uarte: Low power wait for RX drop

embassy-nrf/src/uarte.rs

@@ -17,8 +17,8 @@ use crate::hal::gpio::Port as GpioPort;
 use crate::hal::pac;
 use crate::hal::prelude::*;
 use crate::hal::target_constants::EASY_DMA_SIZE;
-use crate::interrupt;
 use crate::interrupt::OwnedInterrupt;
+use crate::{interrupt, util};
 
 pub use crate::hal::uarte::Pins;
 // Re-export SVD variants to allow user to directly set values.
@@ -275,7 +275,9 @@ where
                 .instance
                 .tasks_stoptx
                 .write(|w| unsafe { w.bits(1) });
-            T::state().tx_done.blocking_wait();
+
+            // TX is stopped almost instantly, spinning is fine.
+            while !T::state().tx_done.signaled() {}
         }
     }
 }
@@ -342,7 +344,8 @@ where
                 .instance
                 .tasks_stoprx
                 .write(|w| unsafe { w.bits(1) });
-            T::state().rx_done.blocking_wait();
+
+            util::low_power_wait_until(|| T::state().rx_done.signaled())
         }
     }
 }
@@ -394,7 +397,7 @@ where
     T: Instance,
 {
     /// Stops the ongoing reception and returns the number of bytes received.
-    pub async fn stop(mut self) -> usize {
+    pub async fn stop(self) -> usize {
         let len = if self.uarte.rx_started() {
             trace!("stoprx (stop)");
 

embassy-nrf/src/util/mod.rs

@@ -1,2 +1,12 @@
 pub mod peripheral;
 pub mod ring_buffer;
+
+/// Low power blocking wait loop using WFE/SEV.
+pub fn low_power_wait_until(mut condition: impl FnMut() -> bool) {
+    while !condition() {
+        // WFE might "eat" an event that would have otherwise woken the executor.
+        cortex_m::asm::wfe();
+    }
+    // Retrigger an event to be transparent to the executor.
+    cortex_m::asm::sev();
+}

embassy/src/util/signal.rs

@@ -63,12 +63,7 @@ impl<T: Send> Signal<T> {
         futures::future::poll_fn(move |cx| self.poll_wait(cx))
     }
 
-    /// Blocks until the signal has been received.
-    ///
-    /// Returns immediately when [`poll_wait()`] has not been called before.
-    pub fn blocking_wait(&self) {
-        while cortex_m::interrupt::free(|_| {
-            matches!(unsafe { &*self.state.get() }, State::Waiting(_))
-        }) {}
+    pub fn signaled(&self) -> bool {
+        cortex_m::interrupt::free(|_| matches!(unsafe { &*self.state.get() }, State::Signaled(_)))
     }
 }