Implementing a Kernel Test

This guide covers how to write in-kernel tests of hardware functionality. For example, if you have implemented a chip peripheral, you may want to write in-kernel tests of that peripheral to test peripheral-specific functionality that will not be exposed via the HIL for that peripheral. This guide outlines the general steps for implementing kernel tests.

Setup

This guide assumes you have existing chip, board, or architecture specific code that you wish to test from within the kernel.

Note: If you wish to test kernel code with no hardware dependencies at all, such as a ring buffer implementation, you can use cargo's test framework instead. These tests can be run by simply calling cargo test within the crate that the test is located, and will be executed during CI for all tests merged into upstream Tock. An example of this approach can be found in kernel/src/common/ring_buffer.rs.

Overview

The general steps you will follow are:

  1. Determine the board(s) you want to run your tests on
  2. Add a test file in boards/{board}/src/
  3. Determine where to write actual tests -- in the test file or a capsule test
  4. Write your tests
  5. Call the test from main.rs
  6. Document the expected output from the test at the top of the test file

This guide will walk through how to do each of these steps.

Background

Kernel tests allow for testing of hardware-specific functionality that is not exposed to userspace, and allows for fail-fast tests at boot that otherwise would not be exposed until apps are loaded. Kernel tests can be useful to test chip peripherals prior to exposing these peripherals outside the Kernel. Kernel tests can also be included as required tests run prior to releases, to ensure there have been no regressions for a particular component. Additionally, kernel tests can be useful for testing capsule functionality from within the kernel, such as when unsafe is required to verify the results of tests, or for testing virtualization capsules in a controlled environment.

Kernel tests are generally implemented on an as-needed basis, and are not required for all chip peripherals in Tock. In general, they are not expected to be run in the default case, though they should always be included from main.rs so they are compiled. These tests are allowed to use unsafe as needed, and are permitted to conflict with normal operation, by stealing callbacks from drivers or modifying global state.

Notably, your specific use case may differ some from the one outline here. It is always recommended to attempt to copy from existing Tock code when developing your own solutions. A good collection of kernel tests can be found in boards/imix/src/tests/ for that purpose.

Step-by-Step Guide

The steps from the overview are elaborated on here.

  1. Determine the board(s) you want to run your test on.

    If you are testing chip or architecture specific functionality, you simply need to choose a board that uses that chip or architecture. For board specific functionality you of course need to choose that board. If you are testing a virtualization capsule, then any board that implements the underlying resource being virtualized is acceptable. Currently, most kernel tests are implemented for the Imix platform, and can be found in boards/imix/src/tests/

    Checkpoint: You have identified the board you will implement your test for.

  2. Add a test file in boards/{board}/src/

    To start implementing the test, you should create a new source file inside the boards/{board}/src directory. For boards with lots of tests, like the Imix board, there may be a tests subdirectory -- if so, the test should go in tests instead, and be added to the tests/mod.rs file. The name of this test file generally should indicate the functionality being tested.

    Note: If the board you select is one of the nrf52dk variants (nrf52840_dongle, nrf52840dk, or nrf52dk), tests should be moved into the nrf52dk_base/src/ folder, and called from lib.rs.

    Checkpoint: You have chosen a board for your test and created a test file.

  3. Determine where to write actual tests -- in the test file or a capsule test.

    Depending on what you are testing, it may be best practice to write a capsule test that you call from the test file you created in the previous step.

    Writing a capsule test is best practice if your test meets the following criteria:

    1. Test does not require unsafe
    2. The test is for a peripheral available on multiple boards
    3. A HIL or capsule exists for that peripheral, so it is accessible from the capsules crate
    4. The test relies only on functionality exposed via the HIL or a capsule
    5. You care about being able to call this test from multiple boards

    Examples:

    • UART Virtualization (all boards support UART, there is a HIL for UART devices and a capsule for the virtual_uart)
    • Alarm test (all boards will have some form of hardware alarm, there is an Alarm HIL)
    • Other examples: see capsules/src/test

    If your test meets the criteria for writing a capsule test, follow these steps:

    Add a file in capsules/src/test/, and then add the filename to capsules/src/mod.rs like this:

    
    # #![allow(unused_variables)]
    #fn main() {
    pub mod virtual_uart;
    #}

    Next, create a test struct in this file that can be instantiated by any board using this test capsule. This struct should implement a new() function so it can be instantiated from the test file in boards, and a run() function that will run the actual tests. An example for UART follows:

    
    # #![allow(unused_variables)]
    #fn main() {
    //! capsules/src/test/virtual_uart.rs
    
    pub struct TestVirtualUartReceive {
        device: &'static UartDevice<'static>,
        buffer: TakeCell<'static, [u8]>,
    }
    
    impl TestVirtualUartReceive {
        pub fn new(device: &'static UartDevice<'static>, buffer: &'static mut [u8]) -> Self {
            TestVirtualUartReceive {
                device: device,
                buffer: TakeCell::new(buffer),
            }
        }
    
        pub fn run(&self) {
            // TODO: See Next Step
        }
    }
    #}

    If your test does not meet the above requirements, you can simply implement your tests in the file that you created in step 2. This can involve creating a test structure with test methods. The UDP test file takes this approach, by definining a number of self-contained tests. One such example follows:

    
    # #![allow(unused_variables)]
    #fn main() {
    //! boards/imix/src/test/udp_lowpan_test.rs
    
    pub struct LowpanTest {
        port_table: &'static UdpPortManager,
        // ...
    }
    
    impl LowpanTest {
    
        // This test ensures that an app and capsule cant bind to the same port
        // but can bind to different ports
        fn bind_test(&self) {
            let create_cap = create_capability!(NetworkCapabilityCreationCapability);
            let net_cap = unsafe {
                static_init!(
                    NetworkCapability,
                    NetworkCapability::new(AddrRange::Any, PortRange::Any, PortRange::Any, &create_cap)
                )
            };
            let mut socket1 = self.port_table.create_socket().unwrap();
            // Attempt to bind to a port that has already been bound by an app.
            let result = self.port_table.bind(socket1, 1000, net_cap);
            assert!(result.is_err());
            socket1 = result.unwrap_err(); // Get the socket back
    
            //now bind to an open port
            let (_send_bind, _recv_bind) = self
                .port_table
                .bind(socket1, 1001, net_cap)
                .expect("UDP Bind fail");
    
            debug!("bind_test passed");
        }
        // ...
    }
    #}

    Checkpoint: There is a test capsule with new() and run() implementations.

  4. Write your tests

    The first part of this step takes place in the test file you just created -- writing the actual tests. This part is highly dependent on the funcitonality being verified. If you are writing your tests in test capsule, this should all be triggered from the run() function.

    Depending on the specifics of your test, you may need to implement additional functions or traits in this file to make your test functional. One example is implementing a client trait on the test struct so that the test can receive the results of asynchronous operations. Our UART example requires implementing the uart::RecieveClient on the test struct.

    
    # #![allow(unused_variables)]
    #fn main() {
    //! boards/imix/src/test/virtual_uart_rx_test.rs
    
    impl TestVirtualUartReceive {
        // ...
    
        pub fn run(&self) {
            let buf = self.buffer.take().unwrap();
            let len = buf.len();
            debug!("Starting receive of length {}", len);
            let (err, _opt) = self.device.receive_buffer(buf, len);
            if err != ReturnCode::SUCCESS {
                panic!(
                    "Calling receive_buffer() in virtual_uart test failed: {:?}",
                    err
                );
            }
        }
    }
    
    impl uart::ReceiveClient for TestVirtualUartReceive {
        fn received_buffer(
            &self,
            rx_buffer: &'static mut [u8],
            rx_len: usize,
            rcode: ReturnCode,
            _error: uart::Error,
        ) {
            debug!("Virtual uart read complete: {:?}: ", rcode);
            for i in 0..rx_len {
                debug!("{:02x} ", rx_buffer[i]);
            }
            debug!("Starting receive of length {}", rx_len);
            let (err, _opt) = self.device.receive_buffer(rx_buffer, rx_len);
            if err != ReturnCode::SUCCESS {
                panic!(
                    "Calling receive_buffer() in virtual_uart test failed: {:?}",
                    err
                );
            }
        }
    }
    #}

    Note that the above test calls panic!() in the case of failure. This pattern, or the similar use of assert!() statements, is the preferred way to communicate test failures. If communicating errors in this way is not possible, tests can indicate success/failure by printing different results to the console in each case and asking users to verify the actual output matches the expected output.

    The next step in this process is determining all of the parameters that need to be passed to the test. It is preferred that all logically related tests be called from a single pub unsafe fn run(/* optional args */) to maintain convention. This ensures that all tests can be run by adding a single line to main.rs. Many tests require a reference to an alarm in order to seperate tests in time, or a reference to a virtualization capsule that is being tested. Notably, the run() function should initialize any components itself that would not have already been created in main.rs. As an example, the below function is a starting point for the virtual_uart_receive test for Imix:

    
    # #![allow(unused_variables)]
    #fn main() {
    pub unsafe fn run_virtual_uart_receive(mux: &'static MuxUart<'static>) {
        debug!("Starting virtual reads.");
    }
    #}

    Next, a test function should initialize any objects required to run tests. This is best split out into subfunctions, like the following:

    
    # #![allow(unused_variables)]
    #fn main() {
    unsafe fn static_init_test_receive_small(
        mux: &'static MuxUart<'static>,
    ) -> &'static TestVirtualUartReceive {
        static mut SMALL: [u8; 3] = [0; 3];
        let device = static_init!(UartDevice<'static>, UartDevice::new(mux, true));
        device.setup();
        let test = static_init!(
            TestVirtualUartReceive,
            TestVirtualUartReceive::new(device, &mut SMALL)
        );
        device.set_receive_client(test);
        test
    }
    #}

    This initializes an instance of the test capsule we constructed earlier. Simpler tests (such as those not relying on capsule tests) might simply use static_init!() to initialize normal capsules directly and test them. The log test does this, for example:

    
    # #![allow(unused_variables)]
    #fn main() {
    //! boards/imix/src/test/log_test.rs
    
    pub unsafe fn run(
        mux_alarm: &'static MuxAlarm<'static, Ast>,
        deferred_caller: &'static DynamicDeferredCall,
    ) {
        // Set up flash controller.
        flashcalw::FLASH_CONTROLLER.configure();
        static mut PAGEBUFFER: flashcalw::Sam4lPage = flashcalw::Sam4lPage::new();
    
        // Create actual log storage abstraction on top of flash.
        let log = static_init!(
            Log,
            log::Log::new(
                &TEST_LOG,
                &mut flashcalw::FLASH_CONTROLLER,
                &mut PAGEBUFFER,
                deferred_caller,
                true
            )
        );
        flash::HasClient::set_client(&flashcalw::FLASH_CONTROLLER, log);
        log.initialize_callback_handle(
            deferred_caller
                .register(log)
                .expect("no deferred call slot available for log storage"),
        );
    
        // ...
    }
    #}

    Finally, your run() function should call the actual tests. This may involve simply calling a run() function on a capsule test, or may involve calling test functions written in the board specific test file. The virtual UART test run() looks like this:

    
    # #![allow(unused_variables)]
    #fn main() {
    pub unsafe fn run_virtual_uart_receive(mux: &'static MuxUart<'static>) {
        debug!("Starting virtual reads.");
        let small = static_init_test_receive_small(mux);
        let large = static_init_test_receive_large(mux);
        small.run();
        large.run();
    }
    #}

    As you develop your kernel tests, you may not immediately know what functions are required in your test capsule -- this is okay! It is often easiest to start with a basic test and expand this file to test additional functionality once basic tests are working.

    Checkpoint: Your tests are written, and can be called from a single run() function.

  5. Call the test from main.rs, and iterate on it until it works

    Next, you should run your test by calling it from the reset_handler() in main.rs. In order to do so, you will also need it import it into the file by adding a line like this:

    
    # #![allow(unused_variables)]
    #fn main() {
    #[allow(dead_code)]
    mod virtual_uart_test;
    #}

    However, if your test is located inside a test module this is not needed -- your test will already be included.

    Typically, tests are called after completing setup of the board, immediately before the call to load_processes():

    
    # #![allow(unused_variables)]
    #fn main() {
    virtual_uart_rx_test::run_virtual_uart_receive(uart_mux);
    debug!("Initialization complete. Entering main loop");
    
    extern "C" {
        /// Beginning of the ROM region containing app images.
        static _sapps: u8;
    
        /// End of the ROM region containing app images.
        ///
        /// This symbol is defined in the linker script.
        static _eapps: u8;
    }
    kernel::procs::load_processes(
      // ...
    #}

    Observe your results, and tune or add tests as needed.

    Before you submit a PR including any kernel tests, however, please remove or comment out any lines of code that call these tests.

    Checkpoint: You have a functional test that can be called in a single line from main.rs

  6. Document the expected output from the test at the top of the test file

    For tests that will be merged to upstream, it is good practice to document how to run a test and what the expected output of a test is. This is best done using
    document level coments (//!) at the top of the test file. The documentation for the virtual UART test follows:

    
    # #![allow(unused_variables)]
    #fn main() {
    //! Test reception on the virtualized UART by creating two readers that
    //! read in parallel. To add this test, include the line
    //! ```
    //!    virtual_uart_rx_test::run_virtual_uart_receive(uart_mux);
    //! ```
    //! to the imix boot sequence, where `uart_mux` is a
    //! `capsules::virtual_uart::MuxUart`.  There is a 3-byte and a 7-byte
    //! read running in parallel. Test that they are both working by typing
    //! and seeing that they both get all characters. If you repeatedly
    //! type 'a', for example (0x61), you should see something like:
    //! ```
    //! Starting receive of length 3
    //! Virtual uart read complete: CommandComplete:
    //! 61
    //! 61
    //! 61
    //! 61
    //! 61
    //! 61
    //! 61
    //! Starting receive of length 7
    //! Virtual uart read complete: CommandComplete:
    //! 61
    //! 61
    //! 61
    //! ```
    #}

    Checkpoint: You have documented your tests

Wrap-Up

Congratulations! You have written a kernel test for Tock! We encourage you to submit a pull request to upstream this to the Tock repository.