Monotonic rewrite (#874)

* Rework timer_queue and monotonic architecture

Goals:
 * make Monotonic purely internal
 * make Monotonic purely tick passed, no fugit involved
 * create a wrapper struct in the user's code via a macro that then
   converts the "now" from the tick based monotonic to a fugit based
   timestamp

We need to proxy the delay functions of the timer queue anyway,
so we could simply perform the conversion in those proxy functions.

* Update cargo.lock

* Update readme of rtic-time

* CI: ESP32: Redact esp_image: Too volatile

* Fixup: Changelog double entry rebase mistake

---------

Co-authored-by: Henrik Tjäder <henrik@tjaders.com>

2 files changed, 93 insertions, 0 deletions

diff --git a/rtic-time/src/timer_queue/backend.rs b/rtic-time/src/timer_queue/backend.rs
new file mode 100644
index 0000000..5b71b7c
--- /dev/null
+++ b/rtic-time/src/timer_queue/backend.rs
@@ -0,0 +1,44 @@
+use super::{TimerQueue, TimerQueueTicks};
+
+/// A backend definition for a monotonic clock/counter.
+pub trait TimerQueueBackend: 'static + Sized {
+    /// The type for ticks.
+    type Ticks: TimerQueueTicks;
+
+    /// Get the current time.
+    fn now() -> Self::Ticks;
+
+    /// Set the compare value of the timer interrupt.
+    ///
+    /// **Note:** This method does not need to handle race conditions of the monotonic, the timer
+    /// queue in RTIC checks this.
+    fn set_compare(instant: Self::Ticks);
+
+    /// Clear the compare interrupt flag.
+    fn clear_compare_flag();
+
+    /// Pend the timer's interrupt.
+    fn pend_interrupt();
+
+    /// Optional. Runs on interrupt before any timer queue handling.
+    fn on_interrupt() {}
+
+    /// Optional. This is used to save power, this is called when the timer queue is not empty.
+    ///
+    /// Enabling and disabling the monotonic needs to propagate to `now` so that an instant
+    /// based of `now()` is still valid.
+    ///
+    /// NOTE: This may be called more than once.
+    fn enable_timer() {}
+
+    /// Optional. This is used to save power, this is called when the timer queue is empty.
+    ///
+    /// Enabling and disabling the monotonic needs to propagate to `now` so that an instant
+    /// based of `now()` is still valid.
+    ///
+    /// NOTE: This may be called more than once.
+    fn disable_timer() {}
+
+    /// Returns a reference to the underlying timer queue.
+    fn timer_queue() -> &'static TimerQueue<Self>;
+}
diff --git a/rtic-time/src/timer_queue/tick_type.rs b/rtic-time/src/timer_queue/tick_type.rs
new file mode 100644
index 0000000..571dfc2
--- /dev/null
+++ b/rtic-time/src/timer_queue/tick_type.rs
@@ -0,0 +1,49 @@
+use core::cmp;
+
+/// The ticks of a timer.
+pub trait TimerQueueTicks: Copy + PartialEq + Eq {
+    /// Represents a single tick.
+    const ONE_TICK: Self;
+
+    /// Compares to another tick count.
+    ///
+    /// Takes into account timer wrapping; if the difference is more than
+    /// half the value range, the result will be flipped.
+    fn compare(self, other: Self) -> cmp::Ordering;
+
+    /// True if `self` is at the same time as `other` or later.
+    ///
+    /// Takes into account timer wrapping; if the difference is more than
+    /// half the value range, the result will be negated.
+    fn is_at_least(self, other: Self) -> bool {
+        match self.compare(other) {
+            cmp::Ordering::Less => false,
+            cmp::Ordering::Equal => true,
+            cmp::Ordering::Greater => true,
+        }
+    }
+
+    /// Wrapping addition.
+    fn wrapping_add(self, other: Self) -> Self;
+}
+
+impl TimerQueueTicks for u32 {
+    const ONE_TICK: Self = 1;
+
+    fn compare(self, other: Self) -> cmp::Ordering {
+        (self.wrapping_sub(other) as i32).cmp(&0)
+    }
+    fn wrapping_add(self, other: Self) -> Self {
+        u32::wrapping_add(self, other)
+    }
+}
+impl TimerQueueTicks for u64 {
+    const ONE_TICK: Self = 1;
+
+    fn compare(self, other: Self) -> cmp::Ordering {
+        (self.wrapping_sub(other) as i64).cmp(&0)
+    }
+    fn wrapping_add(self, other: Self) -> Self {
+        u64::wrapping_add(self, other)
+    }
+}