[PATCH net-next v3 6/7] docs: netlink: document struct support for genetlink-legacy

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

Describe the genetlink-legacy support for using struct definitions
for fixed headers and for binary attributes.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
---
 .../netlink/genetlink-legacy.rst              | 54 ++++++++++++++++++-
 1 file changed, 52 insertions(+), 2 deletions(-)

diff --git a/Documentation/userspace-api/netlink/genetlink-legacy.rst b/Documentation/userspace-api/netlink/genetlink-legacy.rst
index 3bf0bcdf21d8..6b385a9e6d0b 100644
--- a/Documentation/userspace-api/netlink/genetlink-legacy.rst
+++ b/Documentation/userspace-api/netlink/genetlink-legacy.rst
@@ -163,8 +163,58 @@ Structures
 ----------
 
 Legacy families can define C structures both to be used as the contents
-of an attribute and as a fixed message header. The plan is to define
-the structs in ``definitions`` and link the appropriate attrs.
+of an attribute and as a fixed message header. Structs are defined
+in ``definitions`` and referenced in operations or attributes.
+
+.. code-block:: yaml
+
+  definitions:
+    -
+      name: message-header
+      type: struct
+      members:
+        -
+          name: a
+          type: u32
+        -
+          name: b
+          type: string
+
+Fixed Headers
+~~~~~~~~~~~~~
+
+Fixed message headers can be added to operations using ``fixed-header``.
+The default ``fixed-header`` can be set in ``operations`` and it can be set
+or overridden for each operation.
+
+.. code-block:: yaml
+
+  operations:
+    fixed-header: message-header
+    list:
+      -
+        name: get
+        fixed-header: custom-header
+        attribute-set: message-attrs
+
+Attributes
+~~~~~~~~~~
+
+A ``binary`` attribute can be interpreted as a C structure using a
+``struct`` property with the name of the structure definition. The
+``struct`` property implies ``sub-type: struct`` so it is not necessary to
+specify a sub-type.
+
+.. code-block:: yaml
+
+  attribute-sets:
+    -
+      name: stats-attrs
+      attributes:
+        -
+          name: stats
+          type: binary
+          struct: vport-stats
 
 Multi-message DO
 ----------------
-- 
2.39.0