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Unit testing coreboot

Preface

First part of this document, Introduction, comprises disambiguation for what unit testing is and what is not. This definition will be a basis for the whole paper.

Next, Rationale, explains why to use unit testing and how coreboot specifically may benefit from it.

This is followed by evaluation of different available free C unit test frameworks. Firstly, collection of requirements is provided. Secondly, there is a description of a few selected candidates. Finally, requirements are applied to candidates to see if they might be a good fit.

Fourth part is a summary of evaluation, with proposal of unit test framework for coreboot to be used.

Finally, Implementation proposal paragraph touches how build system and coreboot codebase in general should be organized, in order to support unit testing. This comprises couple of design considerations which need to be addressed.

Introduction

A unit test is supposed to test a single unit of code in isolation. In C language (in contrary to OOP) unit usually means a function. One may also consider unit under test to be a single compilation unit which exposes some API (set of functions). A function, talking to some external component can be tested if this component can be mocked out.

In other words (looking from C compilation angle), there should be no extra dependencies (executables) required beside unit under test and test harness in order to compile unit test binary. Test harness, beside code examining a routines, may comprise test framework implementation.

It is hard to apply this strict definition of unit test to firmware code in practice, mostly due to constraints on speed of execution and size of final executable. coreboot codebase often cannot be adjusted to be testable. Because of this, coreboot unit testing subsystem should allow to include some additional source object files beside unit under test. That being said, the default and goal wherever possible, should be to isolate unit under test from other parts.

Unit testing is not an integration testing and it doesn't replace it. First of all, integration tests cover larger set of components and interactions between them. Positive integration test result gives more confidence than a positive unit test does. Furthermore, unit tests are running on the build machine, while integration tests usually are executed on the target (or simulator).

Rationale

Considering above, what is the benefit of unit testing, especially keeping in mind that coreboot is low-level firmware? Unit tests should be quick, thus may be executed frequently during development process. It is much easier to build and run a unit test on a build machine, than any integration test. This in turn may be used by dev to gather extra confidence early during code development process. Actually developer may even write unit tests earlier than the code - see TDD concept.

That being said, unit testing embedded C code is a difficult task, due to significant amount of dependencies on underlying hardware. Mocking can handle some hardware dependencies. However, complex mocks make the unit test susceptible to failing and can require significant development effort.

Writing unit tests for a code (both new and currently existing) may be favorable for the code quality. It is not only about finding bugs, but in general - easily testable code is a good code.

coreboot benefits the most from testing common libraries (lib/, commonlib/, payloads/libpayload) and coreboot infrastructure (console/, device/, security/).

Evaluation of unit testing frameworks

Requirements

Requirements for unit testing frameworks:

Easy to use
Few dependencies

Standard C library is all we should need
Isolation between tests
Support for mocking
Support for some machine parsable output
Compiler similarity

Compiler for the host must support the same language standards as the target compiler. Ideally the same toolchain should be used for building firmware executables and test binaries, however the host compiler will be used to build unit tests, whereas the coreboot toolchain will be used for building the firmware executables. For some targets, the host compiler and the target compiler could be the same, but this is not a requirement.
Same language for tests and code

Unit tests will be written in C, because coreboot code is also written in C

Desirables

Easy to integrate with build system/build tools

Ideally JUnit-like XML output format for Jenkins
Popularity is a plus

We want a larger community for a couple of reasons. Firstly, easier access to people with knowledge and tutorials. Secondly, bug fixes for the top of tree are more frequent and known issues are usually shorter in the pending state. Last but not least, larger reviewer pool means better and easier upstream improvements that we would like to submit.
Extra features may be a plus
Compatible license

This should not be a blocker, since test binaries are not distributed. However ideally compatible with GPL.
IDE integration

Candidates

There is a lot of frameworks which allow unit testing C code (list from Wikipedia). While not all of them were evaluated, because that would take an excessive amount of time, couple of them were selected based on the good opinions among C devs, popularity and fitting above criteria.

SputUnit
GoogleTest
Cmocka
Unity (CMock, Ceedling)

We looked at several other test frameworks, but decided not to do a full evaluation for various reasons such as functionality, size of the developer community, or compatibility.

Evaluation

SputUnit
- Pros
  - No dependencies, one header file to include - that’s all
  - Pure C
  - Very easy to use
  - BSD license
- Cons
  - Main repo doesn’t have support for generating JUnit XML reports for Jenkins to consume - this feature is available only on the fork from SputUnit called “Sput_report”. It makes it niche in a niche, so there are some reservations whether support for this will be satisfactory
  - No support for mocks
  - Not too popular
  - No automatic test registration
GoogleTest
- Pros
  - Automatic test registration
  - Support for different output formats (including XML for Jenkins)
  - Good support, widely used, the biggest and the most active community out of all frameworks that were investigated
  - Available as a package in the most common distributions
  - Test fixtures easily available
  - Well documented
  - Easy to integrate with an IDE
  - BSD license
- Cons
  - Requires C++11 compiler
  - To make most out of it (use GMock) C++ knowledge is required
Cmocka
- Pros
  - Self-contained, autonomous framework
  - Pure C
  - API is well documented
  - Multiple output formats (including XML for Jenkins)
  - Available as a package in the most common distributions
  - Used in some popular open source projects (libssh, OpenVPN, Samba)
  - Test fixtures available
  - Support for exception handling
- Cons
  - No automatic test registration
  - It will require some effort to make it work from within an IDE
  - Apache 2.0 license (not compatible with GPLv2)
Unity (CMock, Ceedling)
- Pros
  - Pure C (Unity testing framework itself, not test runner)
  - Support for different output formats (including XML for Jenkins)
  - There are some (rather easy) hints how to use this from an IDE (e.g. Eclipse)
  - MIT license
- Cons
  - Test runner (Ceedling) is not written in C - uses Ruby
  - Mocking/Exception handling functionalities are actually separate tools
  - No automatic test registration
  - Not too popular

Summary & framework proposal

After research, we propose using the Cmocka unit test framework. Cmocka fulfills all stated evaluation criteria. It is rather easy to use, doesn’t have extra dependencies, written fully in C, allows for tests fixtures and some popular open source projects already are using it. Cmocka also includes support for mocks.

Cmocka's limitations, such as the lack of automatic test registration, are considered minor issues that will require only minimal additional work from a developer. At the same time, it may be worth to propose improvement to Cmocka community or simply apply some extra wrapper with demanded functionality.

Implementation

Framework as a submodule or external package

Unit test frameworks may be either compiled from source (from a git submodule under 3rdparty/) or pre-compiled as a package. The second option seems to be easier to maintain, while at the same time may bring some unwanted consequences (different version across distributions, frequent changes in API). It makes sense to initially experiment with packages and check how it works. If this will cause any issues, then it is always possible to switch to submodule approach.

Integration with build system

To get the most out of unit testing framework, it should be integrated with Jenkins automation server. Verification of all unit tests for new changes may improve code reliability to some extent.

Build configuration (Kconfig)

While building unit under test object file, it is necessary to apply some configuration (config) just like when building usual firmware. For simplicity, there will be one default tests .config qemu_x86_i440fx for all unit tests. At the same time, some tests may require running with different values of particular config. This should be handled by adding extra header, included after config.h. This header will comprise #undef of old CONFIG values and #define of the required value. When unit testing will be integrated with Jenkins, it may be preferred to use every available config for periodic builds.

Directory structure

Tests should be kept separate from the code, while at the same time it must be easy to match code with test harness.

We create new directory for test files ($(toplevel)/tests/) and mimic the structure of src/ directory.

Test object files (test harness, unit under tests and any additional executables are stored under build/tests/<test_name> directory.

Below example shows how directory structure is organized for the two test cases: tests/lib/string-test and tests/device/i2c-test:

├── src
│   ├── lib
│   │   ├── string.c <- unit under test
│   │
│   ├── device
│       ├── i2c.c
│
├── tests
│   ├── include
│   │   ├── mocks <- mock headers, which replace original headers
│   │
│   ├── Makefile.mk <- top Makefile for unit tests subsystem
│   ├── lib
│   │   ├── Makefile.mk
│   │   ├── string-test.c <- test code for src/lib/string.c
│   │   │
│   ├── device
│   │   ├── Makefile.mk
│       ├── i2c-test.c
│
├── build
│   ├── tests <-all test-related executables
        ├── config.h <- default config used for tests builds
        ├── lib
        │   ├── string-test <- all string-test executables
        │   │   ├── run     <- final test binary
        │   │   ├── tests   <- all test harness executables
        │   │       ├── lib
        │   │           ├── string-test.o  <-test harness executable
        │   │   ├── src    <- unit under test and other src executables
        │   │       ├── lib
        │   │           ├── string.o       <- unit under test executable
        ├── device
            ├── i2c-test
                ├── run
                ├── tests
                │   ├── device
                │       ├── i2c-test.o
                ├── src
                    ├── device
                        ├── i2c.o

Writing new tests

Our tutorial series has detailed guidelines for writing unit tests.

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