cxl/mem: Arrange for always-synchronous memdev attach

In preparation for CXL accelerator drivers that have a hard dependency on
CXL capability initialization, arrange for cxl_mem_probe() to always run
synchronous with the device_add() of cxl_memdev instances. I.e.
cxl_mem_driver registration is always complete before the first memdev
creation event.

At present, cxl_pci does not care about the attach state of the cxl_memdev
because all generic memory expansion functionality can be handled by the
cxl_core. For accelerators, however, that driver needs to perform driver
specific initialization if CXL is available, or execute a fallback to PCIe
only operation.

This synchronous attach guarantee is also needed for Soft Reserve Recovery,
which is an effort that needs to assert that devices have had a chance to
attach before making a go / no-go decision on proceeding with CXL subsystem
initialization.

By moving devm_cxl_add_memdev() to cxl_mem.ko it removes async module
loading as one reason that a memdev may not be attached upon return from
devm_cxl_add_memdev().

Cc: Smita Koralahalli <Smita.KoralahalliChannabasappa@amd.com>
Cc: Alejandro Lucero <alucerop@amd.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Tested-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Ben Cheatham <benjamin.cheatham@amd.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Tested-by: Alejandro Lucero <alucerop@amd.com>
Link: https://patch.msgid.link/20251216005616.3090129-3-dan.j.williams@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

Author: djbw

drivers/cxl/Kconfig

@@ -22,6 +22,7 @@ if CXL_BUS
 config CXL_PCI
 	tristate "PCI manageability"
 	default CXL_BUS
+	select CXL_MEM
 	help
 	  The CXL specification defines a "CXL memory device" sub-class in the
 	  PCI "memory controller" base class of devices. Device's identified by
@@ -89,7 +90,6 @@ config CXL_PMEM
 
 config CXL_MEM
 	tristate "CXL: Memory Expansion"
-	depends on CXL_PCI
 	default CXL_BUS
 	help
 	  The CXL.mem protocol allows a device to act as a provider of "System

drivers/cxl/core/memdev.c

@@ -1050,8 +1050,12 @@ static const struct file_operations cxl_memdev_fops = {
 	.llseek = noop_llseek,
 };
 
-struct cxl_memdev *devm_cxl_add_memdev(struct device *host,
-				       struct cxl_dev_state *cxlds)
+/*
+ * Core helper for devm_cxl_add_memdev() that wants to both create a device and
+ * assert to the caller that upon return cxl_mem::probe() has been invoked.
+ */
+struct cxl_memdev *__devm_cxl_add_memdev(struct device *host,
+					 struct cxl_dev_state *cxlds)
 {
 	struct cxl_memdev *cxlmd;
 	struct device *dev;
@@ -1093,7 +1097,7 @@ struct cxl_memdev *devm_cxl_add_memdev(struct device *host,
 	put_device(dev);
 	return ERR_PTR(rc);
 }
-EXPORT_SYMBOL_NS_GPL(devm_cxl_add_memdev, "CXL");
+EXPORT_SYMBOL_FOR_MODULES(__devm_cxl_add_memdev, "cxl_mem");
 
 static void sanitize_teardown_notifier(void *data)
 {

drivers/cxl/cxlmem.h

@@ -95,6 +95,8 @@ static inline bool is_cxl_endpoint(struct cxl_port *port)
 	return is_cxl_memdev(port->uport_dev);
 }
 
+struct cxl_memdev *__devm_cxl_add_memdev(struct device *host,
+					 struct cxl_dev_state *cxlds);
 struct cxl_memdev *devm_cxl_add_memdev(struct device *host,
 				       struct cxl_dev_state *cxlds);
 int devm_cxl_sanitize_setup_notifier(struct device *host,

drivers/cxl/mem.c

@@ -201,6 +201,22 @@ static int cxl_mem_probe(struct device *dev)
 	return devm_add_action_or_reset(dev, enable_suspend, NULL);
 }
 
+/**
+ * devm_cxl_add_memdev - Add a CXL memory device
+ * @host: devres alloc/release context and parent for the memdev
+ * @cxlds: CXL device state to associate with the memdev
+ *
+ * Upon return the device will have had a chance to attach to the
+ * cxl_mem driver, but may fail if the CXL topology is not ready
+ * (hardware CXL link down, or software platform CXL root not attached)
+ */
+struct cxl_memdev *devm_cxl_add_memdev(struct device *host,
+				       struct cxl_dev_state *cxlds)
+{
+	return __devm_cxl_add_memdev(host, cxlds);
+}
+EXPORT_SYMBOL_NS_GPL(devm_cxl_add_memdev, "CXL");
+
 static ssize_t trigger_poison_list_store(struct device *dev,
 					 struct device_attribute *attr,
 					 const char *buf, size_t len)
@@ -248,6 +264,7 @@ static struct cxl_driver cxl_mem_driver = {
 	.probe = cxl_mem_probe,
 	.id = CXL_DEVICE_MEMORY_EXPANDER,
 	.drv = {
+		.probe_type = PROBE_FORCE_SYNCHRONOUS,
 		.dev_groups = cxl_mem_groups,
 	},
 };