Re: [PATCH v9 05/18] PCI/P2PDMA: Document directional ACS routing

[Bjorn Helgaas]

On Thu, Oct 01, 2026 at 02:55:13PM +0300, Leon Romanovsky wrote:
> From: Leon Romanovsky <leonro@xxxxxxxxxx>
> 
> 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 <logang@xxxxxxxxxxxx>
> Tested-by: Tushar Dave <tdave@xxxxxxxxxx>
> Signed-off-by: Leon Romanovsky <leonro@xxxxxxxxxx>
> ---
>  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
>