Re: [PATCH v9 10/18] PCI/P2PDMA: Route Translated Requests under Direct Translated P2P

[Leon Romanovsky <leon@kernel.org>]

On Tue, Oct 06, 2026 at 05:28:45PM -0500, Bjorn Helgaas wrote:
> On Thu, Oct 01, 2026 at 02:55:18PM +0300, Leon Romanovsky wrote:
> > From: Leon Romanovsky <leonro@nvidia.com>
> > 
> > A Downstream Port with ACS Direct Translated P2P enabled routes a Request
> > whose Address Type is Translated "to the peer Egress Port without
> > redirection, regardless of ACS P2P Request Redirect and ACS P2P Egress
> > Control", per PCIe r7.0 sec 6.12.3. P2PDMA assumes every Request carries an
> > Untranslated address, so it sends an ATS client through the host bridge
> > even where the fabric would route it straight to the peer.
> 
> "sends an ATS client through the host bridge" -- I assume this really
> means "we told the caller that Requests would be routed through the
> host bridge" when in reality they wouldn't?  I don't think this
> actually changes any routing in the fabric, does it?

I'd say it does. Before this patch, we didn't return `bus_address` here nor
took the P2P path.

Thanks

> 
> So essentially we told the caller that P2P between A and B was, e.g.,
> 5 hops when it was really only 2?
> 
> > Add PCI_P2PDMA_TLP_TRANSLATED and consult Direct Translated P2P for the
> > Requests it describes.
> > 
> > Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
> > Tested-by: Tushar Dave <tdave@nvidia.com>
> > Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> > ---
> >  drivers/pci/p2pdma.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> > index 569a74de3b3a..3fd2cb8d16f0 100644
> > --- a/drivers/pci/p2pdma.c
> > +++ b/drivers/pci/p2pdma.c
> > @@ -548,6 +548,15 @@ pci_acs_p2pdma_request(u16 ctrl, unsigned int tlp_flags)
> >  		 */
> >  		if (ctrl & PCI_ACS_TB)
> >  			return PCI_ACS_P2PDMA_BLOCKED;
> > +
> > +		/*
> > +		 * PCIe r7.0 sec 6.12.3: ACS Direct Translated P2P routes a
> > +		 * Request carrying a Translated address to the peer "without
> > +		 * redirection, regardless of ACS P2P Request Redirect and ACS
> > +		 * P2P Egress Control settings".
> > +		 */
> > +		if (ctrl & PCI_ACS_DT)
> > +			return PCI_ACS_P2PDMA_DIRECT;
> >  	}
> >  
> >  	return ctrl & (PCI_ACS_RR | PCI_ACS_EC) ?
> > 
> > -- 
> > 2.55.0
> >