[PATCH v6 06/13] firmware: arm_scmi: Add notification callbacks-registration

[Cristian Marussi]

Add core SCMI Notifications callbacks-registration support: allow users
to register their own callbacks against the desired events.
Whenever a registration request is issued against a still non existent
event, mark such request as pending for later processing, in order to
account for possible late initializations of SCMI Protocols associated
to loadable drivers.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
Signed-off-by: Cristian Marussi <cristian.marussi@xxxxxxx>
---
V4 --> V5
- fix kernel-doc
- reviewed REVT_NOTIFY_ENABLE macro
- added matching barrier for late_init
V3 --> V4
- split registered_handlers hashtable on a per-protocol basis to reduce
  unneeded contention
- introduced pending_handlers table and related late_init worker to finalize
  handlers registration upon effective protocols' registrations
- introduced further safe accessors macros for registered_protocols
  and registered_events arrays
V2 --> V3
- refactored get/put event_handler
- removed generic non-handle-based API
V1 --> V2
- splitted out of V1 patch 04
- moved from IDR maps to real HashTables to store event_handlers
- added proper enable_events refcounting via __scmi_enable_evt()
  [was broken in V1 when using ALL_SRCIDs notification chains]
- reviewed hashtable cleanup strategy in scmi_notifications_exit()
- added scmi_register_event_notifier()/scmi_unregister_event_notifier()
  to include/linux/scmi_protocol.h as a candidate user API
  [no EXPORTs still]
- added notify_ops to handle during initialization as an additional
  internal API for scmi_drivers
---
 drivers/firmware/arm_scmi/notify.c | 689 +++++++++++++++++++++++++++++
 drivers/firmware/arm_scmi/notify.h |  12 +
 include/linux/scmi_protocol.h      |  47 ++
 3 files changed, 748 insertions(+)

diff --git a/drivers/firmware/arm_scmi/notify.c b/drivers/firmware/arm_scmi/notify.c
index 15097bf2b52a..f1374860dc66 100644
--- a/drivers/firmware/arm_scmi/notify.c
+++ b/drivers/firmware/arm_scmi/notify.c
@@ -19,18 +19,50 @@
  * this core its set of supported events using scmi_register_protocol_events():
  * all the needed descriptors are stored in the &struct registered_protocols and
  * &struct registered_events arrays.
+ *
+ * Kernel users interested in some specific event can register their callbacks
+ * providing the usual notifier_block descriptor, since this core implements
+ * events' delivery using the standard Kernel notification chains machinery.
+ *
+ * Given the number of possible events defined by SCMI and the extensibility
+ * of the SCMI Protocol itself, the underlying notification chains are created
+ * and destroyed dynamically on demand depending on the number of users
+ * effectively registered for an event, so that no support structures or chains
+ * are allocated until at least one user has registered a notifier_block for
+ * such event. Similarly, events' generation itself is enabled at the platform
+ * level only after at least one user has registered, and it is shutdown after
+ * the last user for that event has gone.
+ *
+ * All users provided callbacks and allocated notification-chains are stored in
+ * the @registered_events_handlers hashtable. Callbacks' registration requests
+ * for still to be registered events are instead kept in the dedicated common
+ * hashtable @pending_events_handlers.
+ *
+ * An event is identified univocally by the tuple (proto_id, evt_id, src_id)
+ * and is served by its own dedicated notification chain; information contained
+ * in such tuples is used, in a few different ways, to generate the needed
+ * hash-keys.
+ *
+ * Here proto_id and evt_id are simply the protocol_id and message_id numbers
+ * as described in the SCMI Protocol specification, while src_id represents an
+ * optional, protocol dependent, source identifier (like domain_id, perf_id
+ * or sensor_id and so forth).
  */
 
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <linux/atomic.h>
+#include <linux/bitfield.h>
 #include <linux/bug.h>
 #include <linux/compiler.h>
 #include <linux/device.h>
 #include <linux/err.h>
+#include <linux/hashtable.h>
 #include <linux/kernel.h>
 #include <linux/kfifo.h>
+#include <linux/list.h>
 #include <linux/mutex.h>
+#include <linux/notifier.h>
 #include <linux/refcount.h>
 #include <linux/scmi_protocol.h>
 #include <linux/slab.h>
@@ -50,6 +82,74 @@
 #define MAKE_ALL_SRCS_KEY(p, e)			\
 	MAKE_HASH_KEY((p), (e), SCMI_ALL_SRC_IDS)
 
+/*
+ * Assumes that the stored obj includes its own hash-key in a field named 'key':
+ * with this simplification this macro can be equally used for all the objects'
+ * types hashed by this implementation.
+ *
+ * @__ht: The hashtable name
+ * @__obj: A pointer to the object type to be retrieved from the hashtable;
+ *	   it will be used as a cursor while scanning the hastable and it will
+ *	   be possibly left as NULL when @__k is not found
+ * @__k: The key to search for
+ */
+#define KEY_FIND(__ht, __obj, __k)				\
+({								\
+	hash_for_each_possible((__ht), (__obj), hash, (__k))	\
+		if (likely((__obj)->key == (__k)))		\
+			break;					\
+	__obj;							\
+})
+
+#define PROTO_ID_MASK			GENMASK(31, 24)
+#define EVT_ID_MASK			GENMASK(23, 16)
+#define SRC_ID_MASK			GENMASK(15, 0)
+#define KEY_XTRACT_PROTO_ID(key)	FIELD_GET(PROTO_ID_MASK, (key))
+#define KEY_XTRACT_EVT_ID(key)		FIELD_GET(EVT_ID_MASK, (key))
+#define KEY_XTRACT_SRC_ID(key)		FIELD_GET(SRC_ID_MASK, (key))
+
+/*
+ * A set of macros used to access safely @registered_protocols and
+ * @registered_events arrays; these are fixed in size and each entry is possibly
+ * populated at protocols' registration time and then only read but NEVER
+ * modified or removed.
+ */
+#define SCMI_GET_PROTO(__ni, __pid)					\
+({									\
+	struct scmi_registered_protocol_events_desc *__pd = NULL;	\
+									\
+	if ((__ni) && (__pid) < SCMI_MAX_PROTO)				\
+		__pd = READ_ONCE((__ni)->registered_protocols[(__pid)]);\
+	__pd;								\
+})
+
+#define SCMI_GET_REVT_FROM_PD(__pd, __eid)				\
+({									\
+	struct scmi_registered_event *__revt = NULL;			\
+									\
+	if ((__pd) && (__eid) < (__pd)->num_events)			\
+		__revt = READ_ONCE((__pd)->registered_events[(__eid)]);	\
+	__revt;								\
+})
+
+#define SCMI_GET_REVT(__ni, __pid, __eid)				\
+({									\
+	struct scmi_registered_event *__revt = NULL;			\
+	struct scmi_registered_protocol_events_desc *__pd = NULL;	\
+									\
+	__pd = SCMI_GET_PROTO((__ni), (__pid));				\
+	__revt = SCMI_GET_REVT_FROM_PD(__pd, (__eid));			\
+	__revt;								\
+})
+
+/* A couple of utility macros to limit cruft when calling protocols' helpers */
+#define REVT_NOTIFY_ENABLE(revt, eid, sid)				       \
+	((revt)->proto->ops->set_notify_enabled((revt)->proto->ni->handle,     \
+						(eid), (sid), true))
+#define REVT_NOTIFY_DISABLE(revt, eid, sid)				       \
+	((revt)->proto->ops->set_notify_enabled((revt)->proto->ni->handle,     \
+						(eid), (sid), false))
+
 struct scmi_registered_protocol_events_desc;
 
 /**
@@ -60,9 +160,13 @@ struct scmi_registered_protocol_events_desc;
  * @initialized: A flag that indicates if the core resources have been allocated
  *		 and protocols are allowed to register their supported events
  * @enabled: A flag to indicate events can be enabled and start flowing
+ * @init_work: A work item to perform final initializations of pending handlers
+ * @pending_mtx: A mutex to protect @pending_events_handlers
  * @registered_protocols: A statically allocated array containing pointers to
  *			  all the registered protocol-level specific information
  *			  related to events' handling
+ * @pending_events_handlers: An hashtable containing all pending events'
+ *			     handlers descriptors
  *
  * Each platform instance, represented by a handle, has its own instance of
  * the notification subsystem represented by this structure.
@@ -72,7 +176,12 @@ struct scmi_notify_instance {
 	struct scmi_handle				*handle;
 	atomic_t					initialized;
 	atomic_t					enabled;
+
+	struct work_struct				init_work;
+
+	struct mutex					pending_mtx;
 	struct scmi_registered_protocol_events_desc	**registered_protocols;
+	DECLARE_HASHTABLE(pending_events_handlers, 8);
 };
 
 /**
@@ -121,6 +230,9 @@ struct scmi_registered_event;
  * @registered_events: A dynamically allocated array holding all the registered
  *		       events' descriptors, whose fixed-size is determined at
  *		       compile time.
+ * @registered_mtx: A mutex to protect @registered_events_handlers
+ * @registered_events_handlers: An hashtable containing all events' handlers
+ *				descriptors registered for this protocol
  *
  * All protocols that register at least one event have their protocol-specific
  * information stored here, together with the embedded allocated events_queue.
@@ -141,6 +253,8 @@ struct scmi_registered_protocol_events_desc {
 	void					*in_flight;
 	int					num_events;
 	struct scmi_registered_event		**registered_events;
+	struct mutex				registered_mtx;
+	DECLARE_HASHTABLE(registered_events_handlers, 8);
 };
 
 /**
@@ -172,6 +286,38 @@ struct scmi_registered_event {
 	struct mutex					sources_mtx;
 };
 
+/**
+ * struct scmi_event_handler  - Event handler information
+ * @key: The used hashkey
+ * @users: A reference count for number of active users for this handler
+ * @r_evt: A reference to the associated registered event; when this is NULL
+ *	   this handler is pending, which means that identifies a set of
+ *	   callbacks intended to be attached to an event which is still not
+ *	   known nor registered by any protocol at that point in time
+ * @chain: The notification chain dedicated to this specific event tuple
+ * @hash: The hlist_node used for collision handling
+ * @enabled: A boolean which records if event's generation has been already
+ *	     enabled for this handler as a whole
+ *
+ * This structure collects all the information needed to process a received
+ * event identified by the tuple (proto_id, evt_id, src_id).
+ * These descriptors are stored in a per-protocol @registered_events_handlers
+ * table using as a key a value derived from that tuple.
+ */
+struct scmi_event_handler {
+	u32				key;
+	refcount_t			users;
+	struct scmi_registered_event	*r_evt;
+	struct blocking_notifier_head	chain;
+	struct hlist_node		hash;
+	bool				enabled;
+};
+
+#define IS_HNDL_PENDING(hndl)	((hndl)->r_evt == NULL)
+
+static void scmi_put_handler_unlocked(struct scmi_notify_instance *ni,
+				      struct scmi_event_handler *hndl);
+
 /**
  * scmi_kfifo_free()  - Devres action helper to free the kfifo
  * @kfifo: The kfifo to free
@@ -259,6 +405,10 @@ scmi_allocate_registered_protocol_desc(struct scmi_notify_instance *ni,
 		return ERR_PTR(-ENOMEM);
 	pd->num_events = num_events;
 
+	/* Initialize per protocol handlers table */
+	mutex_init(&pd->registered_mtx);
+	hash_init(pd->registered_events_handlers);
+
 	return pd;
 }
 
@@ -348,6 +498,12 @@ int scmi_register_protocol_events(const struct scmi_handle *handle,
 
 	devres_close_group(ni->handle->dev, ni->gid);
 
+	/*
+	 * Finalize any pending events' handler which could have been waiting
+	 * for this protocol's events registration.
+	 */
+	schedule_work(&ni->init_work);
+
 	return 0;
 
 err:
@@ -359,6 +515,533 @@ int scmi_register_protocol_events(const struct scmi_handle *handle,
 	return -ENOMEM;
 }
 
+/**
+ * scmi_allocate_event_handler()  - Allocate Event handler
+ * @ni: A reference to the notification instance to use
+ * @evt_key: 32bit key uniquely bind to the event identified by the tuple
+ *	     (proto_id, evt_id, src_id)
+ *
+ * Allocate an event handler and related notification chain associated with
+ * the provided event handler key.
+ * Note that, at this point, a related registered_event is still to be
+ * associated to this handler descriptor (hndl->r_evt == NULL), so the handler
+ * is initialized as pending.
+ *
+ * Context: Assumes to be called with @pending_mtx already acquired.
+ * Return: the freshly allocated structure on Success
+ */
+static struct scmi_event_handler *
+scmi_allocate_event_handler(struct scmi_notify_instance *ni, u32 evt_key)
+{
+	struct scmi_event_handler *hndl;
+
+	hndl = kzalloc(sizeof(*hndl), GFP_KERNEL);
+	if (!hndl)
+		return ERR_PTR(-ENOMEM);
+	hndl->key = evt_key;
+	BLOCKING_INIT_NOTIFIER_HEAD(&hndl->chain);
+	refcount_set(&hndl->users, 1);
+	/* New handlers are created pending */
+	hash_add(ni->pending_events_handlers, &hndl->hash, hndl->key);
+
+	return hndl;
+}
+
+/**
+ * scmi_free_event_handler()  - Free the provided Event handler
+ * @hndl: The event handler structure to free
+ *
+ * Context: Assumes to be called with proper locking acquired depending
+ *	    on the situation.
+ */
+static void scmi_free_event_handler(struct scmi_event_handler *hndl)
+{
+	hash_del(&hndl->hash);
+	kfree(hndl);
+}
+
+/**
+ * scmi_bind_event_handler()  - Helper to attempt binding an handler to an event
+ * @ni: A reference to the notification instance to use
+ * @hndl: The event handler to bind
+ *
+ * If an associated registered event is found, move the handler from the pending
+ * into the registered table.
+ *
+ * Context: Assumes to be called with @pending_mtx already acquired.
+ * Return: True if bind was successful, False otherwise
+ */
+static inline bool scmi_bind_event_handler(struct scmi_notify_instance *ni,
+					   struct scmi_event_handler *hndl)
+{
+	struct scmi_registered_event *r_evt;
+
+
+	r_evt = SCMI_GET_REVT(ni, KEY_XTRACT_PROTO_ID(hndl->key),
+			      KEY_XTRACT_EVT_ID(hndl->key));
+	if (unlikely(!r_evt))
+		return false;
+
+	/* Remove from pending and insert into registered */
+	hash_del(&hndl->hash);
+	hndl->r_evt = r_evt;
+	mutex_lock(&r_evt->proto->registered_mtx);
+	hash_add(r_evt->proto->registered_events_handlers,
+		 &hndl->hash, hndl->key);
+	mutex_unlock(&r_evt->proto->registered_mtx);
+
+	return true;
+}
+
+/**
+ * scmi_valid_pending_handler()  - Helper to check pending status of handlers
+ * @ni: A reference to the notification instance to use
+ * @hndl: The event handler to check
+ *
+ * An handler is considered pending when its r_evt == NULL, because the related
+ * event was still unknown at handler's registration time; anyway, since all
+ * protocols register their supported events once for all at protocols'
+ * initialization time, a pending handler cannot be considered valid anymore if
+ * the underlying event (which it is waiting for), belongs to an already
+ * initialized and registered protocol.
+ *
+ * Return: True if pending registration is still valid, False otherwise.
+ */
+static inline bool scmi_valid_pending_handler(struct scmi_notify_instance *ni,
+					      struct scmi_event_handler *hndl)
+{
+	struct scmi_registered_protocol_events_desc *pd;
+
+	if (unlikely(!IS_HNDL_PENDING(hndl)))
+		return false;
+
+	pd = SCMI_GET_PROTO(ni, KEY_XTRACT_PROTO_ID(hndl->key));
+	if (pd)
+		return false;
+
+	return true;
+}
+
+/**
+ * scmi_register_event_handler()  - Register whenever possible an Event handler
+ * @ni: A reference to the notification instance to use
+ * @hndl: The event handler to register
+ *
+ * At first try to bind an event handler to its associated event, then check if
+ * it was at least a valid pending handler: if it was not bound nor valid return
+ * false.
+ *
+ * Valid pending incomplete bindings will be periodically retried by a dedicated
+ * worker which is kicked each time a new protocol completes its own
+ * registration phase.
+ *
+ * Context: Assumes to be called with @pending_mtx acquired.
+ * Return: True if a normal or a valid pending registration has been completed,
+ *	   False otherwise
+ */
+static bool scmi_register_event_handler(struct scmi_notify_instance *ni,
+					struct scmi_event_handler *hndl)
+{
+	bool ret;
+
+	ret = scmi_bind_event_handler(ni, hndl);
+	if (ret) {
+		pr_info("SCMI Notifications: registered NEW handler - key:%X\n",
+			hndl->key);
+	} else {
+		ret = scmi_valid_pending_handler(ni, hndl);
+		if (ret)
+			pr_info("SCMI Notifications: registered PENDING handler - key:%X\n",
+				hndl->key);
+	}
+
+	return ret;
+}
+
+/**
+ * __scmi_event_handler_get_ops()  - Utility to get or create an event handler
+ * @ni: A reference to the notification instance to use
+ * @evt_key: The event key to use
+ * @create: A boolean flag to specify if a handler must be created when
+ *	    not already existent
+ *
+ * Search for the desired handler matching the key in both the per-protocol
+ * registered table and the common pending table:
+ * * if found adjust users refcount
+ * * if not found and @create is true, create and register the new handler:
+ *   handler could end up being registered as pending if no matching event
+ *   could be found.
+ *
+ * An handler is guaranteed to reside in one and only one of the tables at
+ * any one time; to ensure this the whole search and create is performed
+ * holding the @pending_mtx lock, with @registered_mtx additionally acquired
+ * if needed.
+ *
+ * Note that when a nested acquisition of these mutexes is needed the locking
+ * order is always (same as in @init_work):
+ * 1. pending_mtx
+ * 2. registered_mtx
+ *
+ * Events generation is NOT enabled right after creation within this routine
+ * since at creation time we usually want to have all setup and ready before
+ * events really start flowing.
+ *
+ * Return: A properly refcounted handler on Success, NULL on Failure
+ */
+static inline struct scmi_event_handler *
+__scmi_event_handler_get_ops(struct scmi_notify_instance *ni,
+			     u32 evt_key, bool create)
+{
+	struct scmi_registered_event *r_evt;
+	struct scmi_event_handler *hndl = NULL;
+
+	r_evt = SCMI_GET_REVT(ni, KEY_XTRACT_PROTO_ID(evt_key),
+			      KEY_XTRACT_EVT_ID(evt_key));
+
+	mutex_lock(&ni->pending_mtx);
+	/* Search registered events at first ... if possible at all */
+	if (likely(r_evt)) {
+		mutex_lock(&r_evt->proto->registered_mtx);
+		hndl = KEY_FIND(r_evt->proto->registered_events_handlers,
+				hndl, evt_key);
+		if (likely(hndl))
+			refcount_inc(&hndl->users);
+		mutex_unlock(&r_evt->proto->registered_mtx);
+	}
+
+	/* ...then amongst pending. */
+	if (unlikely(!hndl)) {
+		hndl = KEY_FIND(ni->pending_events_handlers, hndl, evt_key);
+		if (likely(hndl))
+			refcount_inc(&hndl->users);
+	}
+
+	/* Create if still not found and required */
+	if (!hndl && create) {
+		hndl = scmi_allocate_event_handler(ni, evt_key);
+		if (!IS_ERR_OR_NULL(hndl)) {
+			if (!scmi_register_event_handler(ni, hndl)) {
+				pr_info("SCMI Notifications: purging UNKNOWN handler - key:%X\n",
+					hndl->key);
+				/* this hndl can be only a pending one */
+				scmi_put_handler_unlocked(ni, hndl);
+				hndl = NULL;
+			}
+		}
+	}
+	mutex_unlock(&ni->pending_mtx);
+
+	return hndl;
+}
+
+static struct scmi_event_handler *
+scmi_get_handler(struct scmi_notify_instance *ni, u32 evt_key)
+{
+	return __scmi_event_handler_get_ops(ni, evt_key, false);
+}
+
+static struct scmi_event_handler *
+scmi_get_or_create_handler(struct scmi_notify_instance *ni, u32 evt_key)
+{
+	return __scmi_event_handler_get_ops(ni, evt_key, true);
+}
+
+/**
+ * __scmi_enable_evt()  - Enable/disable events generation
+ * @r_evt: The registered event to act upon
+ * @src_id: The src_id to act upon
+ * @enable: The action to perform: true->Enable, false->Disable
+ *
+ * Takes care of proper refcounting while performing enable/disable: handles
+ * the special case of ALL sources requests by itself.
+ *
+ * Return: True when the required action has been successfully executed
+ */
+static inline bool __scmi_enable_evt(struct scmi_registered_event *r_evt,
+				     u32 src_id, bool enable)
+{
+	int ret = 0;
+	u32 num_sources;
+	refcount_t *sid;
+
+	if (src_id == SCMI_ALL_SRC_IDS) {
+		src_id = 0;
+		num_sources = r_evt->num_sources;
+	} else if (src_id < r_evt->num_sources) {
+		num_sources = 1;
+	} else {
+		return ret;
+	}
+
+	mutex_lock(&r_evt->sources_mtx);
+	if (enable) {
+		for (; num_sources; src_id++, num_sources--) {
+			bool r;
+
+			sid = &r_evt->sources[src_id];
+			if (refcount_read(sid) == 0) {
+				r = REVT_NOTIFY_ENABLE(r_evt,
+						       r_evt->evt->id, src_id);
+				if (r)
+					refcount_set(sid, 1);
+			} else {
+				refcount_inc(sid);
+				r = true;
+			}
+			ret += r;
+		}
+	} else {
+		for (; num_sources; src_id++, num_sources--) {
+			sid = &r_evt->sources[src_id];
+			if (refcount_dec_and_test(sid))
+				REVT_NOTIFY_DISABLE(r_evt,
+						    r_evt->evt->id, src_id);
+		}
+		ret = 1;
+	}
+	mutex_unlock(&r_evt->sources_mtx);
+
+	return ret;
+}
+
+static bool scmi_enable_events(struct scmi_event_handler *hndl)
+{
+	if (!hndl->enabled)
+		hndl->enabled = __scmi_enable_evt(hndl->r_evt,
+						  KEY_XTRACT_SRC_ID(hndl->key),
+						  true);
+	return hndl->enabled;
+}
+
+static bool scmi_disable_events(struct scmi_event_handler *hndl)
+{
+	if (hndl->enabled)
+		hndl->enabled = !__scmi_enable_evt(hndl->r_evt,
+						   KEY_XTRACT_SRC_ID(hndl->key),
+						   false);
+	return !hndl->enabled;
+}
+
+/**
+ * scmi_put_handler_unlocked()  - Put an event handler
+ * @ni: A reference to the notification instance to use
+ * @hndl: The event handler to act upon
+ *
+ * After having got exclusive access to the registered handlers hashtable,
+ * update the refcount and if @hndl is no more in use by anyone:
+ * * ask for events' generation disabling
+ * * unregister and free the handler itself
+ *
+ * Context: Assumes all the proper locking has been managed by the caller.
+ */
+static void
+scmi_put_handler_unlocked(struct scmi_notify_instance *ni,
+				struct scmi_event_handler *hndl)
+{
+	if (refcount_dec_and_test(&hndl->users)) {
+		if (likely(!IS_HNDL_PENDING(hndl)))
+			scmi_disable_events(hndl);
+		scmi_free_event_handler(hndl);
+	}
+}
+
+static void scmi_put_handler(struct scmi_notify_instance *ni,
+			     struct scmi_event_handler *hndl)
+{
+	struct scmi_registered_event *r_evt = hndl->r_evt;
+
+	mutex_lock(&ni->pending_mtx);
+	if (r_evt)
+		mutex_lock(&r_evt->proto->registered_mtx);
+
+	scmi_put_handler_unlocked(ni, hndl);
+
+	if (r_evt)
+		mutex_unlock(&r_evt->proto->registered_mtx);
+	mutex_unlock(&ni->pending_mtx);
+}
+
+/**
+ * scmi_event_handler_enable_events()  - Enable events associated to an handler
+ * @hndl: The Event handler to act upon
+ *
+ * Return: True on success
+ */
+static bool scmi_event_handler_enable_events(struct scmi_event_handler *hndl)
+{
+	if (!scmi_enable_events(hndl)) {
+		pr_err("SCMI Notifications: Failed to ENABLE events for key:%X !\n",
+		       hndl->key);
+		return false;
+	}
+
+	return true;
+}
+
+/**
+ * scmi_register_notifier()  - Register a notifier_block for an event
+ * @handle: The handle identifying the platform instance against which the
+ *	    callback is registered
+ * @proto_id: Protocol ID
+ * @evt_id: Event ID
+ * @src_id: Source ID, when NULL register for events coming form ALL possible
+ *	    sources
+ * @nb: A standard notifier block to register for the specified event
+ *
+ * Generic helper to register a notifier_block against a protocol event.
+ *
+ * A notifier_block @nb will be registered for each distinct event identified
+ * by the tuple (proto_id, evt_id, src_id) on a dedicated notification chain
+ * so that:
+ *
+ *	(proto_X, evt_Y, src_Z) --> chain_X_Y_Z
+ *
+ * @src_id meaning is protocol specific and identifies the origin of the event
+ * (like domain_id, sensor_id and so forth).
+ *
+ * @src_id can be NULL to signify that the caller is interested in receiving
+ * notifications from ALL the available sources for that protocol OR simply that
+ * the protocol does not support distinct sources.
+ *
+ * As soon as one user for the specified tuple appears, an handler is created,
+ * and that specific event's generation is enabled at the platform level, unless
+ * an associated registered event is found missing, meaning that the needed
+ * protocol is still to be initialized and the handler has just been registered
+ * as still pending.
+ *
+ * Return: Return 0 on Success
+ */
+static int scmi_register_notifier(const struct scmi_handle *handle,
+				  u8 proto_id, u8 evt_id, u32 *src_id,
+				  struct notifier_block *nb)
+{
+	int ret = 0;
+	u32 evt_key;
+	struct scmi_event_handler *hndl;
+	struct scmi_notify_instance *ni = handle->notify_priv;
+
+	if (unlikely(!ni || !atomic_read(&ni->initialized)))
+		return 0;
+
+	evt_key = MAKE_HASH_KEY(proto_id, evt_id,
+				src_id ? *src_id : SCMI_ALL_SRC_IDS);
+	hndl = scmi_get_or_create_handler(ni, evt_key);
+	if (IS_ERR_OR_NULL(hndl))
+		return PTR_ERR(hndl);
+
+	blocking_notifier_chain_register(&hndl->chain, nb);
+
+	/* Enable events for not pending handlers */
+	if (likely(!IS_HNDL_PENDING(hndl))) {
+		if (!scmi_event_handler_enable_events(hndl)) {
+			scmi_put_handler(ni, hndl);
+			ret = -EINVAL;
+		}
+	}
+
+	return ret;
+}
+
+/**
+ * scmi_unregister_notifier()  - Unregister a notifier_block for an event
+ * @handle: The handle identifying the platform instance against which the
+ *	    callback is unregistered
+ * @proto_id: Protocol ID
+ * @evt_id: Event ID
+ * @src_id: Source ID
+ * @nb: The notifier_block to unregister
+ *
+ * Takes care to unregister the provided @nb from the notification chain
+ * associated to the specified event and, if there are no more users for the
+ * event handler, frees also the associated event handler structures.
+ * (this could possibly cause disabling of event's generation at platform level)
+ *
+ * Return: 0 on Success
+ */
+static int scmi_unregister_notifier(const struct scmi_handle *handle,
+				    u8 proto_id, u8 evt_id, u32 *src_id,
+				    struct notifier_block *nb)
+{
+	u32 evt_key;
+	struct scmi_event_handler *hndl;
+	struct scmi_notify_instance *ni = handle->notify_priv;
+
+	if (unlikely(!ni || !atomic_read(&ni->initialized)))
+		return 0;
+
+	evt_key = MAKE_HASH_KEY(proto_id, evt_id,
+				src_id ? *src_id : SCMI_ALL_SRC_IDS);
+	hndl = scmi_get_handler(ni, evt_key);
+	if (IS_ERR_OR_NULL(hndl))
+		return -EINVAL;
+
+	blocking_notifier_chain_unregister(&hndl->chain, nb);
+	scmi_put_handler(ni, hndl);
+
+	/*
+	 * Free the handler (and stop events) if this happens to be the last
+	 * known user callback for this handler; a possible concurrently ongoing
+	 * run of @scmi_lookup_and_call_event_chain will cause this to happen
+	 * in that context safely instead.
+	 */
+	scmi_put_handler(ni, hndl);
+
+	return 0;
+}
+
+/**
+ * scmi_protocols_late_init()  - Worker for late initialization
+ * @work: The work item to use associated to the proper SCMI instance
+ *
+ * This kicks in whenever a new protocol has completed its own registration via
+ * scmi_register_protocol_events(): it is in charge of scanning the table of
+ * pending handlers (registered by users while the related protocol was still
+ * not initialized) and finalizing their initialization whenever possible;
+ * invalid pending handlers are purged at this point in time.
+ */
+static void scmi_protocols_late_init(struct work_struct *work)
+{
+	int bkt;
+	struct scmi_event_handler *hndl;
+	struct scmi_notify_instance *ni;
+	struct hlist_node *tmp;
+
+	ni = container_of(work, struct scmi_notify_instance, init_work);
+
+	/* Ensure protocols and events are up to date */
+	smp_rmb();
+
+	mutex_lock(&ni->pending_mtx);
+	hash_for_each_safe(ni->pending_events_handlers, bkt, tmp, hndl, hash) {
+		bool ret;
+
+		ret = scmi_bind_event_handler(ni, hndl);
+		if (ret) {
+			pr_info("SCMI Notifications: finalized PENDING handler - key:%X\n",
+				hndl->key);
+			ret = scmi_event_handler_enable_events(hndl);
+		} else {
+			ret = scmi_valid_pending_handler(ni, hndl);
+		}
+		if (!ret) {
+			pr_info("SCMI Notifications: purging PENDING handler - key:%X\n",
+				hndl->key);
+			/* this hndl can be only a pending one */
+			scmi_put_handler_unlocked(ni, hndl);
+		}
+	}
+	mutex_unlock(&ni->pending_mtx);
+}
+
+/*
+ * notify_ops are attached to the handle so that can be accessed
+ * directly from an scmi_driver to register its own notifiers.
+ */
+static struct scmi_notify_ops notify_ops = {
+	.register_event_notifier = scmi_register_notifier,
+	.unregister_event_notifier = scmi_unregister_notifier,
+};
+
 /**
  * scmi_notification_init()  - Initializes Notification Core Support
  * @handle: The handle identifying the platform instance to initialize
@@ -406,7 +1089,13 @@ int scmi_notification_init(struct scmi_handle *handle)
 	if (!ni->registered_protocols)
 		goto err;
 
+	mutex_init(&ni->pending_mtx);
+	hash_init(ni->pending_events_handlers);
+
+	INIT_WORK(&ni->init_work, scmi_protocols_late_init);
+
 	handle->notify_priv = ni;
+	handle->notify_ops = &notify_ops;
 
 	atomic_set(&ni->initialized, 1);
 	atomic_set(&ni->enabled, 1);
diff --git a/drivers/firmware/arm_scmi/notify.h b/drivers/firmware/arm_scmi/notify.h
index 54094aaf812a..f0561fb30970 100644
--- a/drivers/firmware/arm_scmi/notify.h
+++ b/drivers/firmware/arm_scmi/notify.h
@@ -9,9 +9,21 @@
 #ifndef _SCMI_NOTIFY_H
 #define _SCMI_NOTIFY_H
 
+#include <linux/bug.h>
 #include <linux/device.h>
 #include <linux/types.h>
 
+#define MAP_EVT_TO_ENABLE_CMD(id, map)			\
+({							\
+	int ret = -1;					\
+							\
+	if (likely((id) < ARRAY_SIZE((map))))		\
+		ret = (map)[(id)];			\
+	else						\
+		WARN(1, "UN-KNOWN evt_id:%d\n", (id));	\
+	ret;						\
+})
+
 /**
  * struct scmi_event  - Describes an event to be supported
  * @id: Event ID
diff --git a/include/linux/scmi_protocol.h b/include/linux/scmi_protocol.h
index 0679f10ab05e..934b91d29311 100644
--- a/include/linux/scmi_protocol.h
+++ b/include/linux/scmi_protocol.h
@@ -9,6 +9,8 @@
 #define _LINUX_SCMI_PROTOCOL_H
 
 #include <linux/device.h>
+#include <linux/ktime.h>
+#include <linux/notifier.h>
 #include <linux/types.h>
 
 #define SCMI_MAX_STR_SIZE	16
@@ -211,6 +213,49 @@ struct scmi_reset_ops {
 	int (*deassert)(const struct scmi_handle *handle, u32 domain);
 };
 
+/**
+ * struct scmi_notify_ops  - represents notifications' operations provided by
+ * SCMI core
+ * @register_event_notifier: Register a notifier_block for the requested event
+ * @unregister_event_notifier: Unregister a notifier_block for the requested
+ *			       event
+ *
+ * A user can register/unregister its own notifier_block against the wanted
+ * platform instance regarding the desired event identified by the
+ * tuple: (proto_id, evt_id, src_id) using the provided register/unregister
+ * interface where:
+ *
+ * @handle: The handle identifying the platform instance to use
+ * @proto_id: The protocol ID as in SCMI Specification
+ * @evt_id: The message ID of the desired event as in SCMI Specification
+ * @src_id: A pointer to the desired source ID if different sources are
+ *	    possible for the protocol (like domain_id, sensor_id...etc)
+ *
+ * @src_id can be provided as NULL if it simply does NOT make sense for
+ * the protocol at hand, OR if the user is explicitly interested in
+ * receiving notifications from ANY existent source associated to the
+ * specified proto_id / evt_id.
+ *
+ * Received notifications are finally delivered to the registered users,
+ * invoking the callback provided with the notifier_block *nb as follows:
+ *
+ *	int user_cb(nb, evt_id, report)
+ *
+ * with:
+ *
+ * @nb: The notifier block provided by the user
+ * @evt_id: The message ID of the delivered event
+ * @report: A custom struct describing the specific event delivered
+ */
+struct scmi_notify_ops {
+	int (*register_event_notifier)(const struct scmi_handle *handle,
+				       u8 proto_id, u8 evt_id, u32 *src_id,
+				       struct notifier_block *nb);
+	int (*unregister_event_notifier)(const struct scmi_handle *handle,
+					 u8 proto_id, u8 evt_id, u32 *src_id,
+					 struct notifier_block *nb);
+};
+
 /**
  * struct scmi_handle - Handle returned to ARM SCMI clients for usage.
  *
@@ -221,6 +266,7 @@ struct scmi_reset_ops {
  * @clk_ops: pointer to set of clock protocol operations
  * @sensor_ops: pointer to set of sensor protocol operations
  * @reset_ops: pointer to set of reset protocol operations
+ * @notify_ops: pointer to set of notifications related operations
  * @perf_priv: pointer to private data structure specific to performance
  *	protocol(for internal use only)
  * @clk_priv: pointer to private data structure specific to clock
@@ -242,6 +288,7 @@ struct scmi_handle {
 	struct scmi_power_ops *power_ops;
 	struct scmi_sensor_ops *sensor_ops;
 	struct scmi_reset_ops *reset_ops;
+	struct scmi_notify_ops *notify_ops;
 	/* for protocol internal use */
 	void *perf_priv;
 	void *clk_priv;
-- 
2.17.1