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Date:   Thu, 17 Nov 2022 19:13:11 +0100
From:   Robert Richter <rrichter@amd.com>
To:     Dan Williams <dan.j.williams@intel.com>
CC:     Alison Schofield <alison.schofield@intel.com>,
        Vishal Verma <vishal.l.verma@intel.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Ben Widawsky <bwidawsk@kernel.org>,
        <linux-cxl@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Len Brown <lenb@kernel.org>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Dave Jiang <dave.jiang@intel.com>
Subject: Re: [PATCH v3 3/9] cxl/mem: Adjust cxl_mem_find_port() to find an
 RCH's port
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On 15.11.22 10:06:28, Dan Williams wrote:
> Robert Richter wrote:
> > On 14.11.22 15:45:19, Dan Williams wrote:
> > > 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".
> > 
> > find_cxl_port() uses the dport_dev, not the uport_dev. A lookup of
> > pci0000:38 gives the cxl root (ACPI.CXL).
> 
> ...but that is what I would expect. I.e. that RCDs appear directly
> connected to the cxl root with no intervening cxl_port instance, and RCH
> host-bridges only serve the role of dport devices. This also matches the
> diagram from 9.11.8 CXL Devices Attached to an RCH where a
> downstream-port RCRB in the host bridge is directly connected to the
> RCIEP endpoint.

All devices connected to ACPI.CXL? IMO this needs to be bound to the
host bridge. The hierarchy should show a host/device mapping. In fact,
in a VH the root port is also not part of the mapping, instead the
host bridge shows up there. Also, there can be multiple RCHs.

-Robert