of/overlay: add of overlay notifications

This patch add of overlay notifications.

When DT overlays are being added, some drivers/subsystems
need to see device tree overlays before the changes go into
the live tree.

This is distinct from reconfig notifiers that are
post-apply or post-remove and which issue very granular
notifications without providing access to the context
of a whole overlay.

The following 4 notificatons are issued:
  OF_OVERLAY_PRE_APPLY
  OF_OVERLAY_POST_APPLY
  OF_OVERLAY_PRE_REMOVE
  OF_OVERLAY_POST_REMOVE

In the case of pre-apply notification, if the notifier
returns error, the overlay will be rejected.

This patch exports two functions for registering/unregistering
notifications:
  of_overlay_notifier_register(struct notifier_block *nb)
  of_overlay_notifier_unregister(struct notifier_block *nb)

The of_mutex is held during these notifications. The
notification data includes pointers to the overlay target
and the overlay:

struct of_overlay_notify_data {
       struct device_node *overlay;
       struct device_node *target;
};
Signed-off-by: Alan Tull <atull@opensource.altera.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Moritz Fischer <moritz.fischer@ettus.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/of/overlay.c

@@ -58,6 +58,41 @@ struct of_overlay {
 static int of_overlay_apply_one(struct of_overlay *ov,
 		struct device_node *target, const struct device_node *overlay);
 
+static BLOCKING_NOTIFIER_HEAD(of_overlay_chain);
+
+int of_overlay_notifier_register(struct notifier_block *nb)
+{
+	return blocking_notifier_chain_register(&of_overlay_chain, nb);
+}
+EXPORT_SYMBOL_GPL(of_overlay_notifier_register);
+
+int of_overlay_notifier_unregister(struct notifier_block *nb)
+{
+	return blocking_notifier_chain_unregister(&of_overlay_chain, nb);
+}
+EXPORT_SYMBOL_GPL(of_overlay_notifier_unregister);
+
+static int of_overlay_notify(struct of_overlay *ov,
+			     enum of_overlay_notify_action action)
+{
+	struct of_overlay_notify_data nd;
+	int i, ret;
+
+	for (i = 0; i < ov->count; i++) {
+		struct of_overlay_info *ovinfo = &ov->ovinfo_tab[i];
+
+		nd.target = ovinfo->target;
+		nd.overlay = ovinfo->overlay;
+
+		ret = blocking_notifier_call_chain(&of_overlay_chain,
+						   action, &nd);
+		if (ret)
+			return notifier_to_errno(ret);
+	}
+
+	return 0;
+}
+
 static int of_overlay_apply_single_property(struct of_overlay *ov,
 		struct device_node *target, struct property *prop)
 {
@@ -368,6 +403,13 @@ int of_overlay_create(struct device_node *tree)
 		goto err_free_idr;
 	}
 
+	err = of_overlay_notify(ov, OF_OVERLAY_PRE_APPLY);
+	if (err < 0) {
+		pr_err("%s: Pre-apply notifier failed (err=%d)\n",
+		       __func__, err);
+		goto err_free_idr;
+	}
+
 	/* apply the overlay */
 	err = of_overlay_apply(ov);
 	if (err)
@@ -382,6 +424,8 @@ int of_overlay_create(struct device_node *tree)
 	/* add to the tail of the overlay list */
 	list_add_tail(&ov->node, &ov_list);
 
+	of_overlay_notify(ov, OF_OVERLAY_POST_APPLY);
+
 	mutex_unlock(&of_mutex);
 
 	return id;
@@ -498,9 +542,10 @@ int of_overlay_destroy(int id)
 		goto out;
 	}
 
-
+	of_overlay_notify(ov, OF_OVERLAY_PRE_REMOVE);
 	list_del(&ov->node);
 	__of_changeset_revert(&ov->cset);
+	of_overlay_notify(ov, OF_OVERLAY_POST_REMOVE);
 	of_free_overlay_info(ov);
 	idr_remove(&ov_idr, id);
 	of_changeset_destroy(&ov->cset);

include/linux/of.h

@@ -1266,6 +1266,18 @@ static inline bool of_device_is_system_power_controller(const struct device_node
  * Overlay support
  */
 
+enum of_overlay_notify_action {
+	OF_OVERLAY_PRE_APPLY,
+	OF_OVERLAY_POST_APPLY,
+	OF_OVERLAY_PRE_REMOVE,
+	OF_OVERLAY_POST_REMOVE,
+};
+
+struct of_overlay_notify_data {
+	struct device_node *overlay;
+	struct device_node *target;
+};
+
 #ifdef CONFIG_OF_OVERLAY
 
 /* ID based overlays; the API for external users */
@@ -1273,6 +1285,9 @@ int of_overlay_create(struct device_node *tree);
 int of_overlay_destroy(int id);
 int of_overlay_destroy_all(void);
 
+int of_overlay_notifier_register(struct notifier_block *nb);
+int of_overlay_notifier_unregister(struct notifier_block *nb);
+
 #else
 
 static inline int of_overlay_create(struct device_node *tree)
@@ -1290,6 +1305,16 @@ static inline int of_overlay_destroy_all(void)
 	return -ENOTSUPP;
 }
 
+static inline int of_overlay_notifier_register(struct notifier_block *nb)
+{
+	return 0;
+}
+
+static inline int of_overlay_notifier_unregister(struct notifier_block *nb)
+{
+	return 0;
+}
+
 #endif
 
 #endif /* _LINUX_OF_H */