netdev - [PATCH net-next v5 7/7] docs: netlink: document the sub-type attribute property

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Date:   Mon, 27 Mar 2023 09:31:38 +0100
From:   Donald Hunter <donald.hunter@...il.com>
To:     netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org
Cc:     donald.hunter@...hat.com, Donald Hunter <donald.hunter@...il.com>
Subject: [PATCH net-next v5 7/7] docs: netlink: document the sub-type attribute property

Add a definition for sub-type to the protocol spec doc and a description of
its usage for C arrays in genetlink-legacy.

Signed-off-by: Donald Hunter <donald.hunter@...il.com>
---
 .../userspace-api/netlink/genetlink-legacy.rst     | 14 ++++++++++++++
 Documentation/userspace-api/netlink/specs.rst      | 10 ++++++++++
 2 files changed, 24 insertions(+)

diff --git a/Documentation/userspace-api/netlink/genetlink-legacy.rst b/Documentation/userspace-api/netlink/genetlink-legacy.rst
index b8fdcf7f6615..802875a37a27 100644
--- a/Documentation/userspace-api/netlink/genetlink-legacy.rst
+++ b/Documentation/userspace-api/netlink/genetlink-legacy.rst
@@ -234,6 +234,20 @@ specify a sub-type.
           type: binary
           struct: vport-stats
 
+C Arrays
+--------
+
+Legacy families also use ``binary`` attributes to encapsulate C arrays. The
+``sub-type`` is used to identify the type of scalar to extract.
+
+.. code-block:: yaml
+
+  attributes:
+    -
+      name: ports
+      type: binary
+      sub-type: u32
+
 Multi-message DO
 ----------------
 
diff --git a/Documentation/userspace-api/netlink/specs.rst b/Documentation/userspace-api/netlink/specs.rst
index a22442ba1d30..2e4acde890b7 100644
--- a/Documentation/userspace-api/netlink/specs.rst
+++ b/Documentation/userspace-api/netlink/specs.rst
@@ -254,6 +254,16 @@ rather than depend on what is specified in the spec file.
 The validation policy in the kernel is formed by combining the type
 definition (``type`` and ``nested-attributes``) and the ``checks``.
 
+sub-type
+~~~~~~~~
+
+Legacy families have special ways of expressing arrays. ``sub-type`` can be
+used to define the type of array members in case array members are not
+fully defined as attributes (in a bona fide attribute space). For instance
+a C array of u32 values can be specified with ``type: binary`` and
+``sub-type: u32``. Binary types and legacy array formats are described in
+more detail in :doc:`genetlink-legacy`.
+
 operations
 ----------
 
-- 
2.39.0