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

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Message-ID: <c43c630e-cd3c-43bf-b390-659c23f7c711@samsung.com>
Date: Thu, 8 Jan 2026 14:57:48 +0100
From: Marek Szyprowski <m.szyprowski@...sung.com>
To: "Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org
Cc: Cong Wang <xiyou.wangcong@...il.com>, Jonathan Corbet <corbet@....net>,
	Olivia Mackall <olivia@...enic.com>, Herbert Xu
	<herbert@...dor.apana.org.au>, Jason Wang <jasowang@...hat.com>, Paolo
	Bonzini <pbonzini@...hat.com>, Stefan Hajnoczi <stefanha@...hat.com>,
	Eugenio Pérez <eperezma@...hat.com>, "James E.J. Bottomley"
	<James.Bottomley@...senpartnership.com>, "Martin K. Petersen"
	<martin.petersen@...cle.com>, Gerd Hoffmann <kraxel@...hat.com>, Xuan Zhuo
	<xuanzhuo@...ux.alibaba.com>, Robin Murphy <robin.murphy@....com>, Stefano
	Garzarella <sgarzare@...hat.com>, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo
	Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, Petr Tesarik
	<ptesarik@...e.com>, Leon Romanovsky <leon@...nel.org>, Jason Gunthorpe
	<jgg@...pe.ca>, Bartosz Golaszewski <brgl@...nel.org>,
	linux-doc@...r.kernel.org, linux-crypto@...r.kernel.org,
	virtualization@...ts.linux.dev, linux-scsi@...r.kernel.org,
	iommu@...ts.linux.dev, kvm@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v2 03/15] dma-mapping: add DMA_ATTR_CPU_CACHE_CLEAN

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@...hat.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@...sung.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