cxl: Add helper to detect top of CXL device topology

Add a helper to replace the open code detection of CXL device hierarchy
root, or the host bridge. The helper will be used for delayed downstream
port (dport) creation.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Li Ming <ming.li@zohomail.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Robert Richter <rrichter@amd.com>
Tested-by: Robert Richter <rrichter@amd.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

Author: davejiang

drivers/cxl/core/port.c

@@ -33,6 +33,15 @@
 static DEFINE_IDA(cxl_port_ida);
 static DEFINE_XARRAY(cxl_root_buses);
 
+/*
+ * The terminal device in PCI is NULL and @platform_bus
+ * for platform devices (for cxl_test)
+ */
+static bool is_cxl_host_bridge(struct device *dev)
+{
+	return (!dev || dev == &platform_bus);
+}
+
 int cxl_num_decoders_committed(struct cxl_port *port)
 {
 	lockdep_assert_held(&cxl_rwsem.region);
@@ -1541,7 +1550,7 @@ static int add_port_attach_ep(struct cxl_memdev *cxlmd,
 	resource_size_t component_reg_phys;
 	int rc;
 
-	if (!dparent) {
+	if (is_cxl_host_bridge(dparent)) {
 		/*
 		 * The iteration reached the topology root without finding the
 		 * CXL-root 'cxl_port' on a previous iteration, fail for now to
@@ -1629,11 +1638,7 @@ int devm_cxl_enumerate_ports(struct cxl_memdev *cxlmd)
 		struct device *uport_dev;
 		struct cxl_dport *dport;
 
-		/*
-		 * The terminal "grandparent" in PCI is NULL and @platform_bus
-		 * for platform devices
-		 */
-		if (!dport_dev || dport_dev == &platform_bus)
+		if (is_cxl_host_bridge(dport_dev))
 			return 0;
 
 		uport_dev = dport_dev->parent;