expo: Add documentation for the configuration editor

This is mentioned in passing in the 'cedit' command. Its file format is
described under `expo`. But it would be better if it had its own entry
in the documentation.

Add a new 'cedit' entry with a few details about this feature.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/develop/cedit.rst

@@ -0,0 +1,147 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Configuration Editor
+====================
+
+Introduction
+------------
+
+U-Boot provides a configuration editor which allows settings to be changed in
+a GUI or text environment.
+
+
+This feature is still in development and has a number of limitations. For
+example, cedit only supports menu items (there is no numeric or text entry),
+provides no support for colour text and does not support scrolling. Still it is
+possible to use it for simple applications.
+
+
+Overview
+--------
+
+The configuration editor makes use of :doc:`expo` to build a description of the
+configuration screens and allow user to interact with it.
+
+To create a single-scene cedit for your application:
+
+#. Design the scene, i.e. the objects that need to be present and what their
+   possible values are
+
+#. Enter this in .dts format
+
+#. Create a header file containing the IDs
+
+#. Run the 'expo.py' tool to generate a .dtb file containing the layout, which
+   can be used by U-Boot
+
+#. Use the :doc:`../usage/cmd/cedit` to create the cedit, read the settings,
+   present the cedit to the user and save the settings afterwards.
+
+Each of these is described in a separate section. See :ref:`expo_example` for
+an example file.
+
+
+Design a scene
+--------------
+
+Using a piece of paper or a drawing tool, lay out the objects you want in your
+scene. Typically you will use the default layout engine, which simply puts items
+one after the other from top to bottom. So use a single column and show the
+prompt and value for each object.
+
+For menu items, show one of the values, but keep in mind what else you need.
+
+
+Create an expo-format file
+--------------------------
+
+The description is in the form of a devicetree file, as documented at
+:ref:`expo_format`. Since everything in an expo has an ID number (an integer
+greater than 1) the description is written terms of these IDs. They each have
+an enum value. which is typically taken care of by the `expo.py` tool.
+
+The expo should have a `scenes` node with a named scene as a subnode. Within the
+scene, add properties for the scene, then a subnode for each object in the
+scene.
+
+All object nodes require an `id` value and a `type` property. Other properties
+depend on the type. For example, a menu has a `title` and an `item-label` list
+proving the text for the menu items, as well as an `item-id` list providing the
+ID of each menu item, so it can be selected.
+
+Text properties may have two variants. For example `title` specifies the title
+of a menu, but you can instead use `title-id` to specify the string ID to use as
+the title. String are defined in a separate area, common to the whole expo,
+which contains a subnode for each string. Within that subnode are the ID and the
+`value` (i.e. the text). For now only English is supported, but in future it may
+be possible to append a language identifier to provide other values (e.g.
+'value-es' for Spanish).
+
+
+Create an ID header-file
+------------------------
+
+Expo needs to know the integer value to use for every ID referenced in your
+expo-format file. For example, if you have defined a `cpu-speed` node with an
+id of `ID_CPU_SPEED`, then Expo needs to know the value of `ID_CPU_SPEED`.
+
+When you write C code to use the expo, you may need to know the IDs. For
+example, to find which value the user selected in `cpu-speed` menu, you must
+use the `ID_CPU_SPEED` ID. The ID is the only way to refer to anything in Expo.
+
+Since we need a shared set of IDs, it is best to have a header file containing
+them. Expo supports doing this with an enum, where every ID is listed in the
+enum::
+
+    enum {
+        ZERO,
+
+        ID_PROMPT,
+
+        ID_SCENE1,
+        ID_SCENE1_TITLE,
+        ...
+    };
+
+The C compiler can parse this directly. The `expo.py` tool parses it for expo.
+
+Create a header file containing every ID mentioned in your expo. Try to group
+related things together.
+
+
+Build the expo layout
+---------------------
+
+Use the `expo.py` tool to build a .dtb for your expo::
+
+    ./tools/expo.py -e expo_ids.h -l expo_layout.dts -o expo.dtb
+
+This uses the enum in the provided header file to get the ID numbers, grabs
+the `.dts` file, inserts the ID numbers and then uses the devicetree compiler to
+build a `.dtb` file.
+
+If you get an error::
+
+    Devicetree compiler error:
+    Error: <stdin>:9.19-20 syntax error
+    FATAL ERROR: Unable to parse input tree
+
+that means that something is wrong with your syntax, or perhaps you have an ID
+in the `.dts` file that is not mentioned in your enum. Check both files and try
+again.
+
+
+Use the command interface
+-------------------------
+
+See the :doc:`../usage/cmd/cedit` command for information on available commands.
+Typically you will use `cedit load` to load the `.dtb` file and `cedit run` to
+let the user interact with it.
+
+
+Multiple scenes
+---------------
+
+Expo supports multiple scenes but has no pre-determined way of moving between
+them. You could use selection of a menu item as a signal to change the scene,
+but this is not currently implemented in the cedit code (see `cedit_run()`).

doc/develop/expo.rst

@@ -358,6 +358,9 @@ The `expo_arrange()` function can be called to arrange the expo objects in a
 suitable manner. For each scene it puts the title at the top, the prompt at the
 bottom and the objects in order from top to bottom.
 
+
+.. _expo_example:
+
 Expo format example
 ~~~~~~~~~~~~~~~~~~~
 

doc/develop/index.rst

@@ -38,6 +38,7 @@ Implementation
    driver-model/index
    environment
    expo
+   cedit
    event
    global_data
    logging

doc/usage/cmd/cedit.rst

@@ -22,6 +22,8 @@ It makes use of the expo subsystem.
 The description is in the form of a devicetree file, as documented at
 :ref:`expo_format`.
 
+See :doc:`../../develop/cedit` for information about the configuration editor.
+
 Example
 -------