cxl/core/port: Add switch port enumeration

So far the platorm level CXL resources have been enumerated by the
cxl_acpi driver, and cxl_pci has gathered all the pre-requisite
information it needs to fire up a cxl_mem driver. However, the first
thing the cxl_mem driver will be tasked to do is validate that all the
PCIe Switches in its ancestry also have CXL capabilities and an CXL.mem
link established.

Provide a common mechanism for a CXL.mem endpoint driver to enumerate
all the ancestor CXL ports in the topology and validate CXL.mem
connectivity.

Multiple endpoints may end up racing to establish a shared port in the
topology. This race is resolved via taking the device-lock on a parent
CXL Port before establishing a new child. The winner of the race
establishes the port, the loser simply registers its interest in the
port via 'struct cxl_ep' place-holder reference.

At endpoint teardown the same parent port lock is taken as 'struct
cxl_ep' references are deleted. Last endpoint to drop its reference
unregisters the port.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/164398731146.902644.1029761300481366248.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/cxl/acpi.c

@@ -130,21 +130,6 @@ static int cxl_parse_cfmws(union acpi_subtable_headers *header, void *arg,
 	return 0;
 }
 
-static struct cxl_dport *find_dport_by_dev(struct cxl_port *port, struct device *dev)
-{
-	struct cxl_dport *dport;
-
-	cxl_device_lock(&port->dev);
-	list_for_each_entry(dport, &port->dports, list)
-		if (dport->dport == dev) {
-			cxl_device_unlock(&port->dev);
-			return dport;
-		}
-
-	cxl_device_unlock(&port->dev);
-	return NULL;
-}
-
 __mock struct acpi_device *to_cxl_host_bridge(struct device *host,
 					      struct device *dev)
 {
@@ -175,7 +160,7 @@ static int add_host_bridge_uport(struct device *match, void *arg)
 	if (!bridge)
 		return 0;
 
-	dport = find_dport_by_dev(root_port, match);
+	dport = cxl_find_dport_by_dev(root_port, match);
 	if (!dport) {
 		dev_dbg(host, "host bridge expected and not found\n");
 		return 0;