[PATCH net-next v2 06/11] doc/netlink: Document the sub-message format for netlink-raw

[Donald Hunter <donald.hunter@gmail.com>]

Document the spec format used by netlink-raw families like rt and tc.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
---
 .../userspace-api/netlink/netlink-raw.rst     | 75 ++++++++++++++++++-
 1 file changed, 74 insertions(+), 1 deletion(-)

diff --git a/Documentation/userspace-api/netlink/netlink-raw.rst b/Documentation/userspace-api/netlink/netlink-raw.rst
index f07fb9b9c101..e1d3e5ecfc6a 100644
--- a/Documentation/userspace-api/netlink/netlink-raw.rst
+++ b/Documentation/userspace-api/netlink/netlink-raw.rst
@@ -14,7 +14,8 @@ Specification
 The netlink-raw schema extends the :doc:`genetlink-legacy <genetlink-legacy>`
 schema with properties that are needed to specify the protocol numbers and
 multicast IDs used by raw netlink families. See :ref:`classic_netlink` for more
-information.
+information. The raw netlink families also make use of type-specific
+sub-messages.
 
 Globals
 -------
@@ -56,3 +57,75 @@ group registration.
       -
         name: rtnlgrp-mctp-ifaddr
         value: 34
+
+Sub-messages
+------------
+
+Several raw netlink families such as rt_link and tc have type-specific
+sub-messages. These sub-messages can appear as an attribute in a top-level or a
+nested attribute space.
+
+A sub-message attribute uses the value of another attribute as a selector key to
+choose the right sub-message format. For example if the following attribute has
+already been decoded:
+
+.. code-block:: json
+
+  { "kind": "gre" }
+
+and we encounter the following attribute spec:
+
+.. code-block:: yaml
+
+  -
+    name: data
+    type: sub-message
+    sub-message: linkinfo-data-msg
+    selector: kind
+
+Then we look for a sub-message definition called 'linkinfo-data-msg' and use the
+value of the 'kind' attribute i.e. 'gre' as the key to choose the correct format
+for the sub-message:
+
+.. code-block:: yaml
+
+  sub-messages:
+    name: linkinfo-data-msg
+    formats:
+      -
+        value: bridge
+        attribute-set: linkinfo-bridge-attrs
+      -
+        value: gre
+        attribute-set: linkinfo-gre-attrs
+      -
+        value: geneve
+        attribute-set: linkinfo-geneve-attrs
+
+This would decode the attribute value as a sub-message with the attribute-set
+called 'linkinfo-gre-attrs' as the attribute space.
+
+A sub-message can have an optional 'fixed-header' followed by zero or more
+attributes from an attribute-set. For example the following 'tc-options-msg'
+sub-message defines message formats that use a mixture of fixed-header,
+attribute-set or both together:
+
+.. code-block:: yaml
+
+  sub-messages:
+    -
+      name: tc-options-msg
+      formats:
+        -
+          value: bfifo
+          fixed-header: tc-fifo-qopt
+        -
+          value: cake
+          attribute-set: tc-cake-attrs
+        -
+          value: netem
+          fixed-header: tc-netem-qopt
+          attribute-set: tc-netem-attrs
+
+Note that a selector attribute must appear in a netlink message before any
+sub-message attributes that depend on it.
-- 
2.42.0