[PATCH v2 6/6] cxl/mem: Introduce cxl_memdev_attach for CXL-dependent operation

Dan Williams
2025-12-16T00:55:36Z

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.

Allow for a driver to pass a routine to be called in cxl_mem_probe()
context. This ability is inspired by and mirrors the semantics of
faux_device_create(). It allows for the caller to run CXL-topology
attach-dependent logic on behalf of the caller.

The probe callback runs after the port topology is successfully attached
for the given memdev.

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.

Cc: Smita Koralahalli <Smita.KoralahalliChannabasappa@amd.com>
Reviewed-by: Ben Cheatham <benjamin.cheatham@amd.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com> (✓ DKIM/intel.com)
Tested-by: Alejandro Lucero <alucerop@amd.com> (✓ DKIM/amd.com)
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
drivers/cxl/cxlmem.h | 12 ++++++++++--
drivers/cxl/core/memdev.c | 33 +++++++++++++++++++++++++++++----
drivers/cxl/mem.c | 20 ++++++++++++++++----
drivers/cxl/pci.c | 2 +-
tools/testing/cxl/test/mem.c | 2 +-
5 files changed, 57 insertions(+), 12 deletions(-)

diff --git a/drivers/cxl/cxlmem.h b/drivers/cxl/cxlmem.h
index 9db31c7993c4..ef202b34e5ea 100644
--- a/drivers/cxl/cxlmem.h
+++ b/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;
diff --git a/drivers/cxl/core/memdev.c b/drivers/cxl/core/memdev.c
index 63da2bd4436e..3ab4cd8f19ed 100644
--- a/drivers/cxl/core/memdev.c
+++ b/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;

diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
index 677996c65272..333c366b69e7 100644
--- a/drivers/cxl/mem.c
+++ b/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");

diff --git a/drivers/cxl/pci.c b/drivers/cxl/pci.c
index 1c6fc5334806..549368a9c868 100644
--- a/drivers/cxl/pci.c
+++ b/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);

diff --git a/tools/testing/cxl/test/mem.c b/tools/testing/cxl/test/mem.c
index 8a22b7601627..cb87e8c0e63c 100644
--- a/tools/testing/cxl/test/mem.c
+++ b/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);

--
2.51.1