From 0631623b511d2906993bf2fa3799ea7eec3d7f4a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timo=20Kr=C3=B6ger?= <timokroeger93@gmail.com>
Date: Sun, 3 Jan 2021 17:05:04 +0100
Subject: [PATCH] uarte: Low power wait for RX drop

---
 embassy-nrf/src/uarte.rs    | 13 ++++++++-----
 embassy-nrf/src/util/mod.rs | 10 ++++++++++
 embassy/src/util/signal.rs  |  9 ++-------
 3 files changed, 20 insertions(+), 12 deletions(-)

diff --git a/embassy-nrf/src/uarte.rs b/embassy-nrf/src/uarte.rs
index dc2093c3..bfebf5ca 100644
--- a/embassy-nrf/src/uarte.rs
+++ b/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())
         }
     }
 }
@@ -361,7 +364,7 @@ where
                 let ptr = buf.as_ptr();
                 let len = buf.len();
                 assert!(len <= EASY_DMA_SIZE);
-                
+
                 uarte.enable();
 
                 compiler_fence(Ordering::SeqCst);
@@ -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)");
 
diff --git a/embassy-nrf/src/util/mod.rs b/embassy-nrf/src/util/mod.rs
index 2fd5453d..cf330654 100644
--- a/embassy-nrf/src/util/mod.rs
+++ b/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();
+}
diff --git a/embassy/src/util/signal.rs b/embassy/src/util/signal.rs
index cb641061..8e778d1e 100644
--- a/embassy/src/util/signal.rs
+++ b/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(_)))
     }
 }