Refactor race condition free timer helper (#850)

* Implement half_period_counter in rtic-time

* Rename compute_now to calculate_now, use it in stm32 and imxrt

* Add more tests

* Add some docs

* Fix clippy warning, add imxrt timer to monotonics tests

* Bump dependency version to make sure monotonics will build properly

* Add changelog to rtic-monotonics

* Add more docs

* Add more docs

* Finish documentation

* Fix typos

* Switch from atomic-polyfill to portable-atomic

* Some more doc fixes

* More doc fixes

* Minor doc fix

* Minor doc fix

* Fix Atomics not existing

* Fix example

* Minor example improvement

* Revert back to atomic-polyfill

* Fix cargo.toml formatting

* Remove atomic-polyfill

* Attempt to fix unused macro warning

* Remove atomics completely from half period counter

* Minor doc fix

* Doc fixes

* Doc fixes

* Remove obsolete comment

* Fix ordering in monotonic initialization sequence

1 files changed, 9 insertions, 15 deletions

diff --git a/rtic-monotonics/src/stm32.rs b/rtic-monotonics/src/stm32.rs
index 254f130..c86005e 100644
--- a/rtic-monotonics/src/stm32.rs
+++ b/rtic-monotonics/src/stm32.rs
@@ -35,8 +35,9 @@
 //! ```
 
 use crate::{Monotonic, TimeoutError, TimerQueue};
-use atomic_polyfill::{compiler_fence, AtomicU64, Ordering};
+use atomic_polyfill::{AtomicU64, Ordering};
 pub use fugit::{self, ExtU64, ExtU64Ceil};
+use rtic_time::half_period_counter::calculate_now;
 use stm32_metapac as pac;
 
 mod _generated {
@@ -166,13 +167,14 @@ macro_rules! make_timer {
                 // Since this is not the case, it should be cleared.
                 $timer.sr().modify(|r| r.set_uif(false));
 
+                $tq.initialize(Self {});
+                $overflow.store(0, Ordering::SeqCst);
+
                 // Start the counter.
                 $timer.cr1().modify(|r| {
                     r.set_cen(true);
                 });
 
-                $tq.initialize(Self {});
-
                 // SAFETY: We take full ownership of the peripheral and interrupt vector,
                 // plus we are not using any external shared resources so we won't impact
                 // basepri/source masking based critical sections.
@@ -231,18 +233,10 @@ macro_rules! make_timer {
             const TICK_PERIOD: Self::Duration = Self::Duration::from_ticks(1);
 
             fn now() -> Self::Instant {
-                // Credits to the `time-driver` of `embassy-stm32`.
-                // For more info, see the `imxrt` driver.
-                fn calc_now(period: u64, counter: $bits) -> u64 {
-                    (period << ($bits::BITS - 1)) + u64::from(counter ^ (((period & 1) as $bits) << ($bits::BITS - 1)))
-                }
-
-                // Important: period **must** be read first.
-                let period = $overflow.load(Ordering::Relaxed);
-                compiler_fence(Ordering::Acquire);
-                let counter = $timer.cnt().read().cnt();
-
-                Self::Instant::from_ticks(calc_now(period, counter))
+                Self::Instant::from_ticks(calculate_now(
+                    $overflow.load(Ordering::Relaxed),
+                    || $timer.cnt().read().cnt()
+                ))
             }
 
             fn set_compare(instant: Self::Instant) {