Apache Mynewt Operating System Kernel

The Mynewt Core OS is a multitasking, preemptive real-time operating system combining a scheduler with typical RTOS features such as mutexes, semaphores, memory pools, etc. The Mynewt Core OS also provides a number of useful utilities such as a task watchdog, networking stack memory buffers and time management API. Each of these features is described in detail in its own section of the manual.

A multitasking, preemptive operating system is one in which a number of different tasks can be instantiated and assigned a priority, with higher priority tasks running before lower priority tasks. Furthermore, if a lower priority task is running and a higher priority task wants to run, the lower priority task is halted and the higher priority task is allowed to run. In other words, the lower priority task is preempted by the higher priority task.

Why use an OS?

You may ask yourself “why do I need a multitasking preemptive OS”? The answer may indeed be that you do not. Some applications are simple and only require a polling loop. Others are more complex and may require that certain jobs are executed in a timely manner or before other jobs are executed. If you have a simple polling loop, you cannot move on to service a job until the current job is done being serviced. With the Mynewt OS, the application developer need not worry about certain jobs taking too long or not executing in a timely fashion; the OS provides mechanisms to deal with these situations. Another benefit of using an OS is that it helps shield application developers from other application code being written; the developer does not have to worry (or has to worry less) about other application code behaving badly and causing undesirable behavior or preventing their code from executing properly. Other benefits of using an OS (and the Mynewt OS in particular) is that it also provides features that the developer would otherwise need to create on his/her own.

Basic OS Application Creation

Creating an application using the Mynewt Core OS is a relatively simple task. The main steps are:

  1. Install the basic Newt Tool structure (build structure) for your application.

  2. Create your BSP (Board Support Package).

  3. In your application main() function, call the sysinit() function to initialize the system and packages, perform application specific initialization, then wait and dispatch events from the OS default event queue in an infinite loop. (See Compile-Time Configuration and Initialization for more details.)

Initializing application modules and tasks can get somewhat complicated with RTOS’s similar to Mynewt. Care must be taken that the API provided by a task are initialized prior to being called by another (higher priority) task.

For example, take a simple application with two tasks (tasks 1 and 2, with task 1 higher priority than task 2). Task 2 provides an API which has a semaphore lock and this semaphore is initialized by task 2 when the task handler for task 2 is called. Task 1 is expected to call this API.

Consider the sequence of events when the OS is started. The scheduler starts running and picks the highest priority task (task 1 in this case). The task handler function for task 1 is called and will keep running until it yields execution. Before yielding, code in the task 1 handler function calls the API provided by task 2. The semaphore being accessed in the task 2 API has yet to be initialized since the task 2 handler function has not had a chance to run! This will lead to undesirable behavior and will need to be addressed by the application developer. Note that the Mynewt OS does guard against internal API being called before the OS has started (they will return error) but it does not safeguard application defined objects from access prior to initialization.

Example

One way to avoid initialization issues like the one described above is for the application to initialize the objects that are accessed by multiple tasks before it initializes the tasks with the OS.

The code example shown below illustrates this concept. The application initializes system and packages, calls an application specific “task initialization” function, and dispatches events from the default event queue. The application task initialization function is responsible for initializing all the data objects that each task exposes to the other tasks. The tasks themselves are also initialized at this time (by calling os_task_init()).

In the example, each task works in a ping-pong like fashion: task 1 wakes up, adds a token to semaphore 1 and then waits for a token from semaphore 2. Task 2 waits for a token on semaphore 1 and once it gets it, adds a token to semaphore 2. Notice that the semaphores are initialized by the application specific task initialization functions and not inside the task handler functions. If task 2 (being lower in priority than task 1) had called os_sem_init() for task2_sem inside task2_handler(), task 1 would have called os_sem_pend() using task2_sem before task2_sem was initialized.

    struct os_sem task1_sem;
    struct os_sem task2_sem;

    /* Task 1 handler function */
    void
    task1_handler(void *arg)
    {
        while (1) {
            /* Release semaphore to task 2 */
            os_sem_release(&task1_sem);

            /* Wait for semaphore from task 2 */
            os_sem_pend(&task2_sem, OS_TIMEOUT_NEVER);
        }
    }

    /* Task 2 handler function */
    void
    task2_handler(void *arg)
    {
        struct os_task *t;

        while (1) {
            /* Wait for semaphore from task1 */
            os_sem_pend(&task1_sem, OS_TIMEOUT_NEVER);

            /* Release task2 semaphore */
            os_sem_release(&task2_sem);
        }
    }


    /* Initialize task 1 exposed data objects */
    void
    task1_init(void)
    {
        /* Initialize task1 semaphore */
        os_sem_init(&task1_sem, 0);
    }

    /* Initialize task 2 exposed data objects */
    void
    task2_init(void)
    {
        /* Initialize task1 semaphore */
        os_sem_init(&task2_sem, 0);
    }

    /**
     * init_app_tasks
     *
     * This function performs initializations that are required before tasks run.
     *
     * @return int 0 success; error otherwise.
     */
    static int
    init_app_tasks(void)
    {
        /*
         * Call task specific initialization functions to initialize any shared objects
         * before initializing the tasks with the OS.
         */
        task1_init();
        task2_init();

        /*
         * Initialize tasks 1 and 2 with the OS.
         */
        os_task_init(&task1, "task1", task1_handler, NULL, TASK1_PRIO,
                     OS_WAIT_FOREVER, task1_stack, TASK1_STACK_SIZE);

        os_task_init(&task2, "task2", task2_handler, NULL, TASK2_PRIO,
                     OS_WAIT_FOREVER, task2_stack, TASK2_STACK_SIZE);

        return 0;
    }

    /**
     * main
     *
     * The main function for the application. This function initializes the system and packages,
     * calls the application specific task initialization function, then waits and dispatches
     * events from the OS default event queue in an infinite loop.
     */
    int
    main(int argc, char **arg)
    {

        /* Perform system and package initialization */
        sysinit();

        /* Initialize application specific tasks */
        init_app_tasks();

        while (1) {
           os_eventq_run(os_eventq_dflt_get());
        }
        /* main never returns */
}