driver core: fw_devlink: Allow marking a fwnode link as being part of a cycle

To improve detection and handling of dependency cycles, we need to be
able to mark fwnode links as being part of cycles. fwnode links marked
as being part of a cycle should not block their consumers from probing.

Fixes: 2de9d8e0d2fe ("driver core: fw_devlink: Improve handling of cyclic dependencies")
Signed-off-by: Saravana Kannan <[e-mail]>
Tested-by: Colin Foster <[e-mail]>
Tested-by: Sudeep Holla <[e-mail]>
Tested-by: Douglas Anderson <[e-mail]>
Tested-by: Geert Uytterhoeven <[e-mail]>
Tested-by: Luca Weiss <[e-mail]> # qcom/sm7225-fairphone-fp4
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/base/core.c

···
  * are ignored and there is no reference counting.
  */
 static int __fwnode_link_add(struct fwnode_handle *con,
-			     struct fwnode_handle *sup)
+			     struct fwnode_handle *sup, u8 flags)
 {
 	struct fwnode_link *link;
 
 	list_for_each_entry(link, &sup->consumers, s_hook)
-		if (link->consumer == con)
+		if (link->consumer == con) {
+			link->flags |= flags;
 			return 0;
+		}
 
 	link = kzalloc(sizeof(*link), GFP_KERNEL);
 	if (!link)
···
 	INIT_LIST_HEAD(&link->s_hook);
 	link->consumer = con;
 	INIT_LIST_HEAD(&link->c_hook);
+	link->flags = flags;
 
 	list_add(&link->s_hook, &sup->consumers);
 	list_add(&link->c_hook, &con->suppliers);
···
 	int ret;
 
 	mutex_lock(&fwnode_link_lock);
-	ret = __fwnode_link_add(con, sup);
+	ret = __fwnode_link_add(con, sup, 0);
 	mutex_unlock(&fwnode_link_lock);
 	return ret;
 }
···
 	list_del(&link->s_hook);
 	list_del(&link->c_hook);
 	kfree(link);
+}
+
+/**
+ * __fwnode_link_cycle - Mark a fwnode link as being part of a cycle.
+ * @link: the fwnode_link to be marked
+ *
+ * The fwnode_link_lock needs to be held when this function is called.
+ */
+static void __fwnode_link_cycle(struct fwnode_link *link)
+{
+	pr_debug("%pfwf: Relaxing link with %pfwf\n",
+		 link->consumer, link->supplier);
+	link->flags |= FWLINK_FLAG_CYCLE;
 }
 
 /**
···
 	struct fwnode_link *link, *tmp;
 
 	list_for_each_entry_safe(link, tmp, &from->consumers, s_hook) {
-		__fwnode_link_add(link->consumer, to);
+		__fwnode_link_add(link->consumer, to, link->flags);
 		__fwnode_link_del(link);
 	}
 }
···
 		(dev->fwnode && (dev->fwnode->flags & FWNODE_FLAG_BEST_EFFORT));
 }
 
+static struct fwnode_handle *fwnode_links_check_suppliers(
+						struct fwnode_handle *fwnode)
+{
+	struct fwnode_link *link;
+
+	if (!fwnode || fw_devlink_is_permissive())
+		return NULL;
+
+	list_for_each_entry(link, &fwnode->suppliers, c_hook)
+		if (!(link->flags & FWLINK_FLAG_CYCLE))
+			return link->supplier;
+
+	return NULL;
+}
+
 /**
  * device_links_check_suppliers - Check presence of supplier drivers.
  * @dev: Consumer device.
···
 	 * probe.
 	 */
 	mutex_lock(&fwnode_link_lock);
-	if (dev->fwnode && !list_empty(&dev->fwnode->suppliers) &&
-	    !fw_devlink_is_permissive()) {
-		sup_fw = list_first_entry(&dev->fwnode->suppliers,
-					  struct fwnode_link,
-					  c_hook)->supplier;
+	sup_fw = fwnode_links_check_suppliers(dev->fwnode);
+	if (sup_fw) {
 		if (!dev_is_best_effort(dev)) {
 			fwnode_ret = -EPROBE_DEFER;
 			dev_err_probe(dev, -EPROBE_DEFER,
···
 	bool val;
 
 	device_lock(dev);
-	val = !list_empty(&dev->fwnode->suppliers);
+	mutex_lock(&fwnode_link_lock);
+	val = !!fwnode_links_check_suppliers(dev->fwnode);
+	mutex_unlock(&fwnode_link_lock);
 	device_unlock(dev);
 	return sysfs_emit(buf, "%u\n", val);
 }

include/linux/fwnode.h

···
 struct device;
 
 /*
- * fwnode link flags
+ * fwnode flags
  *
  * LINKS_ADDED:	The fwnode has already be parsed to add fwnode links.
  * NOT_DEVICE:	The fwnode will never be populated as a struct device.
···
 #define FWNODE_FLAG_INITIALIZED			BIT(2)
 #define FWNODE_FLAG_NEEDS_CHILD_BOUND_ON_ADD	BIT(3)
 #define FWNODE_FLAG_BEST_EFFORT			BIT(4)
+#define FWNODE_FLAG_VISITED			BIT(5)
 
 struct fwnode_handle {
 	struct fwnode_handle *secondary;
···
 	u8 flags;
 };
 
+/*
+ * fwnode link flags
+ *
+ * CYCLE:	The fwnode link is part of a cycle. Don't defer probe.
+ */
+#define FWLINK_FLAG_CYCLE			BIT(0)
+
 struct fwnode_link {
 	struct fwnode_handle *supplier;
 	struct list_head s_hook;
 	struct fwnode_handle *consumer;
 	struct list_head c_hook;
+	u8 flags;
 };
 
 /**