eng: Add interface specification how-to

Update #3715.

eng/req/howto.rst

@@ -0,0 +1,88 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. Copyright (C) 2020 embedded brains GmbH (http://www.embedded-brains.de)
+
+How-To
+======
+
+Interface Specification
+-----------------------
+
+.. _ReqEngAddAPIHeaderFile:
+
+Specify an API Header File
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The RTEMS :term:`API` header files are specified under ``spec:/if/rtems/*``.
+Create a subdirectory with a corresponding name for the API, for example in
+:file:`spec/if/rtems/foo` for the `foo` API.  In this new subdirectory place an
+:ref:`SpecTypeInterfaceHeaderFileItemType` item named :file:`header.yml`
+(:file:`spec/if/rtems/foo/header.yml`) and populate it with the required
+attributes.
+
+.. code-block:: yaml
+
+    SPDX-License-Identifier: CC-BY-SA-4.0 OR BSD-2-Clause
+    copyrights:
+    - Copyright (C) 2020 embedded brains GmbH (http://www.embedded-brains.de)
+    enabled-by: true
+    interface-type: header-file
+    links:
+    - role: interface-placement
+      uid: /if/domains/api
+    path: rtems/rtems/foo.h
+    prefix: cpukit/include
+    type: interface
+
+Specify an API Element
+^^^^^^^^^^^^^^^^^^^^^^
+
+Figure out the corresponding header file item.  If it does not exist, see
+:ref:`ReqEngAddAPIHeaderFile`.  Place a specialization of an
+:ref:`SpecTypeInterfaceItemType` item into the directory of the header file
+item, for example :file:`spec/if/rtems/foo/bar.yml` for the :c:func:`bar`
+function.  Add the required attributes for the new interface item.  Do not hard
+code interface names which are used to define the new interface.  Use
+``${uid-of-interface-item:/name}`` instead.  If the referenced interface is
+specified in the same directory, then use a relative UID.  Using interface
+references creates implicit dependencies and helps the header file generator to
+resolve the interface dependencies and header file includes for you.  Use
+:ref:`SpecTypeInterfaceUnspecifiedItemType` items for interface dependencies to
+other domains such as the C language, the compiler, the implementation, or
+user-provided defines.  To avoid cyclic dependencies between types you may use
+an :ref:`SpecTypeInterfaceForwardDeclarationItemType` item.
+
+.. code-block:: yaml
+
+    SPDX-License-Identifier: CC-BY-SA-4.0 OR BSD-2-Clause
+    brief: Tries to create a magic object and returns it.
+    copyrights:
+    - Copyright (C) 2020 embedded brains GmbH (http://www.embedded-brains.de)
+    definition:
+      default:
+        body: null
+        params:
+        - ${magic-wand:/name} ${.:/params[0]/name}
+        return: ${magic-type:/name} *
+      variants: []
+    description: |
+      The magic object is created out of nothing with the help of a magic wand.
+    enabled-by: true
+    interface-type: function
+    links:
+    - role: interface-placement
+      uid: header
+    - role: interface-ingroup
+      uid: /groups/api/classic/foo
+    name: bar
+    notes: null
+    params:
+    - description: is the magic wand.
+      dir: null
+      name: magic_wand
+    return:
+      return: Otherwise, the magic object is returned.
+      return-values:
+      - description: The caller did not have enough magic power.
+        value: ${/if/c/null}
+    type: interface

eng/req/index.rst

@@ -89,3 +89,4 @@ All procedures should be based on a peer review principles.
     traceability
     management
     tooling
+    howto