of: property: Create device links for all child-supplier depencencies

A parent device can have child devices that it adds when it probes. But
this probing of the parent device can happen way after kernel init is done
-- for example, when the parent device's driver is loaded as a module.

In such cases, if the child devices depend on a supplier in the system, we
need to make sure the supplier gets the sync_state() callback only after
these child devices are added and probed.

To achieve this, when creating device links for a device by looking at its
DT node, don't just look at DT references at the top node level. Look at DT
references in all the descendant nodes too and create device links from the
ancestor device to all these supplier devices.

This way, when the parent device probes and adds child devices, the child
devices can then create their own device links to the suppliers and further
delay the supplier's sync_state() callback to after the child devices are
probed.

Example:
In this illustration, -> denotes DT references and indentation
represents child status.

Device node A
	Device node B -> D
	Device node C -> B, D

Device node D

Assume all these devices have their drivers loaded as modules.

Without this patch, this is the sequence of events:
1. D is added.
2. A is added.
3. Device D probes.
4. Device D gets its sync_state() callback.
5. Device B and C might malfunction because their resources got
   altered/turned off before they can make active requests for them.

With this patch, this is the sequence of events:
1. D is added.
2. A is added and creates device links to D.
3. Device link from A to B is not added because A is a parent of B.
4. Device D probes.
5. Device D does not get it's sync_state() callback because consumer A
   hasn't probed yet.
5. Device A probes.
5. a. Devices B and C are added.
5. b. Device links from B and C to D are added.
5. c. Device A's probe completes.
6. Device D does not get it's sync_state() callback because consumer A
   has probed but consumers B and C haven't probed yet.
7. Device B and C probe.
8. Device D gets it's sync_state() callback because all its consumers
   have probed.
9. None of the devices malfunction.

Signed-off-by: Saravana Kannan <saravanak@google.com>
Link: https://lore.kernel.org/r/20190904211126.47518-7-saravanak@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/of/property.c

@@ -1207,6 +1207,10 @@ static int __of_link_to_suppliers(struct device *dev,
 		if (of_link_property(dev, con_np, p->name))
 			ret = -EAGAIN;
 
+	for_each_child_of_node(con_np, child)
+		if (__of_link_to_suppliers(dev, child))
+			ret = -EAGAIN;
+
 	return ret;
 }