linux-kernel - Re: [PATCH v2 01/16] dma-mapping: introduce new DMA attribute to indicate MMIO memory

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Message-ID: <20250814174333.GA8427@unreal>
Date: Thu, 14 Aug 2025 20:43:33 +0300
From: Leon Romanovsky <leon@...nel.org>
To: Randy Dunlap <rdunlap@...radead.org>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>,
	Jason Gunthorpe <jgg@...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 v2 01/16] dma-mapping: introduce new DMA attribute to
 indicate MMIO memory

On Thu, Aug 14, 2025 at 10:37:22AM -0700, Randy Dunlap wrote:
> Hi Leon,
> 
> On 8/14/25 3:13 AM, Leon Romanovsky wrote:
> > diff --git a/Documentation/core-api/dma-attributes.rst b/Documentation/core-api/dma-attributes.rst
> > index 1887d92e8e92..58a1528a9bb9 100644
> > --- a/Documentation/core-api/dma-attributes.rst
> > +++ b/Documentation/core-api/dma-attributes.rst
> > @@ -130,3 +130,21 @@ accesses to DMA buffers in both privileged "supervisor" and unprivileged
> >  subsystem that the buffer is fully accessible at the elevated privilege
> >  level (and ideally inaccessible or at least read-only at the
> >  lesser-privileged levels).
> > +
> > +DMA_ATTR_MMIO
> > +-------------
> > +
> > +This attribute indicates the physical address is not normal system
> > +memory. It may not be used with kmap*()/phys_to_virt()/phys_to_page()
> > +functions, it may not be cachable, and access using CPU load/store
> 
> Usually "cacheable" (git grep -w cacheable counts 1042 hits vs.
> 55 hits for "cachable"). And the $internet agrees.
> 
> > +instructions may not be allowed.
> > +
> > +Usually this will be used to describe MMIO addresses, or other non
> 
> non-cacheable
> 
> > +cachable register addresses. When DMA mapping this sort of address we
> 
> > +call the operation Peer to Peer as a one device is DMA'ing to another
> > +device. For PCI devices the p2pdma APIs must be used to determine if
> > +DMA_ATTR_MMIO is appropriate.
> > +
> > +For architectures that require cache flushing for DMA coherence
> > +DMA_ATTR_MMIO will not perform any cache flushing. The address
> > +provided must never be mapped cachable into the CPU.
> again.

Thanks, I will fix.

> 
> thanks.
> -- 
> ~Randy
> 
>