linux-kernel - Re: [PATCH v4 01/16] dma-mapping: introduce new DMA attribute to indicate MMIO memory

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Message-ID: <20250828130329.GA9469@nvidia.com>
Date: Thu, 28 Aug 2025 10:03:29 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Leon Romanovsky <leon@...nel.org>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>,
	Leon Romanovsky <leonro@...dia.com>,
	Abdiel Janulgue <abdiel.janulgue@...il.com>,
	Alexander Potapenko <glider@...gle.com>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@....de>, Danilo Krummrich <dakr@...nel.org>,
	iommu@...ts.linux.dev, Jason Wang <jasowang@...hat.com>,
	Jens Axboe <axboe@...nel.dk>, Joerg Roedel <joro@...tes.org>,
	Jonathan Corbet <corbet@....net>, Juergen Gross <jgross@...e.com>,
	kasan-dev@...glegroups.com, Keith Busch <kbusch@...nel.org>,
	linux-block@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-nvme@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
	linux-trace-kernel@...r.kernel.org,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Robin Murphy <robin.murphy@....com>, rust-for-linux@...r.kernel.org,
	Sagi Grimberg <sagi@...mberg.me>,
	Stefano Stabellini <sstabellini@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	virtualization@...ts.linux.dev, Will Deacon <will@...nel.org>,
	xen-devel@...ts.xenproject.org
Subject: Re: [PATCH v4 01/16] dma-mapping: introduce new DMA attribute to
 indicate MMIO memory

On Tue, Aug 19, 2025 at 08:36:45PM +0300, Leon Romanovsky wrote:
> From: Leon Romanovsky <leonro@...dia.com>
> 
> This patch introduces the DMA_ATTR_MMIO attribute to mark DMA buffers
> that reside in memory-mapped I/O (MMIO) regions, such as device BARs
> exposed through the host bridge, which are accessible for peer-to-peer
> (P2P) DMA.
> 
> This attribute is especially useful for exporting device memory to other
> devices for DMA without CPU involvement, and avoids unnecessary or
> potentially detrimental CPU cache maintenance calls.
> 
> DMA_ATTR_MMIO is supposed to provide dma_map_resource() functionality
> without need to call to special function and perform branching by
> the callers.

'branching when processing generic containers like bio_vec by the callers'

Many of the existing dma_map_resource() users already know the thing
is MMIO and don't have branching..

> Signed-off-by: Leon Romanovsky <leonro@...dia.com>
> ---
>  Documentation/core-api/dma-attributes.rst | 18 ++++++++++++++++++
>  include/linux/dma-mapping.h               | 20 ++++++++++++++++++++
>  include/trace/events/dma.h                |  3 ++-
>  rust/kernel/dma.rs                        |  3 +++
>  4 files changed, 43 insertions(+), 1 deletion(-)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason