
> On Thu, Oct 09, 2025 at 04:15:08PM +0200, Marek Szyprowski wrote:
>> @@ -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())) {
> Maybe use is_swiotlb_active(entry->dev) instead for completeness. Either
> way:
>
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

Thanks for suggestion. is_swiotlb_active() looks a bit more appropriate 
in this context.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland