cxl/port: Link the 'parent_dport' in portX/ and endpointX/ sysfs

Similar to the justification in:

1b58b4ca ("cxl/port: Record parent dport when adding ports")

...userspace wants to know the routing information for ports for
calculating the memdev order for region creation among other things.
Cache the information the kernel discovers at enumeration time in a
'parent_dport' attribute to save userspace the time of trawling sysfs
to recover the same information.
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/167124082375.1626103.6047000000121298560.stgit@dwillia2-xfh.jf.intel.comSigned-off-by: Dan Williams <dan.j.williams@intel.com>

Documentation/ABI/testing/sysfs-bus-cxl

@@ -90,6 +90,21 @@ Description:
 		capability.
 
 
+What:		/sys/bus/cxl/devices/{port,endpoint}X/parent_dport
+Date:		January, 2023
+KernelVersion:	v6.3
+Contact:	linux-cxl@vger.kernel.org
+Description:
+		(RO) CXL port objects are instantiated for each upstream port in
+		a CXL/PCIe switch, and for each endpoint to map the
+		corresponding memory device into the CXL port hierarchy. When a
+		descendant CXL port (switch or endpoint) is enumerated it is
+		useful to know which 'dport' object in the parent CXL port
+		routes to this descendant. The 'parent_dport' symlink points to
+		the device representing the downstream port of a CXL switch that
+		routes to {port,endpoint}X.
+
+
 What:		/sys/bus/cxl/devices/portX/dportY
 Date:		June, 2021
 KernelVersion:	v5.14

drivers/cxl/core/port.c

@@ -583,6 +583,29 @@ static int devm_cxl_link_uport(struct device *host, struct cxl_port *port)
 	return devm_add_action_or_reset(host, cxl_unlink_uport, port);
 }
 
+static void cxl_unlink_parent_dport(void *_port)
+{
+	struct cxl_port *port = _port;
+
+	sysfs_remove_link(&port->dev.kobj, "parent_dport");
+}
+
+static int devm_cxl_link_parent_dport(struct device *host,
+				      struct cxl_port *port,
+				      struct cxl_dport *parent_dport)
+{
+	int rc;
+
+	if (!parent_dport)
+		return 0;
+
+	rc = sysfs_create_link(&port->dev.kobj, &parent_dport->dport->kobj,
+			       "parent_dport");
+	if (rc)
+		return rc;
+	return devm_add_action_or_reset(host, cxl_unlink_parent_dport, port);
+}
+
 static struct lock_class_key cxl_port_key;
 
 static struct cxl_port *cxl_port_alloc(struct device *uport,
@@ -692,6 +715,10 @@ static struct cxl_port *__devm_cxl_add_port(struct device *host,
 	if (rc)
 		return ERR_PTR(rc);
 
+	rc = devm_cxl_link_parent_dport(host, port, parent_dport);
+	if (rc)
+		return ERR_PTR(rc);
+
 	return port;
 
 err:
@@ -1164,6 +1191,7 @@ static void delete_endpoint(void *data)
 
 	device_lock(parent);
 	if (parent->driver && !endpoint->dead) {
+		devm_release_action(parent, cxl_unlink_parent_dport, endpoint);
 		devm_release_action(parent, cxl_unlink_uport, endpoint);
 		devm_release_action(parent, unregister_port, endpoint);
 	}
@@ -1194,6 +1222,7 @@ EXPORT_SYMBOL_NS_GPL(cxl_endpoint_autoremove, CXL);
  */
 static void delete_switch_port(struct cxl_port *port)
 {
+	devm_release_action(port->dev.parent, cxl_unlink_parent_dport, port);
 	devm_release_action(port->dev.parent, cxl_unlink_uport, port);
 	devm_release_action(port->dev.parent, unregister_port, port);
 }