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Toggle - Hardware and Feature Toggles

by Djones A. Boni

A big issue on embedded software development is the ability to manage changes in hardware and changes in the software itself.

Time to market and backwards compatibility are often concerns, raising the importance of the ability to reuse working/older code in the new hardware revisions and to use new code in working/older hardware.

The software systems are getting larger, with lots of "moving parts", and becoming themselves the moving parts of even bigger systems.

Therefore the design and progression of the software architecture (long term) and of the new features (short term) are indispensable for the maintainability and testability of the code.

This project is an attempt to solve this issue, enabling the continuous evolution, operation and maintenance of long lived embedded software systems.

C/C++ Feature Toggles:

  • Support multiple hardware revisions in a single repository
    • Select the hardware version at build time
  • Support enable/disable features under development
    • Essential for teams using Trunk Based Development
  • Flexibility:
    • Compile-time toggles (macro or constant)
    • Run-time toggles (variable)
    • Testing support (all toggles as variables, allow changes in run-time)

More Features:

  • Automate the generation of characterizations:
    • Simple to add new options/features.
    • Simple to add new characterizations/versions.
    • Avoid the error prone and tedious work of updating several files with similar definitions.
    • Avoid missing some definition.
  • Simple file format, easy to edit, review and source control:
    • Characterizations are defined in two YAML files.
    • YAMLs are simple text files that can be diffed, reviewed, and source controlled.
  • Flexible:
    • Default declaration and definitions.
    • Customize declaration and definitions every option.
  • Characterizations documented with Doxygen comments.

Dependencies:

  • Python 3

Using (manual generation)

Using Toggle is just a matter of editing two YAML files, where you define the options available for the characterizations (yaml/defaults.yaml) and define the characterizations with the values for the options (yaml/char_ids.yaml).

  1. Edit the YAML files:
    • yaml/defaults.yaml: Define the option name, default value, type, declaration type, brief description, full description, custom header declaration, and custom source definition.
    • yaml/char_ids.yaml: Define the characterization (char IDs), brief description, full description, and values for the options.
      • Options not present use the default value.
  2. Run generate.py.
    • Generate include/toggle.h, src/toggle.c, and files specific for the characterizations.
  3. Include toggle.h in the source.
  4. Build and link toggle.c in the executable.
  5. Compile defining the characterization:
    • Add -D CHAR_ID=CHAR_ID_TEST to compile with the test characterization
    • Add -D CHAR_ID=ANOTHER_CHAR_ID to compile with another characterization

See the example/.

Using with CMake (automated generation)

To use Toggle in a CMake project allows automated generation of the characterization code, every time one of the YAML files change.

Add Toggle as an external project (see example/CMakeLists.txt), or add it as a subdirectory (using add_subdirectory()), create a characterization library (using add_toggle_library()), then link the library to the executable.

cmake_minimum_required(VERSION 3.10)
project(toggle_example VERSION 1.0)

# Set default CHAR_ID
set(CHAR_ID
    CHAR_ID_TEST
    CACHE STRING "Characterization")

# Option A:
# Add Toggle as an external project
include(FetchContent)
FetchContent_Declare(
  toggle
  GIT_REPOSITORY https://github.com/embtool/toggle
  GIT_TAG main # or use a tag, eg. v1.0
)
FetchContent_MakeAvailable(toggle)

# Option B:
# Add subdirectory
add_subdirectory(components/characterization)

# Create a characterization library using Toggle
add_toggle_library(characterization ${CMAKE_CURRENT_SOURCE_DIR})

# Link the characterization library to an executable or library
add_executable(main main.c)
target_link_libraries(main PRIVATE characterization)

See the example/.

Details of File: yaml/defaults.yaml

The file yaml/defaults.yaml defines the list of options for the characterizations. The description of the columns is provide below.

  • NAME: Option name
  • DEFAULT: Default value
  • TYPE: OPTION or VALUE
  • DECL: Declaration type
  • BRIEF: Brief description
  • DESCRIPTION: Full description
  • H: Custom header declaration
  • C: Custom source definition
  • TEST_ASSIGN: Custom assignment for tests

Example of yaml/defaults.yaml file:

- NAME: TESTING
  DEFAULT: 0
  TYPE: OPTION
  DECL: MACRO
  BRIEF: Testing support.
  DESCRIPTION: |
    0: dev or prod; 1: unit-test.

- NAME: SERIAL_DEBUG
  DEFAULT: NO_SER_DBG
  TYPE: OPTION
  DECL: MACRO_INT8
  BRIEF: Serial debug.

- NAME: NO_SER_DBG
  DEFAULT: 0
  TYPE: VALUE
  DECL: MACRO_INT8
  BRIEF: No serial debug.
  DESCRIPTION: |
    @see SERIAL_DEBUG.

- NAME: SER_DBG_UART3
  DEFAULT: 1
  TYPE: VALUE
  DECL: MACRO_INT8
  BRIEF: Serial debug on UART3.
  DESCRIPTION: |
    @see SERIAL_DEBUG.

- NAME: SER_DBG_UART2
  DEFAULT: 2
  TYPE: VALUE
  DECL: MACRO_INT8
  BRIEF: Serial debug on UART2.
  DESCRIPTION: |
    @see SERIAL_DEBUG.
NAME DEFAULT TYPE DECL BRIEF DESCRIPTION
TESTING 0 OPTION MACRO Testing support. 0: dev or prod; 1: unit-test.
SERIAL_DEBUG NO_SER_DBG OPTION MACRO_INT8 Serial debug.
NO_SER_DBG 0 VALUE MACRO_INT8 No serial debug. @see SERIAL_DEBUG.
SER_DBG_UART3 1 VALUE MACRO_INT8 Serial debug on UART3. @see SERIAL_DEBUG.
SER_DBG_UART2 2 VALUE MACRO_INT8 Serial debug on UART2. @see SERIAL_DEBUG.

NAME: Option name of the option/feature. Must be a valid C/C++ identifier ([a-zA-Z][a-za-z0-9]*).

DEFAULT: Default value. The default value for the option. This value is used in all characterizations that leave the option empty and for the options that are not present in the characterization file.

When adding a new option, the default value can be selected to allow most of the current characterizations to continue working with the least effort.

Let's say you are in need of a serial port to print debug information and in hardware v1.0 no serial port is routed. You decide then to route the serial port UART3 in the hardware v2.0.

Then you create an option named SERIAL_DEBUG, and a good default value would be 0 (disabled), while 1 (enabled) is defined only in the v2.0 characterization.

yaml/defaults.yaml:

NAME DEFAULT TYPE DECL BRIEF DESCRIPTION
SERIAL_DEBUG 0 OPTION MACRO_INT8 
if (SERIAL_DEBUG)
{
    // Initialize UART3
}

TYPE: OPTION or VALUE. An option can be changed in the characterization, while a value cannot be changed by the characterization.

Values can be used to compare with options and be assured the characterizations cannot change them.

Suppose that in hardware v3.0 you added USB and it uses one of the pins from UART3, so you cannot use it for debug anymore. You decide to route UART2 for this purpose.

Now, just a 0/1 meaning NO/YES is not enough, so it is better to give actual names for the values of the option, adding NO_SER_DBG (0 - meaning disabled, v1.0), SER_DBG_UART3 (1 - meaning to use UART3, v2.0), and SER_DBG_UART2 (2 - meaning to use UART2, V3.0).

yaml/defaults.yaml:

NAME DEFAULT TYPE DECL BRIEF DESCRIPTION
SERIAL_DEBUG NO_SER_DBG OPTION MACRO_INT8
NO_SER_DBG 0 VALUE MACRO_INT8
SER_DBG_UART3 1 VALUE MACRO_INT8
SER_DBG_UART2 2 VALUE MACRO_INT8 
if (SERIAL_DEBUG == SER_DBG_UART3)
{
    // Initialize UART3
}
else if (SERIAL_DEBUG == SER_DBG_UART2)
{
    // Initialize UART2
}
else
{
    // Nothing to initialize
}

DECL: Declaration of the option. Specifies the default declaration (H - for headers), the default definition (C - for sources).

The declaration need a base and a type joined by an underscore _. The base can be:

  • MACRO - Create a macro (non-testable)
  • MACRO_type - Create a macro (testable)
  • CONST_type - Create a constant global variable (testable)
  • VAR_type - Create a global variable (testable)

And the type can be:

  • BOOL - bool - boolean
  • INT32 - int32_t - 32 bit signed integer
  • UINT32 - uint32_t - 32 bit unsigned integer
  • etc (see table below)
DECL Type in the code
MACRO #define @NAME@ @VALUE@
base_BOOL bool
base_CHAR char
base_INT8 int8_t
base_INT16 int16_t
base_INT32 int32_t
base_INT64 int64_t
base_INTPTR intptr_t
base_UINT8 uint8_t
base_UINT16 uint16_t
base_UINT32 uint32_t
base_UINT64 uint64_t
base_UINTPTR uintptr_t
base_FLOAT float
base_DOUBLE double
base_CHAR_ARRAY char []

Type of macro? It is nice to document the expected type for the macro, even if it is not used in the code itself. An additional reason is related to the testability (see below).

Testable and non-testable? In the characterization you can define a special option named TESTING as 1. In this case all options that are "testable macros" (MACRO_type) will be converted to variables of that type and all options that are "constants" (CONST_type) will be converted to non-constants.

Note that only TYPE=OPTION are modified when TESTING=1. TYPE=VALUE are NOT converted, to assure they do not change.

The table below shows the the resulting type of an option or value and marks with a plus (+) when it is is testable (can be changed at run time).

TYPE TESTING MACRO MACRO_type CONST_type VAR_type
OPTION TESTING=0 macro macro const type type (+)
OPTION TESTING=1 macro type (+) type (+) type (+)
VALUE TESTING=0 macro macro const type type (+)
VALUE TESTING=1 macro macro const type type (+)

BRIEF: Brief description to be added as documentation.

DESCRIPTION: Additional description to be added as documentation.

H: Custom header declaration. Specifies how to declare the option in the header toggle.h. Use @NAME@ as a placeholder for the option name, @VALUE@ as a placeholder for the option value, and @CONST@ as a placeholder for the constness.

Therefore, if you are not happy with the default header declaration, you can DIY.

Examples:

  • Macro: #define @NAME@ @VALUE@
  • Boolean: extern @CONST@ bool @NAME@;
  • Integer: extern @CONST@ int @NAME@;
  • String: extern @CONST@ char @NAME@[];

Why a placeholder for constness? When TESTING=1 the constness is removed to enable testing.

C: Custom source definition. Specifies how to define the option in the source toggle.c. Works the same way as in the header, using @NAME@, @VALUE@, and @CONST@ as placeholders.

Examples:

  • Macro: (nothing)
  • Boolean: @CONST@ bool @NAME@ = @VALUE@;
  • Integer: @CONST@ int @NAME@ = @VALUE@;
  • String: @CONST@ char @NAME@[] = @VALUE@;

If there is a need to include or define something at before the definitions (in the C file) write a comment in the form: TOP_C: ....

/* TOP_C: #include <stddef.h> */

/* TOP_C: #ifndef MY_ASSERT
 * TOP_C:     #define MY_ASSERT(x) if (!(x)) { for (;;) {} }
 * TOP_C: #endif
 */

// TOP_C: typedef unsigned char my_size;

TEST_ASSIGN: Custom assignment for tests. Specifies how to assign values to the options, so they can be reset to the original values when testing, allowing the tests to start with consistent options. Works the same way as in the header, using @NAME@, @VALUE@, and @CONST@ as placeholders.

When TESTING=1 is the function testing_reset_toggles() is available and uses this (or the default) assignment recipe to assign the original value to the options.

Examples:

  • Macro: (nothing)
  • Boolean: @NAME@ = @VALUE@;
  • Integer: @NAME@ = @VALUE@;
  • String:
    • Resetting a string is a bit cumbersome. Include <string.h> and use strncpy() or write a loop yourself.
    /* TOP_C: #include <string.h> */
strncpy(@NAME@, @VALUE@, sizeof(@NAME@));

Details of File: yaml/char_ids.yaml

The file yaml/char_ids.yaml defines the list of characterizations and their values. The description of the columns is provide below.

  • CHAR_ID: Characterization name
  • BASED_ON: Base on other characterizations
  • BRIEF: Brief description
  • DESCRIPTION: Full description
  • TESTING: Special option for testing
  • Other options: Additional options set for the characterizations

Example of yaml/char_ids.yaml file:

- CHAR_ID: CHAR_ID_TEST
  BRIEF: Unit-tests.
  DESCRIPTION: Unit-tests.
  TESTING: 1

- CHAR_ID: HW_V1
  BRIEF: Hardware v1.0.
  DESCRIPTION: First version.

- CHAR_ID: HW_V2
  BRIEF: Hardware v2.0.
  DESCRIPTION: Serial debug.
  SERIAL_DEBUG: SER_DBT_UART3

- CHAR_ID: HW_V3
  BRIEF: Hardware v3.0.
  DESCRIPTION: Serial debug on UART2.
  SERIAL_DEBUG: SER_DBT_UART2
CHAR_ID BRIEF DESCRIPTION TESTING SERIAL_DEBUG
CHAR_ID_TEST Unit-tests. 1
HW_V1 Hardware v1.0. First version.
HW_V2 Hardware v2.0. Serial debug. SER_DBT_UART3
HW_V3 Hardware v3.0. Serial debug on UART2. SER_DBT_UART2

CHAR_ID: Characterization name. Must be a valid C/C++ identifier ([a-zA-Z][a-za-z0-9]*).

BASED_ON: Base on other characterizations. Apply the values of each base in order then apply the new characterization values. It is best to use orthogonal bases, bases that touch different options (including their bases). Ex: BASED_ON: HW_V3 or BASED_ON: [HW_V3, CHAR_ID_TEST].

BRIEF: Brief description to be added as documentation.

DESCRIPTION: Full description to be added as documentation.

TESTING: Special option that specifies if the characterization needs testing support.

Options defined as macros or constants are transformed in non-constant variables.

Other options: Additional options set for the characterizations.

Minimal Configuration

  • yaml/defaults.yaml:
    • It is recommended to leave the option "TESTING" in the defaults file.
    • Add other options as you need.
- NAME: TESTING
  DEFAULT: 0
  TYPE: OPTION
  DECL: MACRO
  BRIEF: Specifies if the code needs testing support (bool).
  DESCRIPTION: |
    - 0 Development and production.
    - 1 Unit-tests.
  • yaml/char_ids.yaml:
    • It is recommended to leave the characterization "CHAR_ID_TEST" in the characterizations file.
    • Add characterizations as you need.
- CHAR_ID: CHAR_ID_TEST
  BRIEF: Testing.
  DESCRIPTION: Characterization for unit-tests.
  TESTING: 1