cxl/mem: Introduce cxl_memdev_attach for CXL-dependent operation

Unlike the cxl_pci class driver that opportunistically enables memory
expansion with no other dependent functionality, CXL accelerator drivers
have distinct PCIe-only and CXL-enhanced operation states. If CXL is
available some additional coherent memory/cache operations can be enabled,
otherwise traditional DMA+MMIO over PCIe/CXL.io is a fallback.

This constitutes a new mode of operation where the caller of
devm_cxl_add_memdev() wants to make a "go/no-go" decision about running
in CXL accelerated mode or falling back to PCIe-only operation. Part of
that decision making process likely also includes additional
CXL-acceleration-specific resource setup. Encapsulate both of those
requirements into 'struct cxl_memdev_attach' that provides a ->probe()
callback. The probe callback runs in cxl_mem_probe() context, after the
port topology is successfully attached for the given memdev. It supports
a contract where, upon successful return from devm_cxl_add_memdev(),
everything needed for CXL accelerated operation has been enabled.

Additionally the presence of @cxlmd->attach indicates that the accelerator
driver be detached when CXL operation ends. This conceptually makes a CXL
link loss event mirror a PCIe link loss event which results in triggering
the ->remove() callback of affected devices+drivers. A driver can re-attach
to recover back to PCIe-only operation. Live recovery, i.e. without a
->remove()/->probe() cycle, is left as a future consideration.

[ dj: Repalce with updated commit log from Dan ]

Cc: Smita Koralahalli <Smita.KoralahalliChannabasappa@amd.com>
Reviewed-by: Ben Cheatham <benjamin.cheatham@amd.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Tested-by: Alejandro Lucero <alucerop@amd.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Link: https://patch.msgid.link/20251216005616.3090129-7-dan.j.williams@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

drivers/cxl/core/memdev.c

@@ -641,14 +641,24 @@ static void detach_memdev(struct work_struct *work)
 	struct cxl_memdev *cxlmd;
 
 	cxlmd = container_of(work, typeof(*cxlmd), detach_work);
-	device_release_driver(&cxlmd->dev);
+
+	/*
+	 * When the creator of @cxlmd sets ->attach it indicates CXL operation
+	 * is required. In that case, @cxlmd detach escalates to parent device
+	 * detach.
+	 */
+	if (cxlmd->attach)
+		device_release_driver(cxlmd->dev.parent);
+	else
+		device_release_driver(&cxlmd->dev);
 	put_device(&cxlmd->dev);
 }
 
 static struct lock_class_key cxl_memdev_key;
 
 static struct cxl_memdev *cxl_memdev_alloc(struct cxl_dev_state *cxlds,
-					   const struct file_operations *fops)
+					   const struct file_operations *fops,
+					   const struct cxl_memdev_attach *attach)
 {
 	struct cxl_memdev *cxlmd;
 	struct device *dev;
@@ -664,6 +674,8 @@ static struct cxl_memdev *cxl_memdev_alloc(struct cxl_dev_state *cxlds,
 		goto err;
 	cxlmd->id = rc;
 	cxlmd->depth = -1;
+	cxlmd->attach = attach;
+	cxlmd->endpoint = ERR_PTR(-ENXIO);
 
 	dev = &cxlmd->dev;
 	device_initialize(dev);
@@ -1081,6 +1093,18 @@ static struct cxl_memdev *cxl_memdev_autoremove(struct cxl_memdev *cxlmd)
 {
 	int rc;
 
+	/*
+	 * If @attach is provided fail if the driver is not attached upon
+	 * return. Note that failure here could be the result of a race to
+	 * teardown the CXL port topology. I.e. cxl_mem_probe() could have
+	 * succeeded and then cxl_mem unbound before the lock is acquired.
+	 */
+	guard(device)(&cxlmd->dev);
+	if (cxlmd->attach && !cxlmd->dev.driver) {
+		cxl_memdev_unregister(cxlmd);
+		return ERR_PTR(-ENXIO);
+	}
+
 	rc = devm_add_action_or_reset(cxlmd->cxlds->dev, cxl_memdev_unregister,
 				      cxlmd);
 	if (rc)
@@ -1093,13 +1117,14 @@ static struct cxl_memdev *cxl_memdev_autoremove(struct cxl_memdev *cxlmd)
  * 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 cxl_dev_state *cxlds)
+struct cxl_memdev *__devm_cxl_add_memdev(struct cxl_dev_state *cxlds,
+					 const struct cxl_memdev_attach *attach)
 {
 	struct device *dev;
 	int rc;
 
 	struct cxl_memdev *cxlmd __free(put_cxlmd) =
-		cxl_memdev_alloc(cxlds, &cxl_memdev_fops);
+		cxl_memdev_alloc(cxlds, &cxl_memdev_fops, attach);
 	if (IS_ERR(cxlmd))
 		return cxlmd;
 

drivers/cxl/cxlmem.h

@@ -34,6 +34,10 @@
 	(FIELD_GET(CXLMDEV_RESET_NEEDED_MASK, status) !=                       \
 	 CXLMDEV_RESET_NEEDED_NOT)
 
+struct cxl_memdev_attach {
+	int (*probe)(struct cxl_memdev *cxlmd);
+};
+
 /**
  * struct cxl_memdev - CXL bus object representing a Type-3 Memory Device
  * @dev: driver core device object
@@ -43,6 +47,7 @@
  * @cxl_nvb: coordinate removal of @cxl_nvd if present
  * @cxl_nvd: optional bridge to an nvdimm if the device supports pmem
  * @endpoint: connection to the CXL port topology for this memory device
+ * @attach: creator of this memdev depends on CXL link attach to operate
  * @id: id number of this memdev instance.
  * @depth: endpoint port depth
  * @scrub_cycle: current scrub cycle set for this device
@@ -59,6 +64,7 @@ struct cxl_memdev {
 	struct cxl_nvdimm_bridge *cxl_nvb;
 	struct cxl_nvdimm *cxl_nvd;
 	struct cxl_port *endpoint;
+	const struct cxl_memdev_attach *attach;
 	int id;
 	int depth;
 	u8 scrub_cycle;
@@ -95,8 +101,10 @@ 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 cxl_dev_state *cxlds);
-struct cxl_memdev *devm_cxl_add_memdev(struct cxl_dev_state *cxlds);
+struct cxl_memdev *__devm_cxl_add_memdev(struct cxl_dev_state *cxlds,
+					 const struct cxl_memdev_attach *attach);
+struct cxl_memdev *devm_cxl_add_memdev(struct cxl_dev_state *cxlds,
+				       const struct cxl_memdev_attach *attach);
 int devm_cxl_sanitize_setup_notifier(struct device *host,
 				     struct cxl_memdev *cxlmd);
 struct cxl_memdev_state;

drivers/cxl/mem.c

@@ -142,6 +142,12 @@ static int cxl_mem_probe(struct device *dev)
 			return rc;
 	}
 
+	if (cxlmd->attach) {
+		rc = cxlmd->attach->probe(cxlmd);
+		if (rc)
+			return rc;
+	}
+
 	rc = devm_cxl_memdev_edac_register(cxlmd);
 	if (rc)
 		dev_dbg(dev, "CXL memdev EDAC registration failed rc=%d\n", rc);
@@ -166,17 +172,23 @@ static int cxl_mem_probe(struct device *dev)
 /**
  * devm_cxl_add_memdev - Add a CXL memory device
  * @cxlds: CXL device state to associate with the memdev
+ * @attach: Caller depends on CXL topology attachment
  *
  * 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)
+ * cxl_mem driver, but may fail to attach if the CXL topology is not ready
+ * (hardware CXL link down, or software platform CXL root not attached).
+ *
+ * When @attach is NULL it indicates the caller wants the memdev to remain
+ * registered even if it does not immediately attach to the CXL hierarchy. When
+ * @attach is provided a cxl_mem_probe() failure leads to failure of this routine.
  *
  * The parent of the resulting device and the devm context for allocations is
  * @cxlds->dev.
  */
-struct cxl_memdev *devm_cxl_add_memdev(struct cxl_dev_state *cxlds)
+struct cxl_memdev *devm_cxl_add_memdev(struct cxl_dev_state *cxlds,
+				       const struct cxl_memdev_attach *attach)
 {
-	return __devm_cxl_add_memdev(cxlds);
+	return __devm_cxl_add_memdev(cxlds, attach);
 }
 EXPORT_SYMBOL_NS_GPL(devm_cxl_add_memdev, "CXL");
 

drivers/cxl/pci.c

@@ -1006,7 +1006,7 @@ static int cxl_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
 	if (rc)
 		dev_dbg(&pdev->dev, "No CXL Features discovered\n");
 
-	cxlmd = devm_cxl_add_memdev(cxlds);
+	cxlmd = devm_cxl_add_memdev(cxlds, NULL);
 	if (IS_ERR(cxlmd))
 		return PTR_ERR(cxlmd);
 

tools/testing/cxl/test/mem.c

@@ -1767,7 +1767,7 @@ static int cxl_mock_mem_probe(struct platform_device *pdev)
 
 	cxl_mock_add_event_logs(&mdata->mes);
 
-	cxlmd = devm_cxl_add_memdev(cxlds);
+	cxlmd = devm_cxl_add_memdev(cxlds, NULL);
 	if (IS_ERR(cxlmd))
 		return PTR_ERR(cxlmd);