Re: [RFC PATCH 01/12] dma-buf: Introduce dma_buf_get_pfn_unlocked() kAPI

[Christoph Hellwig]

On Wed, Jan 15, 2025 at 09:34:19AM -0400, Jason Gunthorpe wrote:
> > 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.
> 
> He is talking about private interconnect hidden inside clusters of
> devices.
> 
> Ie the system may have many GPUs and those GPUs have their own private
> interconnect between them. It is not PCI, and packets don't transit
> through the CPU SOC at all, so the IOMMU is not involved.
> 
> DMA can happen on that private interconnect, but from a Linux
> perspective it is not DMA API DMA, and the addresses used to describe
> it are not part of the CPU address space. The initiating device will
> have a way to choose which path the DMA goes through when setting up
> the DMA.

So how is this in any way relevant to dma_buf which operates on
a dma_addr_t right now and thus by definition can't be used for
these?