Documentation: devlink: add nested instance section

Add a part talking about nested devlink instances describing
the helpers and locking ordering.
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/devlink/index.rst

@@ -18,6 +18,30 @@ netlink commands.
 
 Drivers are encouraged to use the devlink instance lock for their own needs.
 
+Nested instances
+----------------
+
+Some objects, like linecards or port functions, could have another
+devlink instances created underneath. In that case, drivers should make
+sure to respect following rules:
+
+ - Lock ordering should be maintained. If driver needs to take instance
+   lock of both nested and parent instances at the same time, devlink
+   instance lock of the parent instance should be taken first, only then
+   instance lock of the nested instance could be taken.
+ - Driver should use object-specific helpers to setup the
+   nested relationship:
+
+   - ``devl_nested_devlink_set()`` - called to setup devlink -> nested
+     devlink relationship (could be user for multiple nested instances.
+   - ``devl_port_fn_devlink_set()`` - called to setup port function ->
+     nested devlink relationship.
+   - ``devlink_linecard_nested_dl_set()`` - called to setup linecard ->
+     nested devlink relationship.
+
+The nested devlink info is exposed to the userspace over object-specific
+attributes of devlink netlink.
+
 Interface documentation
 -----------------------