Re: [PATCH] dma-debug: Skip cacheline tracking for MMIO mappings

[Robin Murphy]

On 13/10/2026 12:44 pm, Leon Romanovsky wrote:
> From: Leon Romanovsky <leonro@xxxxxxxxxx>
>
> Mapping one MMIO dma-buf for many RDMA importers overflows dma-debug's
> cacheline overlap counter and triggers a warning. DMA_ATTR_MMIO mappings
> must never be CPU-cacheable, so CPU cacheline overlap tracking does not
> apply to them.
>
> Skip MMIO entries in both cacheline insertion and removal. Keep them in
> the DMA entry hash so mapping and unmapping checks still run.

Seems fair;

Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>

[...]
> Fixes: 0e74b34dfc33 ("dma-debug: add support for resource mappings")

Although it would be a challenge to backport _that_ far, so more 
realistically I'd probably say:

Fixes: 3aa31a8bb11e ("dma-buf: provide phys_vec to scatter-gather 
mapping routine")

since regular dma_map_resource() users are still never going to hit this 
making their one-off mapping of a UART FIFO or whatever.

Thanks,
Robin.

> Signed-off-by: Leon Romanovsky <leonro@xxxxxxxxxx>
> ---
>   kernel/dma/debug.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
> index 026d50de340e..36c1f7cbb2ae 100644
> --- a/kernel/dma/debug.c
> +++ b/kernel/dma/debug.c
> @@ -489,8 +489,10 @@ static int active_cacheline_insert(struct dma_debug_entry *entry,
>   	/* If the device is not writing memory then we don't have any
>   	 * concerns about the cpu consuming stale data.  This mitigates
>   	 * legitimate usages of overlapping mappings.
> +	 * MMIO mappings are never CPU-cacheable and do not need tracking either.
>   	 */
> -	if (entry->direction == DMA_TO_DEVICE)
> +	if (entry->direction == DMA_TO_DEVICE ||
> +	    entry->attrs & DMA_ATTR_MMIO)
>   		return 0;
>   
>   	spin_lock_irqsave(&radix_lock, flags);
> @@ -519,7 +521,8 @@ static void active_cacheline_remove(struct dma_debug_entry *entry)
>   	unsigned long flags;
>   
>   	/* ...mirror the insert case */
> -	if (entry->direction == DMA_TO_DEVICE)
> +	if (entry->direction == DMA_TO_DEVICE ||
> +	    entry->attrs & DMA_ATTR_MMIO)
>   		return;
>   
>   	spin_lock_irqsave(&radix_lock, flags);
>
> ---
> base-commit: 39d3758108bb0ef9da484062d65288bbe9cada0e
> change-id: 20261013-dma-debug-fixes-5aaf1bb17544
>
> Best regards,
> --
> Leon Romanovsky <leonro@xxxxxxxxxx>