docs: netlink: add a starting guide for working with specs

We have a bit of documentation about the internals of Netlink
and the specs, but really the goal is for most people to not
worry about those. Add a practical guide for beginners who
want to poke at the specs.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/userspace-api/netlink/index.rst

@@ -10,6 +10,7 @@ Netlink documentation for users.
    :maxdepth: 2
 
    intro
+   intro-specs
    specs
    c-code-gen
    genetlink-legacy

Documentation/userspace-api/netlink/intro-specs.rst

+.. SPDX-License-Identifier: BSD-3-Clause
+
+=====================================
+Using Netlink protocol specifications
+=====================================
+
+This document is a quick starting guide for using Netlink protocol
+specifications. For more detailed description of the specs see :doc:`specs`.
+
+Simple CLI
+==========
+
+Kernel comes with a simple CLI tool which should be useful when
+developing Netlink related code. The tool is implemented in Python
+and can use a YAML specification to issue Netlink requests
+to the kernel. Only Generic Netlink is supported.
+
+The tool is located at ``tools/net/ynl/cli.py``. It accepts
+a handul of arguments, the most important ones are:
+
+ - ``--spec`` - point to the spec file
+ - ``--do $name`` / ``--dump $name`` - issue request ``$name``
+ - ``--json $attrs`` - provide attributes for the request
+ - ``--subscribe $group`` - receive notifications from ``$group``
+
+YAML specs can be found under ``Documentation/netlink/specs/``.
+
+Example use::
+
+  $ ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/ethtool.yaml \
+        --do rings-get \
+	--json '{"header":{"dev-index": 18}}'
+  {'header': {'dev-index': 18, 'dev-name': 'eni1np1'},
+   'rx': 0,
+   'rx-jumbo': 0,
+   'rx-jumbo-max': 4096,
+   'rx-max': 4096,
+   'rx-mini': 0,
+   'rx-mini-max': 4096,
+   'tx': 0,
+   'tx-max': 4096,
+   'tx-push': 0}
+
+The input arguments are parsed as JSON, while the output is only
+Python-pretty-printed. This is because some Netlink types can't
+be expressed as JSON directly. If such attributes are needed in
+the input some hacking of the script will be necessary.
+
+The spec and Netlink internals are factored out as a standalone
+library - it should be easy to write Python tools / tests reusing
+code from ``cli.py``.
+
+Generating kernel code
+======================
+
+``tools/net/ynl/ynl-regen.sh`` scans the kernel tree in search of
+auto-generated files which need to be updated. Using this tool is the easiest
+way to generate / update auto-generated code.
+
+By default code is re-generated only if spec is newer than the source,
+to force regeneration use ``-f``.
+
+``ynl-regen.sh`` searches for ``YNL-GEN`` in the contents of files
+(note that it only scans files in the git index, that is only files
+tracked by git!) For instance the ``fou_nl.c`` kernel source contains::
+
+  /*	Documentation/netlink/specs/fou.yaml */
+  /* YNL-GEN kernel source */
+
+``ynl-regen.sh`` will find this marker and replace the file with
+kernel source based on fou.yaml.
+
+The simplest way to generate a new file based on a spec is to add
+the two marker lines like above to a file, add that file to git,
+and run the regeneration tool. Grep the tree for ``YNL-GEN``
+to see other examples.
+
+The code generation itself is performed by ``tools/net/ynl/ynl-gen-c.py``
+but it takes a few arguments so calling it directly for each file
+quickly becomes tedious.

Documentation/userspace-api/netlink/specs.rst

@@ -21,6 +21,9 @@ Internally kernel uses the YAML specs to generate:
 
 YAML specifications can be found under ``Documentation/netlink/specs/``
 
+This document describes details of the schema.
+See :doc:`intro-specs` for a practical starting guide.
+
 Compatibility levels
 ====================