linux-kernel - Re: [PATCH v4 00/16] dma-mapping: migrate to physical address-based API

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Message-ID: <20250908124526.GW616306@nvidia.com>
Date: Mon, 8 Sep 2025 09:45:26 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Marek Szyprowski <m.szyprowski@...sung.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 00/16] dma-mapping: migrate to physical address-based
 API

On Sun, Sep 07, 2025 at 11:25:09PM +0900, Takashi Sakamoto wrote:
> Hi,
> 
> I'm a present maintainer of Linux FireWire subsystem, and recent years
> have been working to modernize the subsystem.
> 
> On Fri, Sep 05, 2025 at 14:43:24PM -0300, Jason Gunthorpe wrote:
> > There is only one user I found of alloc_pages:
> >
> > drivers/firewire/ohci.c:                ctx->pages[i] = dma_alloc_pages(dev, PAGE_SIZE, &dma_addr,
> >
> > And it deliberately uses page->private:
> >
> >		set_page_private(ctx->pages[i], dma_addr);
> >
> > So it is correct to use the struct page API.
> 
> I've already realized it, and it is in my TODO list to use modern
> alternative APIs to replace it (but not yet). If you know some
> candidates for this purpose, it is really helpful to accomplish it.

I think for now it is probably OKish, but in the medium/longer term
this probably wants to have its own memdesc like other cases.

Ie instead of using page->private you'd have a

struct ohci_desc {
	unsigned long __page_flags;
	dma_addr_t dma_addr;
[..]
};

And instead of using page->private you'd use ohci_desc::dma_addr.

This would require changing dma_alloc_pages() to be able to allocate
the frozen memdescs..

Which we are not quite there yet, but maybe come back to this in 2026?

Jason