linux-kernel - Re: [RFC PATCH 01/12] dma-buf: Introduce dma_buf_get_pfn

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Message-ID: <20250115093234.GB6805@lst.de>
Date: Wed, 15 Jan 2025 10:32:34 +0100
From: Christoph Hellwig <hch@....de>
To: Jason Gunthorpe <jgg@...dia.com>, Xu Yilun <yilun.xu@...ux.intel.com>,
	Christian König <christian.koenig@....com>,
	Christoph Hellwig <hch@....de>, Leon Romanovsky <leonro@...dia.com>,
	kvm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
	sumit.semwal@...aro.org, pbonzini@...hat.com, seanjc@...gle.com,
	alex.williamson@...hat.com, vivek.kasireddy@...el.com,
	dan.j.williams@...el.com, aik@....com, yilun.xu@...el.com,
	linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org,
	lukas@...ner.de, yan.y.zhao@...el.com, leon@...nel.org,
	baolu.lu@...ux.intel.com, zhenzhong.duan@...el.com,
	tao1.su@...el.com
Subject: Re: [RFC PATCH 01/12] dma-buf: Introduce
 dma_buf_get_pfn_unlocked() kAPI

On Wed, Jan 15, 2025 at 09:55:29AM +0100, Simona Vetter wrote:
> I think for 90% of exporters pfn would fit, but there's some really funny
> ones where you cannot get a cpu pfn by design. So we need to keep the
> pfn-less interfaces around. But ideally for the pfn-capable exporters we'd
> have helpers/common code that just implements all the other interfaces.

There is no way to have dma address without a PFN in Linux right now.
How would you generate them?  That implies you have an IOMMU that can
generate IOVAs for something that doesn't have a physical address at
all.

Or do you mean some that don't have pages associated with them, and
thus have pfn_valid fail on them?  They still have a PFN, just not
one that is valid to use in most of the Linux MM.