linux-kernel - Re: [PATCH v5 4/9] PCI/P2PDMA: Export pci_p2pdma_map

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Message-ID: <20251020131417.GN316284@nvidia.com>
Date: Mon, 20 Oct 2025 10:14:17 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Christoph Hellwig <hch@...radead.org>
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>, 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 v5 4/9] PCI/P2PDMA: Export pci_p2pdma_map_type() function

On Mon, Oct 20, 2025 at 05:29:06AM -0700, Christoph Hellwig wrote:
> On Fri, Oct 17, 2025 at 09:14:47AM -0300, Jason Gunthorpe wrote:
> > On Thu, Oct 16, 2025 at 11:31:53PM -0700, Christoph Hellwig wrote:
> > > 
> > > Nacked-by: Christoph Hellwig <hch@....de>
> > > 
> > > As explained to you multiple times, pci_p2pdma_map_type is a low-level
> > > helper that absolutely MUST be wrapper in proper accessors. 
> > 
> > You never responded to the discussion:
> > 
> > https://lore.kernel.org/all/20250727190252.GF7551@nvidia.com/
> > 
> > What is the plan here? Is the new DMA API unusable by modules? That
> > seems a little challenging.
> 
> Yes.  These are only intended to be wrapped by subsystems.

Sure, but many subsystems are fully modular too.. RDMA for example.

Well, lets see what comes in the future..

> Yes, that sounds much better.  And dmabuf in general could use some
> deduplicating of their dma mapping patterns (and eventual fixing).

Yes, it certainly could, but I wanted to tackle this later..

I think adding some 'dmabuf create a map for this list of phys on this
provider' is a good simplifed primitive. Simple drivers like VFIO that
only want to expose MMIO can just call it directly.

Once this is settled I want to have RDMA wrap some of its MMIO VMAs in
DMABUF as well, so I can see at least two users of the helper.

Jason