Document the spec format used by netlink-raw families like rt and tc.
Reviewed-by: Jakub Kicinski <kuba@xxxxxxxxxx>
Signed-off-by: Donald Hunter <donald.hunter@xxxxxxxxx>
---
.../userspace-api/netlink/netlink-raw.rst | 96 ++++++++++++++++++-
1 file changed, 95 insertions(+), 1 deletion(-)
diff --git a/Documentation/userspace-api/netlink/netlink-raw.rst b/Documentation/userspace-api/netlink/netlink-raw.rst
index f07fb9b9c101..1e14f5f22b8e 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 +57,96 @@ group registration.
-
name: rtnlgrp-mctp-ifaddr
value: 34
+
+Sub-messages
+------------
+
+Several raw netlink families such as
+:doc:`rt_link<../../networking/netlink_spec/rt_link>` and
+:doc:`tc<../../networking/netlink_spec/tc>` use attribute nesting as an
+abstraction to carry module specific information.
+
+Conceptually it looks as follows::
+
+ [OUTER NEST OR MESSAGE LEVEL]
+ [GENERIC ATTR 1]
+ [GENERIC ATTR 2]
+ [GENERIC ATTR 3]
+ [GENERIC ATTR - wrapper]
+ [MODULE SPECIFIC ATTR 1]
+ [MODULE SPECIFIC ATTR 2]
+
+The ``GENERIC ATTRs`` at the outer level are defined in the core (or rt_link or
+core TC), while specific drivers, TC classifiers, qdiscs etc. can carry their
+own information wrapped in the ``GENERIC ATTR - wrapper``. Even though the
+example above shows attributes nesting inside the wrapper, the modules generally
+have full freedom to define the format of the nest. In practice the payload of
+the wrapper attr has very similar characteristics to a netlink message. It may
+contain a fixed header / structure, netlink attributes, or both. Because of
+those shared characteristics we refer to the payload of the wrapper attribute as
+a sub-message.
+
+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
