driver core: Add device link support for SYNC_STATE_ONLY flag

Parent devices might need to create "proxy" device links from themselves
to supplier devices to make sure the supplier devices don't get a
sync_state() before the child consumer devices get a chance to add
device links to the supplier devices.

However, the parent device has no real dependency on the supplier device
and probing, suspend/resume or runtime PM don't need to be affected by
the supplier device.  To capture these cases, create a SYNC_STATE_ONLY
device link flag that only affects sync_state() behavior and doesn't
affect probing, suspend/resume or runtime PM.
Signed-off-by: Saravana Kannan <saravanak@google.com>
Link: https://lore.kernel.org/r/20191028220027.251605-2-saravanak@google.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/core.c

@@ -131,6 +131,9 @@ static int device_is_dependent(struct device *dev, void *target)
 		return ret;
 
 	list_for_each_entry(link, &dev->links.consumers, s_node) {
+		if (link->flags == (DL_FLAG_SYNC_STATE_ONLY | DL_FLAG_MANAGED))
+			continue;
+
 		if (link->consumer == target)
 			return 1;
 
@@ -200,8 +203,11 @@ static int device_reorder_to_tail(struct device *dev, void *not_used)
 		device_pm_move_last(dev);
 
 	device_for_each_child(dev, NULL, device_reorder_to_tail);
-	list_for_each_entry(link, &dev->links.consumers, s_node)
+	list_for_each_entry(link, &dev->links.consumers, s_node) {
+		if (link->flags == (DL_FLAG_SYNC_STATE_ONLY | DL_FLAG_MANAGED))
+			continue;
 		device_reorder_to_tail(link->consumer, NULL);
+	}
 
 	return 0;
 }
@@ -228,7 +234,8 @@ void device_pm_move_to_tail(struct device *dev)
 
 #define DL_MANAGED_LINK_FLAGS (DL_FLAG_AUTOREMOVE_CONSUMER | \
 			       DL_FLAG_AUTOREMOVE_SUPPLIER | \
-			       DL_FLAG_AUTOPROBE_CONSUMER)
+			       DL_FLAG_AUTOPROBE_CONSUMER  | \
+			       DL_FLAG_SYNC_STATE_ONLY)
 
 #define DL_ADD_VALID_FLAGS (DL_MANAGED_LINK_FLAGS | DL_FLAG_STATELESS | \
 			    DL_FLAG_PM_RUNTIME | DL_FLAG_RPM_ACTIVE)
@@ -296,6 +303,8 @@ struct device_link *device_link_add(struct device *consumer,
 
 	if (!consumer || !supplier || flags & ~DL_ADD_VALID_FLAGS ||
 	    (flags & DL_FLAG_STATELESS && flags & DL_MANAGED_LINK_FLAGS) ||
+	    (flags & DL_FLAG_SYNC_STATE_ONLY &&
+	     flags != DL_FLAG_SYNC_STATE_ONLY) ||
 	    (flags & DL_FLAG_AUTOPROBE_CONSUMER &&
 	     flags & (DL_FLAG_AUTOREMOVE_CONSUMER |
 		      DL_FLAG_AUTOREMOVE_SUPPLIER)))
@@ -316,11 +325,14 @@ struct device_link *device_link_add(struct device *consumer,
 
 	/*
 	 * If the supplier has not been fully registered yet or there is a
-	 * reverse dependency between the consumer and the supplier already in
-	 * the graph, return NULL.
+	 * reverse (non-SYNC_STATE_ONLY) dependency between the consumer and
+	 * the supplier already in the graph, return NULL. If the link is a
+	 * SYNC_STATE_ONLY link, we don't check for reverse dependencies
+	 * because it only affects sync_state() callbacks.
 	 */
 	if (!device_pm_initialized(supplier)
-	    || device_is_dependent(consumer, supplier)) {
+	    || (!(flags & DL_FLAG_SYNC_STATE_ONLY) &&
+		  device_is_dependent(consumer, supplier))) {
 		link = NULL;
 		goto out;
 	}
@@ -347,10 +359,15 @@ struct device_link *device_link_add(struct device *consumer,
 		}
 
 		if (flags & DL_FLAG_STATELESS) {
-			link->flags |= DL_FLAG_STATELESS;
 			kref_get(&link->kref);
+			if (link->flags & DL_FLAG_SYNC_STATE_ONLY &&
+			    !(link->flags & DL_FLAG_STATELESS)) {
+				link->flags |= DL_FLAG_STATELESS;
+				goto reorder;
+			} else {
 				goto out;
 			}
+		}
 
 		/*
 		 * If the life time of the link following from the new flags is
@@ -371,6 +388,12 @@ struct device_link *device_link_add(struct device *consumer,
 			link->flags |= DL_FLAG_MANAGED;
 			device_link_init_status(link, consumer, supplier);
 		}
+		if (link->flags & DL_FLAG_SYNC_STATE_ONLY &&
+		    !(flags & DL_FLAG_SYNC_STATE_ONLY)) {
+			link->flags &= ~DL_FLAG_SYNC_STATE_ONLY;
+			goto reorder;
+		}
+
 		goto out;
 	}
 
@@ -410,6 +433,13 @@ struct device_link *device_link_add(struct device *consumer,
 	    flags & DL_FLAG_PM_RUNTIME)
 		pm_runtime_resume(supplier);
 
+	if (flags & DL_FLAG_SYNC_STATE_ONLY) {
+		dev_dbg(consumer,
+			"Linked as a sync state only consumer to %s\n",
+			dev_name(supplier));
+		goto out;
+	}
+reorder:
 	/*
 	 * Move the consumer and all of the devices depending on it to the end
 	 * of dpm_list and the devices_kset list.
@@ -635,7 +665,8 @@ int device_links_check_suppliers(struct device *dev)
 	device_links_write_lock();
 
 	list_for_each_entry(link, &dev->links.suppliers, c_node) {
-		if (!(link->flags & DL_FLAG_MANAGED))
+		if (!(link->flags & DL_FLAG_MANAGED) ||
+		    link->flags & DL_FLAG_SYNC_STATE_ONLY)
 			continue;
 
 		if (link->status != DL_STATE_AVAILABLE) {
@@ -949,7 +980,8 @@ void device_links_unbind_consumers(struct device *dev)
 	list_for_each_entry(link, &dev->links.consumers, s_node) {
 		enum device_link_state status;
 
-		if (!(link->flags & DL_FLAG_MANAGED))
+		if (!(link->flags & DL_FLAG_MANAGED) ||
+		    link->flags & DL_FLAG_SYNC_STATE_ONLY)
 			continue;
 
 		status = link->status;

include/linux/device.h

@@ -1096,6 +1096,7 @@ enum device_link_state {
  * AUTOREMOVE_SUPPLIER: Remove the link automatically on supplier driver unbind.
  * AUTOPROBE_CONSUMER: Probe consumer driver automatically after supplier binds.
  * MANAGED: The core tracks presence of supplier/consumer drivers (internal).
+ * SYNC_STATE_ONLY: Link only affects sync_state() behavior.
  */
 #define DL_FLAG_STATELESS		BIT(0)
 #define DL_FLAG_AUTOREMOVE_CONSUMER	BIT(1)
@@ -1104,6 +1105,7 @@ enum device_link_state {
 #define DL_FLAG_AUTOREMOVE_SUPPLIER	BIT(4)
 #define DL_FLAG_AUTOPROBE_CONSUMER	BIT(5)
 #define DL_FLAG_MANAGED			BIT(6)
+#define DL_FLAG_SYNC_STATE_ONLY		BIT(7)
 
 /**
  * struct device_link - Device link representation.