U-Boot provides a menu implementation for use with selecting bootflows and changing U-Boot settings. This is in early stages of development.
U-Boot already has a text-based menu system accessed via the :doc:`../usage/cmd/bootmenu`. This works using environment variables, or via some EFI-specific hacks.
The command makes use of a lower-level menu implementation, which is quite flexible and can be used to make menu hierarchies.
However this system is not flexible enough for use with standard boot. It does not support a graphical user interface and cannot currently support anything more than a very simple list of items. While it does support multiple menus in hierarchies, these are implemented by the caller. See for example eficonfig.c.
Another challenge with the current menu implementation is that it controls the event loop, such that bootmenu_loop() does not return until a key is pressed. This makes it difficult to implement dynamic displays or to do other things while the menu is running, such as searching for more bootflows.
For these reasons an attempt has been made to develop a more flexible system which can handle menus as well as other elements. This is called 'expo', short for exposition, in an attempt to avoid common words like display, screen, menu and the like. The primary goal is to support Verified Boot for Embedded (VBE), although it is available to any boot method, using the 'bootflow menu' command.
Efforts have been made to use common code with the existing menu, including key processing in particular.
Previous work looked at integrating Nuklear into U-Boot. This works fine and could provide a way to provide a more flexible UI, perhaps with expo dealing with the interface to Nuklear. But this is quite a big step and it may be years before this becomes desirable, if at all. For now, U-Boot only needs a fairly simple set of menus and options, so rendering them directly is fairly straightforward.
The creator of the expo is here called a controller and it controls most aspects of the expo. This is the code that you must write to use expo.
An expo is a set of scenes which can be presented to the user one at a time, to show information and obtain input from the user.
A scene is a collection of objects which are displayed together on the screen. Only one scene is visible at a time and scenes do not share objects.
A scene object is something that appears in the scene, such as some text, an image or a menu. Objects can be positioned and hidden.
A menu object contains a title, a set of menu items and a pointer to the current item. Menu items consist of a keypress (indicating what to press to select the item), label and description. All three are shown in a single line within the menu. Items can also have a preview image, which is shown when the item is highlighted.
All components have a name. This is purely for debugging, so it is easy to see what object is referred to. Of course the ID numbers can help as well, but they are less easy to distinguish.
While the expo implementation provides support for handling keypresses and rendering on the display or serial port, it does not actually deal with reading input from the user, nor what should be done when a particular menu item is selected. This is deliberate since having the event loop outside the expo is more flexible, particularly in a single-threaded environment like U-Boot.
Everything within an expo has a unique ID number. This is done so that it is easy to refer to things after the expo has been created. The expectation is that the controller declares an enum containing all of the elements in the expo, passing the ID of each object as it is created. When a menu item is selected, its ID is returned. When a object's font or position needs to change, the ID is passed to expo functions to indicate which object it is. It is possible for expo to auto-allocate IDs, but this is not recommended. The use of IDs is a convenience, removing the need for the controller to store pointers to objects, or even the IDs of objects. Programmatic creation of many items in a loop can be handled by allocating space in the enum for a maximum number of items, then adding the loop count to the enum values to obtain unique IDs.
All text strings are stored in a structure attached to the expo, referenced by a text ID. This makes it easier at some point to implement multiple languages or to support Unicode strings.
Menu objects do not have their own text and image objects. Instead they simply refer to objects which have been created. So a menu item is just a collection of IDs of text and image objects. When adding a menu item you must create these objects first, then create the menu item, passing in the relevant IDs.
To create an expo, use expo_new() followed by scene_new() to create a scene. Then add objects to the scene, using functions like scene_txt_str() and scene_menu(). For every menu item, add text and image objects, then create the menu item with scene_menuitem(), referring to those objects.
Individual objects can be positioned using scene_obj_set_pos(). Menu items cannot be positioned manually: this is done by scene_arrange() which is called automatically when something changes. The menu itself determines the position of its items.
Rendering is performed by calling expo_render(). This uses either the vidconsole, if present, or the serial console in text mode. Expo handles presentation automatically in either case, without any change in how the expo is created.
For the vidconsole, Truetype fonts can be used if enabled, to enhance the quality of the display. For text mode, each menu item is shown in a single line, allowing easy selection using arrow keys.
The controller is responsible for collecting keyboard input. A good way to do this is to use cli_ch_process(), since it handles conversion of escape sequences into keys. However, expo has some special menu-key codes for navigating the interface. These are defined in enum bootmenu_key and include BKEY_UP for moving up and BKEY_SELECT for selecting an item. You can use bootmenu_conv_key() to convert an ASCII key into one of these.
Once a keypress is decoded, call expo_send_key() to send it to the expo. This may cause an update to the expo state and may produce an action.
Call expo_action_get() in the event loop to check for any actions that the expo wants to report. These can include selecting a particular menu item, or quitting the menu. Processing of these is the responsibility of your controller.
Expo is intended to be used in an event loop. For an example loop, see bootflow_menu_run(). It is possible to perform other work in your event loop, such as scanning devices for more bootflows.
Expo does not itself support themes. The bootflow_menu implement supposed a basic theme, applying font sizes to the various text objects in the expo.
.. kernel-doc:: include/expo.h
Some ideas for future work:
- Default menu item and a timeout
- Higher-level / automatic / more flexible layout of objects
- Image formats other than BMP
- Use of ANSI sequences to control a serial terminal
- Colour selection
- Better support for handling lots of settings, e.g. with multiple menus and radio/option widgets
- Mouse support
- Integrate Nuklear, NxWidgets or some other library for a richer UI
- Optimise rendering by only updating the display with changes since last render
- Use expo to replace the existing menu implementation
- Add a Kconfig option to drop the names to save code / data space
- Add a Kconfig option to disable vidconsole support to save code / data space
- Support both graphical and text menus at the same time on different devices
- Implement proper measurement of object bounding boxes, to permit more exact layout. This would tidy up the layout when Truetype is not used
- Support unicode
- Support curses for proper serial-terminal menus