On Tue, Oct 06, 2026 at 04:32:05PM -0500, Bjorn Helgaas wrote: > On Thu, Oct 01, 2026 at 02:55:13PM +0300, Leon Romanovsky wrote: > > From: Leon Romanovsky <[email protected]> > > > > P2PDMA documentation describes ACS controls as path-wide, although Request > > and Completion controls apply to different transaction directions and only > > affect peer-versus-upstream decisions at the path divergence. > > > > Document the fixed client and provider roles, the divergence port checked > > for each TLP direction, and the conservative handling of unreadable ACS > > state. Clarify which controls disable_acs_redir changes. > > > > Reviewed-by: Logan Gunthorpe <[email protected]> > > Tested-by: Tushar Dave <[email protected]> > > Signed-off-by: Leon Romanovsky <[email protected]> > > --- > > Documentation/admin-guide/kernel-parameters.txt | 15 +++++++++------ > > Documentation/driver-api/pci/p2pdma.rst | 13 +++++++++++++ > > 2 files changed, 22 insertions(+), 6 deletions(-) > > > > diff --git a/Documentation/admin-guide/kernel-parameters.txt > > b/Documentation/admin-guide/kernel-parameters.txt > > index 68647ff4bdd2..bc83e07dd5fc 100644 > > --- a/Documentation/admin-guide/kernel-parameters.txt > > +++ b/Documentation/admin-guide/kernel-parameters.txt > > @@ -5291,12 +5291,15 @@ Kernel parameters > > disable_acs_redir=<pci_dev>[; ...] > > Specify one or more PCI devices (in the format > > specified above) separated by semicolons. > > - Each device specified will have the PCI ACS > > - redirect capabilities forced off which will > > - allow P2P traffic between devices through > > - bridges without forcing it upstream. Note: > > - this removes isolation between devices and > > - may put more devices in an IOMMU group. > > + Each device specified will have the PCI ACS P2P > > + Request Redirect, Completion Redirect, and > > Egress > > + Control features forced off. This may allow P2P > > + traffic through bridges that would otherwise be > > + redirected upstream. This may allow P2P traffic > > + through bridges that would otherwise be > > redirected > > + upstream and thus this removes isolation between > > + devices and may cause affected devices to share > > + an IOMMU group. > > config_acs= > > Format: > > <ACS flags>@<pci_dev>[; ...] > > diff --git a/Documentation/driver-api/pci/p2pdma.rst > > b/Documentation/driver-api/pci/p2pdma.rst > > index 80f8fec9b0e9..42b18610bf7d 100644 > > --- a/Documentation/driver-api/pci/p2pdma.rst > > +++ b/Documentation/driver-api/pci/p2pdma.rst > > @@ -15,6 +15,19 @@ then based on the ACS settings the transaction can route > > entirely within > > the PCIe hierarchy and never reach the root port. The kernel will evaluate > > the PCIe topology and always permit P2P in these well-defined cases. > > > > +The client remains the PCIe requester when it reads or writes provider > > memory. > > +Where the paths diverge, the kernel therefore evaluates P2P Request > > Redirect > > +and Egress Control on the client-side port, and P2P Completion Redirect on > > the > > +provider-side port for completions from a read. An enabled Egress Control > > is > > +conservatively treated as a Request redirect. > > I think "where the paths diverge" means the point where a port decides > whether to route a TLP up through its Upstream Port (or to the RC) or > back down via a sibling Downstream Port?
Yes. > > I'm not sure p2pdma.rst includes the context to interpret "divergence" > yet. I think the code comments in [4/18] might also need a little > more context about divergence. > > IIUC, this divergence point is basically a static point where the ACS > controls being applied makes a routing difference. Right. Previously, we did not account for ACS bits, so any "junction" automatically caused P2P to be considered unsupported. Thanks > > > +Below the divergence, the route toward the other branch is already > > upstream, > > +so those P2P redirect controls do not affect it. Redirect controls for the > > +reverse transaction directions do not affect the mapping. P2P DMA is routed > > +through the host bridge when either applicable port redirects. If an ACS > > +Control register cannot be read, P2P DMA is rejected because the kernel > > cannot > > +establish a usable route. > > + > > This evaluation assumes clients issue strictly ordered Requests carrying an > > Untranslated address. Its result is not defined when clients use Relaxed > > Ordering or issue ATS-translated Requests because those TLP attributes can > > > > -- > > 2.55.0 > >
