linux-kernel - Re: [PATCH v1 00/17] Provide a new two step DMA mapping API

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Message-ID: <20241108152956.GA12130@lst.de>
Date: Fri, 8 Nov 2024 16:29:56 +0100
From: Christoph Hellwig <hch@....de>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Christoph Hellwig <hch@....de>, Robin Murphy <robin.murphy@....com>,
	Leon Romanovsky <leon@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Sagi Grimberg <sagi@...mberg.me>, Keith Busch <kbusch@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Logan Gunthorpe <logang@...tatee.com>,
	Yishai Hadas <yishaih@...dia.com>,
	Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
	Kevin Tian <kevin.tian@...el.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
	linux-rdma@...r.kernel.org, iommu@...ts.linux.dev,
	linux-nvme@...ts.infradead.org, linux-pci@...r.kernel.org,
	kvm@...r.kernel.org, linux-mm@...ck.org, matthew.brost@...el.com,
	Thomas.Hellstrom@...ux.intel.com, brian.welty@...el.com,
	himal.prasad.ghimiray@...el.com, krishnaiah.bommu@...el.com,
	niranjana.vishwanathapura@...el.com
Subject: Re: [PATCH v1 00/17] Provide a new two step DMA mapping API

On Fri, Nov 08, 2024 at 11:25:37AM -0400, Jason Gunthorpe wrote:
> I'm asking how it will work if you change the struct page argument to
> physical, because today dma_direct_map_page() has:
> 
> 		if (is_pci_p2pdma_page(page))
> 			return DMA_MAPPING_ERROR;
> 
> Which is exactly the sorts of things I'm looking at when when I say to
> get rid of struct page.

It will have to look up the page from the physical address obviously.
But at least only in the error path.

> What I'm thinking about is replacing code like the above with something like:
> 
> 		if (p2p_provider)
> 			return DMA_MAPPING_ERROR;
> 
> And the caller is the one that would have done is_pci_p2pdma_page()
> and either passes p2p_provider=NULL or page->pgmap->p2p_provider.

And where do you get that one from?