Crate lilos

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A simple async RTOS based around Rust Futures.

This provides a minimal operating environment for running async Rust code on ARM Cortex-M microprocessors, plus some useful doodads and gizmos.

lilos design principles

  1. Be compact. Avoid doing things that increase the minimum size of a useful application. In particular, try to avoid designing APIs that need internal asserts/panics, because those are quite costly in text size.

  2. No magic. lilos doesn’t use proc macros or other tricks to hide what it’s doing. Some (normal) macros are available, but only as shorthand, and we’ll explain how to do the thing without the macros. This is important, both for transparency, but also because you might need to customize what’s happening, and you can’t do that if we hide it in a box.

  3. Be portable. lilos doesn’t depend on any vendor-specific hardware, and can run on any ARM Cortex-M processor. This means you don’t need to wait for lilos to be ported to your processor. (lilos is also not inherently ARM-specific – I just don’t have a lot of non-ARM dev hardware. Suggestions welcome!)

  4. Be predictable. lilos doesn’t use dynamic memory allocation, and the task polling behavior is well-defined in the executor. While you can certainly build unpredictable programs on top of lilos, we want to give you a solid predictable foundation.

About the OS

lilos is designed around the notion of a fixed set of concurrent tasks that run forever. To use the OS, your application startup routine calls exec::run_tasks, giving it an array of tasks you’ve defined; run_tasks never returns.

The OS provides cooperative multitasking: while tasks are concurrent, they are not preemptive, and are not “threads” in the traditional sense. Tasks don’t even have their own stacks – they return completely whenever they yield the CPU.

This would be incredibly frustrating to program, were it not for Future and async.

Each task co-routine must be a Future that can be polled but will never complete (because, remember, tasks run forever). The OS provides an executor that manages polling of a set of Futures.

Rust’s async keyword provides a convenient way to have the compiler rewrite a normal function into a co-routine-style Future. This means that writing co-routines to run on this OS looks very much like programming with threads.

Here is the “hello world” of embedded programming, written as a task for this OS. This task blinks an LED attached to port D12 of an STM32F4.

ⓘ
async fn blinky(gpio: &GPIOD) -> Infallible {
    const PERIOD: Duration = Duration::from_millis(500);

    loop {
        gpio.bsrr.write(|w| w.bs12().set_bit());
        lilos::exec::sleep_for(PERIOD).await;
        gpio.bsrr.write(|w| w.br12().set_bit());
        lilos::exec::sleep_for(PERIOD).await;
    }
}

(Note: the natural way to write that function would be with a return type of !, but doing this requires the unstable toolchain, so we rely on core::convert::Infallible instead in this version.)

Composition and dynamic behavior

The notion of a fixed set of tasks might seem limiting, but it’s more flexible than you might expect. Because Futures can be composed, the fixed set of OS tasks can drive a dynamic set of program Futures.

For instance, a task can fork into several concurrent routines using macros like select_biased! or join! from the futures crate.

Concurrency and interrupts

The OS supports the use of interrupt handlers to wake tasks through the Notify mechanism, but most OS facilities are not available in interrupt context.

By default, interrupts are masked when task code is running, so tasks can be confident that they will preempted if, and only if, they await.

Each time through the task polling loop, the OS unmasks interrupts to let any pending interrupts run. Because the Cortex-M collects pending interrupts while interrupts are masked, we don’t run the risk of missing events.

Interrupts are also unmasked whenever the idle processor is woken from sleep, in order to handle the event that woke it up.

If your application requires tighter interrupt response time, you can configure the OS at startup to permit preemption of various kinds – including allowing preemption by only a subset of your interrupts. See the exec module for more details and some customization options.

Cancellation

Co-routine tasks in this OS are just Futures, which means they can be dropped. Futures are typically dropped just after they resolve (often just after an await keyword in the calling code), but it’s also possible to drop a Future while it is pending. This can happen explicitly (by calling drop), or as a side effect of other operations; for example, the macro select_biased! waits for one future to resolve, and then drops the others, whether they’re done or not.

This means it’s useful to consider what cancellation means for any particular task, and to ensure that its results are what you intend. There’s more about this in The Intro Guide and the technical note on Cancellation.

lilos itself tries to make it easier for you to handle cancellation in your programs, by providing APIs that have reasonable behavior on cancel. Specifically,

  • Wherever possible, lilos futures strive for a strict definition of cancel-safety, where dropping a future and retrying the operation that produced it is equivalent to not dropping the future, in terms of visible side effects. (Obviously doing more work will take more CPU cycles; that’s not what we mean by side effects.)

  • Where that is not possible (in a few corner-case APIs) lilos will provide weak cancel-safety, where the behavior of the operation at cancellation is well-defined and documented.

Over time, we’re trying to redesign APIs to move things out of the second category into the first, with the goal of providing a fully cancel-safe OS API. As far as we can tell nobody’s ever done this before, so it might take a bit. If you have suggestions or ideas, please file an issue!

Modules

  • atomic
    A collection of atomic “polyfill” routines, to use a term from JavaScript.
  • exec
    A system for polling an array of tasks forever, plus Notify and other scheduling tools.
  • handoff
    Mechanism for handing data from one task to another, minimizing copies.
  • list
    Doubly-linked intrusive lists for scheduling and waking.
  • mutex
    Fair mutex that must be pinned.
  • spsc
    A queue for moving data from one future/task into another.
  • time
    Timekeeping.
  • util
    Utility code for use by applications.

Macros