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Date: Tue, 15 Nov 2022 14:12:48 +0100
From: Robert Richter <rrichter@....com>
To: Dan Williams <dan.j.williams@...el.com>
CC: Alison Schofield <alison.schofield@...el.com>,
Vishal Verma <vishal.l.verma@...el.com>,
Ira Weiny <ira.weiny@...el.com>,
Ben Widawsky <bwidawsk@...nel.org>,
<linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
Bjorn Helgaas <bhelgaas@...gle.com>,
"Rafael J. Wysocki" <rafael@...nel.org>,
Len Brown <lenb@...nel.org>,
Jonathan Cameron <Jonathan.Cameron@...wei.com>,
Davidlohr Bueso <dave@...olabs.net>,
Dave Jiang <dave.jiang@...el.com>
Subject: Re: [PATCH v3 3/9] cxl/mem: Adjust cxl_mem_find_port() to find an
RCH's port
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@....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). Instead, the endpoint's
device (0000:38:00.0) must be used to get to the bridge
("pci0000:38").
There is a parent missing because there is no Root Port in the RCD
hierarchy, simplified example:
VH mode:
CXL memory device, cxl_memdev endpoint <- cxlmd
└──PCIe Endpoint (type 0), pci_dev |
└──Downstream Port (type 1), pci_dev (1st switch) port1 <- port1: registered as dport at port0
└──Upstream Port (type 1), pci_dev (1st switch) | port1: grandparent(cxlmd)
└──Root Port (type 1), pci_dev | <- pdev: registered as dport at port0
| | pdev: grandparent(port1)
└──PCI host bridge, pci_host_bridge port0 <- find_cxl_port(grandparent(grandparent(cxlmd)))
: | port0 enumerates root ports (pdev) as dports
:..ACPI0017, acpi_dev root
Additional switches add another grandparent() level each (adding an
dport and uport).
RCD mode:
CXL memory device, cxl_memdev endpoint <- cxlmd
└──PCIe Endpoint (type 0), pci_dev | <- pdev: registered as dport at port0
| | pdev: parent(cxlmd)
└──PCI host bridge, pci_host_bridge port0 <- find_cxl_port(parent(endpoint))
: | port0 enumerates endpoint (RCiEP) as dports
:..ACPI0017, acpi_dev root
I hope I could shed some light here.
-Robert
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