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

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Mon, 2026-09-21 at 10:10 -0300, Jason Gunthorpe wrote:
> On Mon, Sep 21, 2026 at 08:40:41AM +0200, Thomas Hellström wrote:
> > On Fri, 2026-09-18 at 14:05 -0300, Jason Gunthorpe wrote:
> > > On Fri, Sep 18, 2026 at 03:42:28PM +0200, Thomas Hellström wrote:
> > > > 
> > > > 1) Xe attachment check if pci_p2pdma_distance() returns OK for
> > > > the
> > > > path. Then Xe always sets up dma-addresses using
> > > > dma_map_resource(). 
> > > 
> > > Open coding pci_p2pdma_distance() in drivers is a hack. Using
> > > dma_map_resource() like this was never "allowed".
> > > 
> > > We've fixed things so these hacks are not needed, the drivers
> > > need to
> > > move over to things like dma_buf_phys_vec_to_sgt() and the hmm
> > > helpers
> > > to use the DMA API correctly.
> > > 
> > > > 1a) Let's say the importer (Xe) has ATS enabled, and the
> > > > exporter
> > > > is
> > > > supposed to map dma_addresses.
> > > > In between there is a switch that allows direct traffic.
> > > > Now with your patch series, How would the exporter know that Xe
> > > > has
> > > > ATS
> > > > enabled and therefore should return IOVA mappings rather than
> > > > the
> > > > BUS
> > > > addresses representing the shortcut.
> > > 
> > > The thing that does the DMA map must be aware of what PCI TLPs
> > > the
> > > PCIe initiator will use to compute the proper P2P mapping mode.
> > > 
> > > ATS always needs an IOVA but that still doesn't mean the P2P will
> > > work.
> > > 
> > > > It seems to me that a pci-device settable flag "ATS always
> > > > enabled"
> > > > should be enough to fix both issues?
> > > 
> > > It should be be per-mapping to support the NIC workflow that
> > > isn't a
> > > global operation.
> > 
> > OK. It looked to me from the series like the NIC workflow was to
> > adapt
> > its ATS settings per mapping depending on the p2p mapping type
> > returned
> > rather than vice versa. If so it would just clear "ATS always
> > enabled".
> > 
> > Admittedly a per-mapping flag would be more flexible, but also
> > would
> > require changed internal APIs?
> 
> Ah you didn't send this to the list, was it intentional?

No, Just hit the wrong reply-button. +lists again.

> 
> mlx5 has both cases, some DMA is flexable and can do either, some is
> non-ATS only.

OK.

> 
> Jason

Thanks,
Thomas