RE: [PATCH v3 3/9] cxl/mem: Adjust cxl_mem_find_port() to find an RCH's port

[Dan Williams <dan.j.williams@intel.com>]

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 with a parent already
> pointing to a PCI bridge (struct pci_host_bridge). In contrast, in VH
> mode an PCI Express Endpoint is a PCI type 0 device with a PCI type 1
> device as parent (struct pci_dev, most of the time a downstream switch
> port, but could also be a root port). The following hierarchy applies
> in VH mode:
> 
>  CXL memory device, cxl_memdev                               endpoint
>  └──PCIe Endpoint (type 0), pci_dev                           |
>     └──Downstream Port (type 1), pci_dev (Nth switch)        portN
>        └──Upstream Port (type 1), pci_dev (Nth switch)        |
>           :                                                   :
>           └──Downstream Port (type 1), pci_dev (1st switch)  port1
>              └──Upstream Port (type 1), pci_dev (1st switch)  |
>                 └──Root Port (type 1), pci_dev                |
>                    └──PCI host bridge, pci_host_bridge       port0
>                       :                                       |
>                       :..ACPI0017, acpi_dev                  root
> 
>  (There can be zero or any other number of switches in between.)
> 
> An iterator through the grandparents takes us to the root port which
> is registered as dport to the bridge. The next port an endpoint is
> connected to can be determined by using the grandparent of the memory
> device as a dport_dev in cxl_mem_find_port().
> 
> The same does not work in RCD mode where only an RCiEP is connected to
> the host bridge:
> 
>  CXL memory device, cxl_memdev                               endpoint
>  └──PCIe Endpoint (type 0), pci_dev                           |
>     └──PCI host bridge, pci_host_bridge                      port0
>        :                                                      |
>        :..ACPI0017, acpi_dev                                 root
> 
> Here, an endpoint is directly connected to the host bridge without a
> type 1 PCI device (root or downstream port) in between. To link the
> endpoint to the correct port, the endpoint's PCI device (parent of the
> memory device) must be taken as dport_dev arg in cxl_mem_find_port().
> 
> Change cxl_mem_find_port() to find an RCH's port.
> 
> Signed-off-by: Robert Richter <rrichter@amd.com>
> ---
>  drivers/cxl/core/port.c | 38 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 38 insertions(+)
> 
> diff --git a/drivers/cxl/core/port.c b/drivers/cxl/core/port.c
> index 0431ed860d8e..d10c3580719b 100644
> --- a/drivers/cxl/core/port.c
> +++ b/drivers/cxl/core/port.c
> @@ -1354,6 +1354,14 @@ static int add_port_attach_ep(struct cxl_memdev *cxlmd,
>  	return rc;
>  }
>  
> +static inline bool is_cxl_restricted(struct cxl_memdev *cxlmd)
> +{
> +	struct device *parent = cxlmd->dev.parent;
> +	if (!dev_is_pci(parent))
> +		return false;
> +	return pci_pcie_type(to_pci_dev(parent)) == PCI_EXP_TYPE_RC_END;
> +}
> +
>  int devm_cxl_enumerate_ports(struct cxl_memdev *cxlmd)
>  {
>  	struct device *dev = &cxlmd->dev;
> @@ -1433,9 +1441,39 @@ int devm_cxl_enumerate_ports(struct cxl_memdev *cxlmd)
>  }
>  EXPORT_SYMBOL_NS_GPL(devm_cxl_enumerate_ports, CXL);
>  
> +/*
> + * CXL memory device and port hierarchy:
> + *
> + * VH mode:
> + *
> + * CXL memory device, cxl_memdev                               endpoint
> + * └──PCIe Endpoint (type 0), pci_dev                           |
> + *    └──Downstream Port (type 1), pci_dev (Nth switch)        portN
> + *       └──Upstream Port (type 1), pci_dev (Nth switch)        |
> + *          :                                                   :
> + *          └──Downstream Port (type 1), pci_dev (1st switch)  port1
> + *             └──Upstream Port (type 1), pci_dev (1st switch)  |
> + *                └──Root Port (type 1), pci_dev                |
> + *                   └──PCI host bridge, pci_host_bridge       port0
> + *                      :                                       |
> + *                      :..ACPI0017, acpi_dev                  root
> + *
> + * (There can be zero or any other number of switches in between.)
> + *
> + * RCD mode:
> + *
> + * CXL memory device, cxl_memdev                               endpoint
> + * └──PCIe Endpoint (type 0), pci_dev                           |
> + *    └──PCI host bridge, pci_host_bridge                      port0
> + *       :                                                      |
> + *       :..ACPI0017, acpi_dev                                 root
> + */
>  struct cxl_port *cxl_mem_find_port(struct cxl_memdev *cxlmd,
>  				   struct cxl_dport **dport)
>  {
> +	if (is_cxl_restricted(cxlmd))
> +		return find_cxl_port(cxlmd->dev.parent, dport);
> +
>  	return find_cxl_port(grandparent(&cxlmd->dev), dport);

I do not see why this change is needed. For example:

# readlink -f /sys/bus/cxl/devices/mem0
/sys/devices/pci0000:38/0000:38:00.0/mem0
# cxl list -BT
[
  {
    "bus":"root0",
    "provider":"ACPI.CXL",
    "nr_dports":1,
    "dports":[
      {
        "dport":"pci0000:38",
        "id":49
      }
    ]
  }
]

...so, in this case, the grandparent of "mem0" is "pci0000:38", and
"pci0000:38" is a dport. Unmodified cxl_mem_find_port() will do the
right thing and find that this CXL RCIEP is directly connected to
"root0".