rtic/src/export.rs

#![allow(clippy::inline_always)]
use core::{
    cell::Cell,
    sync::atomic::{AtomicBool, Ordering},
};

pub use crate::tq::{NotReady, TimerQueue};
pub use bare_metal::CriticalSection;
pub use cortex_m::{
    asm::nop,
    asm::wfi,
    interrupt,
    peripheral::{scb::SystemHandler, DWT, NVIC, SCB, SYST},
    Peripherals,
};
pub use heapless::sorted_linked_list::SortedLinkedList;
pub use heapless::spsc::Queue;
pub use heapless::BinaryHeap;
pub use rtic_monotonic as monotonic;

pub type SCFQ<const N: usize> = Queue<u8, N>;
pub type SCRQ<T, const N: usize> = Queue<(T, u8), N>;

#[cfg(armv7m)]
use cortex_m::register::basepri;

#[cfg(armv7m)]
#[inline(always)]
pub fn run<F>(priority: u8, f: F)
where
    F: FnOnce(),
{
    if priority == 1 {
        // If the priority of this interrupt is `1` then BASEPRI can only be `0`
        f();
        unsafe { basepri::write(0) }
    } else {
        let initial = basepri::read();
        f();
        unsafe { basepri::write(initial) }
    }
}

#[cfg(not(armv7m))]
#[inline(always)]
pub fn run<F>(_priority: u8, f: F)
where
    F: FnOnce(),
{
    f();
}

pub struct Barrier {
    inner: AtomicBool,
}

impl Barrier {
    pub const fn new() -> Self {
        Barrier {
            inner: AtomicBool::new(false),
        }
    }

    pub fn release(&self) {
        self.inner.store(true, Ordering::Release);
    }

    pub fn wait(&self) {
        while !self.inner.load(Ordering::Acquire) {}
    }
}

// Newtype over `Cell` that forbids mutation through a shared reference
pub struct Priority {
    inner: Cell<u8>,
}

impl Priority {
    /// Create a new Priority
    ///
    /// # Safety
    ///
    /// Will overwrite the current Priority
    #[inline(always)]
    pub unsafe fn new(value: u8) -> Self {
        Priority {
            inner: Cell::new(value),
        }
    }

    /// Change the current priority to `value`
    // These two methods are used by `lock` (see below) but can't be used from the RTIC application
    #[inline(always)]
    fn set(&self, value: u8) {
        self.inner.set(value);
    }

    /// Get the current priority
    #[inline(always)]
    fn get(&self) -> u8 {
        self.inner.get()
    }
}

/// Const helper to check architecture
pub const fn is_armv6() -> bool {
    #[cfg(not(armv6m))]
    {
        false
    }

    #[cfg(armv6m)]
    {
        true
    }
}

#[inline(always)]
pub fn assert_send<T>()
where
    T: Send,
{
}

#[inline(always)]
pub fn assert_sync<T>()
where
    T: Sync,
{
}

#[inline(always)]
pub fn assert_monotonic<T>()
where
    T: monotonic::Monotonic,
{
}

/// Lock implementation using BASEPRI and global Critical Section (CS)
///
/// # Safety
///
/// The system ceiling is raised from current to ceiling
/// by either
/// - raising the BASEPRI to the ceiling value, or
/// - disable all interrupts in case we want to
///   mask interrupts with maximum priority
///
/// Dereferencing a raw pointer inside CS
///
/// The priority.set/priority.get can safely be outside the CS
/// as being a context local cell (not affected by preemptions).
/// It is merely used in order to omit masking in case current
/// priority is current priority >= ceiling.
///
/// Lock Efficiency:
/// Experiments validate (sub)-zero cost for CS implementation
/// (Sub)-zero as:
/// - Either zero OH (lock optimized out), or
/// - Amounting to an optimal assembly implementation
///   - The BASEPRI value is folded to a constant at compile time
///   - CS entry, single assembly instruction to write BASEPRI
///   - CS exit, single assembly instruction to write BASEPRI
///   - priority.set/get optimized out (their effect not)
/// - On par or better than any handwritten implementation of SRP
///
/// Limitations:
/// The current implementation reads/writes BASEPRI once
/// even in some edge cases where this may be omitted.
/// Total OH of per task is max 2 clock cycles, negligible in practice
/// but can in theory be fixed.
///
#[cfg(armv7m)]
#[inline(always)]
pub unsafe fn lock<T, R>(
    ptr: *mut T,
    priority: &Priority,
    ceiling: u8,
    nvic_prio_bits: u8,
    _mask: &[u32; 3],
    f: impl FnOnce(&mut T) -> R,
) -> R {
    let current = priority.get();

    if current < ceiling {
        if ceiling == (1 << nvic_prio_bits) {
            priority.set(u8::max_value());
            let r = interrupt::free(|_| f(&mut *ptr));
            priority.set(current);
            r
        } else {
            priority.set(ceiling);
            basepri::write(logical2hw(ceiling, nvic_prio_bits));
            let r = f(&mut *ptr);
            basepri::write(logical2hw(current, nvic_prio_bits));
            priority.set(current);
            r
        }
    } else {
        f(&mut *ptr)
    }
}

/// Lock implementation using interrupt masking
///
/// # Safety
///
/// The system ceiling is raised from current to ceiling
/// by computing a 32 bit `mask` (1 bit per interrupt)
/// 1: ceiling >= priority > current
/// 0: else
///
/// On CS entry, `clear_enable_mask(mask)` disables interrupts
/// On CS exit,  `set_enable_mask(mask)` re-enables interrupts
///
/// The priority.set/priority.get can safely be outside the CS
/// as being a context local cell (not affected by preemptions).
/// It is merely used in order to omit masking in case
/// current priority >= ceiling.
///
/// Dereferencing a raw pointer is done safely inside the CS
///
/// Lock Efficiency:
/// Early experiments validate (sub)-zero cost for CS implementation
/// (Sub)-zero as:
/// - Either zero OH (lock optimized out), or
/// - Amounting to an optimal assembly implementation
///   - if ceiling == (1 << nvic_prio_bits)
///     - we execute the closure in a global critical section (interrupt free)
///     - CS entry cost, single write to core register
///     - CS exit cost, single write to core register
///   else
///     - The `mask` value is folded to a constant at compile time
///     - CS entry, single write of the 32 bit `mask` to the `icer` register
///     - CS exit, single write of the 32 bit `mask` to the `iser` register
/// - priority.set/get optimized out (their effect not)
/// - On par or better than any hand written implementation of SRP
///
/// Limitations:
/// Current implementation does not allow for tasks with shared resources
/// to be bound to exception handlers, as these cannot be masked in HW.
///
/// Possible solutions:
/// - Mask exceptions by global critical sections (interrupt::free)
/// - Temporary lower exception priority
///
/// These possible solutions are set goals for future work
#[cfg(not(armv7m))]
#[inline(always)]
pub unsafe fn lock<T, R>(
    ptr: *mut T,
    priority: &Priority,
    ceiling: u8,
    _nvic_prio_bits: u8,
    masks: &[u32; 3],
    f: impl FnOnce(&mut T) -> R,
) -> R {
    let current = priority.get();
    if current < ceiling {
        if ceiling >= 4 {
            // safe to manipulate outside critical section
            priority.set(ceiling);
            // execute closure under protection of raised system ceiling
            let r = interrupt::free(|_| f(&mut *ptr));
            // safe to manipulate outside critical section
            priority.set(current);
            r
        } else {
            // safe to manipulate outside critical section
            priority.set(ceiling);
            let mask = compute_mask(current, ceiling, masks);
            clear_enable_mask(mask);

            // execute closure under protection of raised system ceiling
            let r = f(&mut *ptr);

            set_enable_mask(mask);

            // safe to manipulate outside critical section
            priority.set(current);
            r
        }
    } else {
        // execute closure without raising system ceiling
        f(&mut *ptr)
    }
}

#[cfg(not(armv7m))]
#[inline(always)]
fn compute_mask(from_prio: u8, to_prio: u8, masks: &[u32; 3]) -> u32 {
    let mut res = 0;
    masks[from_prio as usize..to_prio as usize]
        .iter()
        .for_each(|m| res |= m);
    res
}

// enables interrupts
#[cfg(not(armv7m))]
#[inline(always)]
unsafe fn set_enable_mask(mask: u32) {
    (*NVIC::ptr()).iser[0].write(mask)
}

// disables interrupts
#[cfg(not(armv7m))]
#[inline(always)]
unsafe fn clear_enable_mask(mask: u32) {
    (*NVIC::ptr()).icer[0].write(mask)
}

#[inline]
#[must_use]
pub fn logical2hw(logical: u8, nvic_prio_bits: u8) -> u8 {
    ((1 << nvic_prio_bits) - logical) << (8 - nvic_prio_bits)
}