From: Corey Leavitt <corey@leavitt.info>

Introduce a blocking notifier chain that allows other subsystems to be
informed when a PSE controller is registered or unregistered, and
provide pse_register_notifier() / pse_unregister_notifier() as the
subscriber interface.

Subsequent patches will use this to let the phy subsystem own the
phydev->psec lifecycle directly, decoupling PSE lookup from
fwnode_mdiobus_register_phy() and removing the probe-time
-EPROBE_DEFER coupling that currently exists between mdio, phy and
pse-pd when the PSE controller driver is modular.

A blocking chain (rather than atomic) is used because callbacks will
take rtnl_lock and call back into pse_core via of_pse_control_get().

The enum pse_controller_event is placed outside the
IS_ENABLED(CONFIG_PSE_CONTROLLER) guard so that subscribers compiled
into a kernel without PSE support can still reference the event
values in dead-code paths without breaking the build.

This patch is pure infrastructure: nothing fires events yet, and
nothing subscribes. No observable behavior change.

Signed-off-by: Corey Leavitt <corey@leavitt.info>
---
 drivers/net/pse-pd/pse_core.c | 34 ++++++++++++++++++++++++++++++++++
 include/linux/pse-pd/pse.h    | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+)

diff --git a/drivers/net/pse-pd/pse_core.c b/drivers/net/pse-pd/pse_core.c
index 893ec2185947..80c5c6c1758c 100644
--- a/drivers/net/pse-pd/pse_core.c
+++ b/drivers/net/pse-pd/pse_core.c
@@ -8,6 +8,7 @@
 #include <linux/device.h>
 #include <linux/ethtool.h>
 #include <linux/ethtool_netlink.h>
+#include <linux/notifier.h>
 #include <linux/of.h>
 #include <linux/phy.h>
 #include <linux/pse-pd/pse.h>
@@ -23,6 +24,39 @@ static LIST_HEAD(pse_controller_list);
 static DEFINE_XARRAY_ALLOC(pse_pw_d_map);
 static DEFINE_MUTEX(pse_pw_d_mutex);
 
+static BLOCKING_NOTIFIER_HEAD(pse_controller_notifier);
+
+/**
+ * pse_register_notifier - register a callback for PSE controller events
+ * @nb: notifier block to register
+ *
+ * See enum pse_controller_event for events fired and their subscriber
+ * contract. Callbacks run in process context; they may sleep, take
+ * rtnl, and call of_pse_control_get(). The chain fires synchronously,
+ * so a PSE controller driver's probe/unbind path must not hold any
+ * such lock when calling pse_controller_register() or
+ * pse_controller_unregister().
+ *
+ * Return: 0 on success, negative error code otherwise.
+ */
+int pse_register_notifier(struct notifier_block *nb)
+{
+	return blocking_notifier_chain_register(&pse_controller_notifier, nb);
+}
+EXPORT_SYMBOL_GPL(pse_register_notifier);
+
+/**
+ * pse_unregister_notifier - unregister a previously registered callback
+ * @nb: notifier block previously passed to pse_register_notifier()
+ *
+ * Return: 0 on success, negative error code otherwise.
+ */
+int pse_unregister_notifier(struct notifier_block *nb)
+{
+	return blocking_notifier_chain_unregister(&pse_controller_notifier, nb);
+}
+EXPORT_SYMBOL_GPL(pse_unregister_notifier);
+
 /**
  * struct pse_control - a PSE control
  * @pcdev: a pointer to the PSE controller device
diff --git a/include/linux/pse-pd/pse.h b/include/linux/pse-pd/pse.h
index 4e5696cfade7..78fe3a2b1ea8 100644
--- a/include/linux/pse-pd/pse.h
+++ b/include/linux/pse-pd/pse.h
@@ -21,6 +21,7 @@ struct net_device;
 struct phy_device;
 struct pse_controller_dev;
 struct netlink_ext_ack;
+struct notifier_block;
 
 /* C33 PSE extended state and substate. */
 struct ethtool_c33_pse_ext_state_info {
@@ -337,6 +338,24 @@ enum pse_budget_eval_strategies {
 	PSE_BUDGET_EVAL_STRAT_DYNAMIC	= 1 << 2,
 };
 
+/**
+ * enum pse_controller_event - PSE controller lifecycle events
+ *
+ * Event data in callbacks is always a pointer to the struct
+ * pse_controller_dev firing the event.
+ *
+ * @PSE_REGISTERED: controller added to pse_controller_list and
+ *	resolvable by of_pse_control_get().
+ * @PSE_UNREGISTERED: controller about to be removed from
+ *	pse_controller_list. Subscribers holding pse_control references
+ *	targeting it must drop them before returning and must not
+ *	acquire new references for it.
+ */
+enum pse_controller_event {
+	PSE_REGISTERED,
+	PSE_UNREGISTERED,
+};
+
 #if IS_ENABLED(CONFIG_PSE_CONTROLLER)
 int pse_controller_register(struct pse_controller_dev *pcdev);
 void pse_controller_unregister(struct pse_controller_dev *pcdev);
@@ -366,6 +385,9 @@ int pse_ethtool_set_prio(struct pse_control *psec,
 bool pse_has_podl(struct pse_control *psec);
 bool pse_has_c33(struct pse_control *psec);
 
+int pse_register_notifier(struct notifier_block *nb);
+int pse_unregister_notifier(struct notifier_block *nb);
+
 #else
 
 static inline struct pse_control *of_pse_control_get(struct device_node *node,
@@ -416,6 +438,16 @@ static inline bool pse_has_c33(struct pse_control *psec)
 	return false;
 }
 
+static inline int pse_register_notifier(struct notifier_block *nb)
+{
+	return 0;
+}
+
+static inline int pse_unregister_notifier(struct notifier_block *nb)
+{
+	return 0;
+}
+
 #endif
 
 #endif

-- 
2.53.0