[PATCH 14/23] cxl: Introduce topology host registration

[Ben Widawsky <ben.widawsky@intel.com>]

The description of the CXL topology will be conveyed by a platform
specific entity that is expected to be a singleton. For ACPI based
systems, this is ACPI0017. When the topology host goes away, which as of
now can only be triggered by module unload, it is desirable to have the
entire topology cleaned up. Regular devm unwinding handles most
situations already, but what's missing is the ability to teardown the
root port. Since the root port is platform specific, the core needs a
set of APIs to allow platform specific drivers to register their root
ports. With that, all the automatic teardown can occur.

cxl_test makes for an interesting case. cxl_test creates an alternate
universe where there are possibly two root topology hosts (a real
ACPI0016, and a fake ACPI0016). For this to work in the future, cxl_acpi
(or some future platform host driver) will need to be unloaded first.

Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
---
The topology lock can be used for more things. I'd like to save that as
an add-on patch since it's extra risk for no reward, at this point.
---
 drivers/cxl/acpi.c     | 18 ++++++++++---
 drivers/cxl/core/bus.c | 57 +++++++++++++++++++++++++++++++++++++++---
 drivers/cxl/cxl.h      |  5 +++-
 3 files changed, 73 insertions(+), 7 deletions(-)

diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
index 3415184a2e61..82cc42ab38c6 100644
--- a/drivers/cxl/acpi.c
+++ b/drivers/cxl/acpi.c
@@ -224,8 +224,7 @@ static int add_host_bridge_uport(struct device *match, void *arg)
 		return 0;
 	}
 
-	port = devm_cxl_add_port(host, match, dport->component_reg_phys,
-				 root_port);
+	port = devm_cxl_add_port(match, dport->component_reg_phys, root_port);
 	if (IS_ERR(port))
 		return PTR_ERR(port);
 	dev_dbg(host, "%s: add: %s\n", dev_name(match), dev_name(&port->dev));
@@ -376,6 +375,11 @@ static int add_root_nvdimm_bridge(struct device *match, void *data)
 	return 1;
 }
 
+static void clear_topology_host(void *data)
+{
+	cxl_unregister_topology_host(data);
+}
+
 static int cxl_acpi_probe(struct platform_device *pdev)
 {
 	int rc;
@@ -384,7 +388,15 @@ static int cxl_acpi_probe(struct platform_device *pdev)
 	struct acpi_device *adev = ACPI_COMPANION(host);
 	struct cxl_cfmws_context ctx;
 
-	root_port = devm_cxl_add_port(host, host, CXL_RESOURCE_NONE, NULL);
+	rc = cxl_register_topology_host(host);
+	if (rc)
+		return rc;
+
+	rc = devm_add_action_or_reset(host, clear_topology_host, host);
+	if (rc)
+		return rc;
+
+	root_port = devm_cxl_add_port(host, CXL_RESOURCE_NONE, root_port);
 	if (IS_ERR(root_port))
 		return PTR_ERR(root_port);
 	dev_dbg(host, "add: %s\n", dev_name(&root_port->dev));
diff --git a/drivers/cxl/core/bus.c b/drivers/cxl/core/bus.c
index cd6fe7823c69..2ad38167796d 100644
--- a/drivers/cxl/core/bus.c
+++ b/drivers/cxl/core/bus.c
@@ -25,6 +25,53 @@
  */
 
 static DEFINE_IDA(cxl_port_ida);
+static DECLARE_RWSEM(topology_host_sem);
+
+static struct device *cxl_topology_host;
+
+int cxl_register_topology_host(struct device *host)
+{
+	down_write(&topology_host_sem);
+	if (cxl_topology_host) {
+		up_write(&topology_host_sem);
+		pr_warn("%s host currently in use. Please try unloading %s",
+			dev_name(cxl_topology_host), host->driver->name);
+		return -EBUSY;
+	}
+
+	cxl_topology_host = host;
+	up_write(&topology_host_sem);
+
+	return 0;
+}
+EXPORT_SYMBOL_NS_GPL(cxl_register_topology_host, CXL);
+
+void cxl_unregister_topology_host(struct device *host)
+{
+	down_write(&topology_host_sem);
+	if (cxl_topology_host == host)
+		cxl_topology_host = NULL;
+	else
+		pr_warn("topology host in use by %s\n",
+			cxl_topology_host->driver->name);
+	up_write(&topology_host_sem);
+}
+EXPORT_SYMBOL_NS_GPL(cxl_unregister_topology_host, CXL);
+
+static struct device *get_cxl_topology_host(void)
+{
+	down_read(&topology_host_sem);
+	if (cxl_topology_host)
+		return cxl_topology_host;
+	up_read(&topology_host_sem);
+	return NULL;
+}
+
+static void put_cxl_topology_host(struct device *dev)
+{
+	WARN_ON(dev != cxl_topology_host);
+	up_read(&topology_host_sem);
+}
 
 static ssize_t devtype_show(struct device *dev, struct device_attribute *attr,
 			    char *buf)
@@ -362,17 +409,16 @@ static struct cxl_port *cxl_port_alloc(struct device *uport,
 
 /**
  * devm_cxl_add_port - register a cxl_port in CXL memory decode hierarchy
- * @host: host device for devm operations
  * @uport: "physical" device implementing this upstream port
  * @component_reg_phys: (optional) for configurable cxl_port instances
  * @parent_port: next hop up in the CXL memory decode hierarchy
  */
-struct cxl_port *devm_cxl_add_port(struct device *host, struct device *uport,
+struct cxl_port *devm_cxl_add_port(struct device *uport,
 				   resource_size_t component_reg_phys,
 				   struct cxl_port *parent_port)
 {
+	struct device *dev, *host;
 	struct cxl_port *port;
-	struct device *dev;
 	int rc;
 
 	port = cxl_port_alloc(uport, component_reg_phys, parent_port);
@@ -391,7 +437,12 @@ struct cxl_port *devm_cxl_add_port(struct device *host, struct device *uport,
 	if (rc)
 		goto err;
 
+	host = get_cxl_topology_host();
+	if (!host)
+		return ERR_PTR(-ENODEV);
+
 	rc = devm_add_action_or_reset(host, unregister_port, port);
+	put_cxl_topology_host(host);
 	if (rc)
 		return ERR_PTR(rc);
 
diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
index 2c5627fa8a34..6fac4826d22b 100644
--- a/drivers/cxl/cxl.h
+++ b/drivers/cxl/cxl.h
@@ -152,6 +152,9 @@ int cxl_find_regblock(struct pci_dev *pdev, enum cxl_regloc_type type,
 #define CXL_RESOURCE_NONE ((resource_size_t) -1)
 #define CXL_TARGET_STRLEN 20
 
+int cxl_register_topology_host(struct device *host);
+void cxl_unregister_topology_host(struct device *host);
+
 /*
  * cxl_decoder flags that define the type of memory / devices this
  * decoder supports as well as configuration lock status See "CXL 2.0
@@ -279,7 +282,7 @@ struct cxl_dport {
 };
 
 struct cxl_port *to_cxl_port(struct device *dev);
-struct cxl_port *devm_cxl_add_port(struct device *host, struct device *uport,
+struct cxl_port *devm_cxl_add_port(struct device *uport,
 				   resource_size_t component_reg_phys,
 				   struct cxl_port *parent_port);
 
-- 
2.34.0