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? 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. > +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 >
