cxl/port: Fix find_cxl_root() for RCDs and simplify it

The find_cxl_root() helper is used to lookup root decoders and other CXL
platform topology information for a given endpoint. It turns out that
for RCDs it has never worked. The result of find_cxl_root(&cxlmd->dev)
is always NULL for the RCH topology case because it expects to find a
cxl_port at the host-bridge. RCH topologies only have the root cxl_port
object with the host-bridge as a dport. While there are no reports of
this being a problem to date, by inspection region enumeration should
crash as a result of this problem, and it does in a local unit test for
this scenario.

However, an observation that ever since:

commit f17b558 ("cxl/pmem: Refactor nvdimm device registration, delete the workqueue")

...all callers of find_cxl_root() occur after the memdev connection to
the port topology has been established. That means that find_cxl_root()
can be simplified to a walk of the endpoint port topology to the root.
Switch to that arrangement which also fixes the RCD bug.

Fixes: a32320b ("cxl/region: Add region autodiscovery")
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/168002857715.50647.344876437247313909.stgit@dwillia2-xfh.jf.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/cxl/core/pmem.c

@@ -62,9 +62,9 @@ static int match_nvdimm_bridge(struct device *dev, void *data)
 	return is_cxl_nvdimm_bridge(dev);
 }
 
-struct cxl_nvdimm_bridge *cxl_find_nvdimm_bridge(struct device *start)
+struct cxl_nvdimm_bridge *cxl_find_nvdimm_bridge(struct cxl_memdev *cxlmd)
 {
-	struct cxl_port *port = find_cxl_root(start);
+	struct cxl_port *port = find_cxl_root(dev_get_drvdata(&cxlmd->dev));
 	struct device *dev;
 
 	if (!port)
@@ -253,7 +253,7 @@ int devm_cxl_add_nvdimm(struct cxl_memdev *cxlmd)
 	struct device *dev;
 	int rc;
 
-	cxl_nvb = cxl_find_nvdimm_bridge(&cxlmd->dev);
+	cxl_nvb = cxl_find_nvdimm_bridge(cxlmd);
 	if (!cxl_nvb)
 		return -ENODEV;
 

drivers/cxl/core/port.c

@@ -823,41 +823,17 @@ static bool dev_is_cxl_root_child(struct device *dev)
 	return false;
 }
 
-/* Find a 2nd level CXL port that has a dport that is an ancestor of @match */
-static int match_root_child(struct device *dev, const void *match)
+struct cxl_port *find_cxl_root(struct cxl_port *port)
 {
-	const struct device *iter = NULL;
-	struct cxl_dport *dport;
-	struct cxl_port *port;
-
-	if (!dev_is_cxl_root_child(dev))
-		return 0;
-
-	port = to_cxl_port(dev);
-	iter = match;
-	while (iter) {
-		dport = cxl_find_dport_by_dev(port, iter);
-		if (dport)
-			break;
-		iter = iter->parent;
-	}
-
-	return !!iter;
-}
+	struct cxl_port *iter = port;
 
-struct cxl_port *find_cxl_root(struct device *dev)
-{
-	struct device *port_dev;
-	struct cxl_port *root;
+	while (iter && !is_cxl_root(iter))
+		iter = to_cxl_port(iter->dev.parent);
 
-	port_dev = bus_find_device(&cxl_bus_type, NULL, dev, match_root_child);
-	if (!port_dev)
+	if (!iter)
 		return NULL;
-
-	root = to_cxl_port(port_dev->parent);
-	get_device(&root->dev);
-	put_device(port_dev);
-	return root;
+	get_device(&iter->dev);
+	return iter;
 }
 EXPORT_SYMBOL_NS_GPL(find_cxl_root, CXL);
 

drivers/cxl/core/region.c

@@ -2251,7 +2251,7 @@ static struct cxl_pmem_region *cxl_pmem_region_alloc(struct cxl_region *cxlr)
 		 * bridge for one device is the same for all.
 		 */
 		if (i == 0) {
-			cxl_nvb = cxl_find_nvdimm_bridge(&cxlmd->dev);
+			cxl_nvb = cxl_find_nvdimm_bridge(cxlmd);
 			if (!cxl_nvb) {
 				cxlr_pmem = ERR_PTR(-ENODEV);
 				goto out;

drivers/cxl/cxl.h

@@ -658,7 +658,7 @@ struct pci_bus *cxl_port_to_pci_bus(struct cxl_port *port);
 struct cxl_port *devm_cxl_add_port(struct device *host, struct device *uport,
 				   resource_size_t component_reg_phys,
 				   struct cxl_dport *parent_dport);
-struct cxl_port *find_cxl_root(struct device *dev);
+struct cxl_port *find_cxl_root(struct cxl_port *port);
 int devm_cxl_enumerate_ports(struct cxl_memdev *cxlmd);
 void cxl_bus_rescan(void);
 void cxl_bus_drain(void);
@@ -760,7 +760,7 @@ struct cxl_nvdimm *to_cxl_nvdimm(struct device *dev);
 bool is_cxl_nvdimm(struct device *dev);
 bool is_cxl_nvdimm_bridge(struct device *dev);
 int devm_cxl_add_nvdimm(struct cxl_memdev *cxlmd);
-struct cxl_nvdimm_bridge *cxl_find_nvdimm_bridge(struct device *dev);
+struct cxl_nvdimm_bridge *cxl_find_nvdimm_bridge(struct cxl_memdev *cxlmd);
 
 #ifdef CONFIG_CXL_REGION
 bool is_cxl_pmem_region(struct device *dev);

drivers/cxl/port.c

@@ -119,7 +119,7 @@ static int cxl_endpoint_port_probe(struct cxl_port *port)
 	 * This can't fail in practice as CXL root exit unregisters all
 	 * descendant ports and that in turn synchronizes with cxl_port_probe()
 	 */
-	root = find_cxl_root(&cxlmd->dev);
+	root = find_cxl_root(port);
 
 	/*
 	 * Now that all endpoint decoders are successfully enumerated, try to