RE: [PATCH v3 8/9] cxl/pci: Extend devm_cxl_port_enumerate_dports() to support restricted hosts (RCH)

[Dan Williams]

Robert Richter wrote:
> The PCIe Software View of an RCH and RCD is different to VH mode. An
> RCD is paired with an RCH and shows up as RCiEP. Its downstream and
> upstream ports are hidden to the PCI hierarchy. This different PCI
> topology requires a different handling of RCHs.
> 
> Extend devm_cxl_port_enumerate_dports() to support restricted hosts
> (RCH). If an RCH is detected, register its port as dport to the
> device. An RCH is found if the host's dev 0 func 0 devices is an RCiEP
> with an existing PCIe DVSEC for CXL Devices (ID 0000h).

It is not clear to me what this extra dport represents. Here are the
Linux CXL objects I see in a VH vs an RCH topology:

               VH
          ┌──────────┐
          │ ACPI0017 │
          │  root0   │
          └─────┬────┘
                │
          ┌─────┴────┐
          │  dport0  │
    ┌─────┤ ACPI0016 ├─────┐
    │     │  port1   │     │
    │     └────┬─────┘     │
    │          │           │
 ┌──┴───┐   ┌──┴───┐   ┌───┴──┐
 │dport0│   │dport1│   │dport2│
 │ RP0  │   │ RP1  │   │ RP2  │
 └──────┘   └──┬───┘   └──────┘
               │
           ┌───┴─────┐
           │endpoint0│
           │  port2  │
           └─────────┘


              RCH
          ┌──────────┐
          │ ACPI0017 │
          │  root0   │
          └────┬─────┘
               │
           ┌───┴────┐
           │ dport0 │
           │ACPI0016│
           └───┬────┘
               │
          ┌────┴─────┐
          │endpoint0 │
          │  port1   │
          └──────────┘

So in the RCH case the only dport is the dport that root0 targets, and
then that dport is directly connected to the RCIEP endpoint-port.

In the VH case another level of dports are needed to route from root0 to
the fan out across the CXL root ports.