doc/develop/expo.rst - u-boot - Git at Google

 .. SPDX-License-Identifier: GPL-2.0+

 Expo menu
 =========

 U-Boot provides a menu implementation for use with selecting bootflows and
 changing U-Boot settings. This is in early stages of development.

 Motivation
 ----------

 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.

 Concepts
 --------

 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.

 Creating an expo
 ----------------

 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.

 Layout
 ------

 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
 ---------

 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.

 Input
 -----

 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.

 Actions
 -------

 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.

 Event loop
 ----------

 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.

 Themes
 ------

 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.

 API documentation
 -----------------

 .. kernel-doc:: include/expo.h

 Future ideas
 ------------

 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

 .. Simon Glass <sjg@chromium.org>
 .. 7-Oct-22
	.. SPDX-License-Identifier: GPL-2.0+

	Expo menu
	=========

	U-Boot provides a menu implementation for use with selecting bootflows and
	changing U-Boot settings. This is in early stages of development.

	Motivation
	----------

	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.

	Concepts
	--------

	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.

	Creating an expo
	----------------

	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.

	Layout
	------

	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
	---------

	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.

	Input
	-----

	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.

	Actions
	-------

	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.

	Event loop
	----------

	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.

	Themes
	------

	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.

	API documentation
	-----------------

	.. kernel-doc:: include/expo.h

	Future ideas
	------------

	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

	.. Simon Glass <sjg@chromium.org>
	.. 7-Oct-22