linux-kernel - Re: [PATCH 05/10] PCI/P2PDMA: Export pci_p2pdma_map

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Message-ID: <20250727190252.GF7551@nvidia.com>
Date: Sun, 27 Jul 2025 16:02:52 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Christoph Hellwig <hch@....de>
Cc: Leon Romanovsky <leon@...nel.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	Leon Romanovsky <leonro@...dia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Christian König <christian.koenig@....com>,
	dri-devel@...ts.freedesktop.org, iommu@...ts.linux.dev,
	Jens Axboe <axboe@...nel.dk>,
	Jérôme Glisse <jglisse@...hat.com>,
	Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
	linaro-mm-sig@...ts.linaro.org, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
	linux-mm@...ck.org, linux-pci@...r.kernel.org,
	Logan Gunthorpe <logang@...tatee.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Vivek Kasireddy <vivek.kasireddy@...el.com>,
	Will Deacon <will@...nel.org>
Subject: Re: [PATCH 05/10] PCI/P2PDMA: Export pci_p2pdma_map_type() function

On Thu, Jul 24, 2025 at 10:03:13AM +0200, Christoph Hellwig wrote:
> On Wed, Jul 23, 2025 at 04:00:06PM +0300, Leon Romanovsky wrote:
> > From: Leon Romanovsky <leonro@...dia.com>
> > 
> > Export the pci_p2pdma_map_type() function to allow external modules
> > and subsystems to determine the appropriate mapping type for P2PDMA
> > transfers between a provider and target device.
> 
> External modules have no business doing this.

So what's the plan?

Today the new DMA API broadly has the pattern:

        switch (pci_p2pdma_state(p2pdma_state, dev, page)) {
[..]
        if (dma_use_iova(state)) {
                ret = dma_iova_link(dev, state, paddr, offset,
[..]
        } else {
                dma_addr = dma_map_page(dev, page, 0, map->dma_entry_size,
[..]

You can't fully use the new API flow without calling
pci_p2pdma_state(), which is also not exported today.

Is the idea the full new DMA API flow should not be available to
modules? We did export dma_iova_link().

Otherwise, the p2p step needs two functions - a struct page-full and a
struct page-less version, and they need to be exported.

The names here are not so good, it would be nicer to have them be a
dma_* prefixed function since they are used with the other dma_
functions.

Jason