Re: [PATCH v2 03/15] dma-mapping: add DMA_ATTR_CPU_CACHE_CLEAN

Marek Szyprowski
2026-01-08T13:57:54Z

On 05.01.2026 09:23, Michael S. Tsirkin wrote:
> When multiple small DMA_FROM_DEVICE or DMA_BIDIRECTIONAL buffers share a
> cacheline, and DMA_API_DEBUG is enabled, we get this warning:
> cacheline tracking EEXIST, overlapping mappings aren't supported.
>
> This is because when one of the mappings is removed, while another one
> is active, CPU might write into the buffer.
>
> Add an attribute for the driver to promise not to do this, making the
> overlapping safe, and suppressing the warning.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

It is somehow similar to DMA_ATTR_SKIP_CPU_SYNC in its concept, so I see
no reason not to accept it.

Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>

> ---
> include/linux/dma-mapping.h | 7 +++++++
> kernel/dma/debug.c | 3 ++-
> 2 files changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 29ad2ce700f0..29973baa0581 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -79,6 +79,13 @@
> */
> #define DMA_ATTR_MMIO (1UL << 10)
>
> +/*
> + * DMA_ATTR_CPU_CACHE_CLEAN: Indicates the CPU will not dirty any cacheline
> + * overlapping this buffer while it is mapped for DMA. All mappings sharing
> + * a cacheline must have this attribute for this to be considered safe.
> + */
> +#define DMA_ATTR_CPU_CACHE_CLEAN (1UL << 11)
> +
> /*
> * A dma_addr_t can hold any valid DMA or bus address for the platform. It can
> * be given to a device to use as a DMA source or target. It is specific to a
> diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
> index 138ede653de4..7e66d863d573 100644
> --- a/kernel/dma/debug.c
> +++ b/kernel/dma/debug.c
> @@ -595,7 +595,8 @@ 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 | DMA_ATTR_CPU_CACHE_CLEAN)) &&
> !(IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) &&
> is_swiotlb_active(entry->dev))) {
> err_printk(entry->dev, entry,

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