Re: [PATCH v6 18/18] RDMA/mlx5: Ask P2PDMA whether ATS takes a direct peer-to-peer route

[Thomas Hellström]

On Thu, 2026-09-17 at 11:02 -0300, Jason Gunthorpe wrote:
> On Thu, Sep 17, 2026 at 03:56:12PM +0200, Thomas Hellström wrote:
> 
> > That is, a flag to tell the topology check that some transactions
> > *will* take the host-bridge path due to IOVA being used, and that
> > the
> > computations including pci_p2pdma_distance() need to check whether
> > that
> > is possible (checking whitelist etc.) and return the corresponding
> > THRU_HOST_BRIDGE mapping type. Translated transactions taking a
> > short-
> > cut using the bus-address would then be hidden from the driver.
> 
> I imagined it would work by the driver passing in what modes it
> supports and Intel GPU would simply pass in ATS only.
> 
> Even something like mlx has variation, only some operations can opt
> into ATS, many others are never-ATS.
> 
> Driver should not make statements like " *will* take the host-bridge
> path" since it has no idea.

Yes, agreed, but anyway DMA-BUF would then, as part of the
dynamic_attach negotiation have an interface that takes an ATS only
flag and use it to determine whether P2P is possible at all, and use
them at mapping time to set up the correct SG list (IOVA or BUS
address).

Similarly the dma_map_page() needs to know these flags for p2pdma pages
so that it never sets up a BUS_ADDRESS to an ATS-enabled device that
didn't explicitly ask for it?

And this all (and in addition the previous discussion around pcie
subsystems calls from within dma-buf, and data-types for mapping
information) makes me think that abstracting fast dma-buf interconnects
along with our previous discussions is a good thing. Then interconnect
details and cross-subsystem dependencies can be hidden in the
interconnect implementations, suitably placed in the tree.


Thanks,
Thomas


> 
> Jason