implement sleep using WFE

closes #1

async-cortex-m/Cargo.toml

@@ -7,8 +7,9 @@ publish = false
 version = "0.0.0-alpha.0"
 
 [dependencies]
+cortex-m = "0.6.2"
 cortex-m-udf = { path = "../cortex-m-udf" }
 generic-array = "0.13.2"
 heapless = { git = "https://github.com/japaric/heapless", branch = "slab" }
 pin-utils = "0.1.0-alpha.4"
-typenum = "1.11.2"
+typenum = "1.11.2"

async-cortex-m/src/executor.rs

@@ -7,6 +7,7 @@ use core::{
     task::{Context, Poll, RawWaker, RawWakerVTable, Waker},
 };
 
+use cortex_m::asm;
 use heapless::Vec;
 use pin_utils::pin_mut;
 
@@ -107,13 +108,9 @@ impl Executor {
                 }
             }
 
-            // TODO try to sleep (WFI) here
-            // just calling WFI here will result in missed notifications because
-            // a `Waker` (called from an interrupt handler) may run just after
-            // we exit the `for` loop but before WFI is called. Then the system
-            // won't wake up until the *next* interrupt handler is triggered
-            // (which could end up never running if the applications used only a
-            // single interrupt)
+            // try to sleep; this will be a no-op if any of the previous tasks generated a SEV or an
+            // interrupt ran (regardless of whether it generated a wake-up or not)
+            asm::wfe();
         };
         self.in_block_on.set(false);
         val

async-cortex-m/src/task.rs

@@ -6,6 +6,8 @@ use core::{
     task::{Context, Poll},
 };
 
+use cortex_m::asm;
+
 use crate::executor;
 
 /// Drives the future `f` to completion
@@ -39,7 +41,9 @@ pub async fn r#yield() {
                 Poll::Ready(())
             } else {
                 self.yielded = true;
+                // wake ourselves
                 cx.waker().wake_by_ref();
+                asm::sev();
                 Poll::Pending
             }
         }

async-cortex-m/src/unsync/channel.rs

@@ -12,6 +12,7 @@ use core::{
     task::{Context, Poll},
 };
 
+use cortex_m::asm;
 use generic_array::{typenum::Unsigned, GenericArray};
 
 use super::waker_set::WakerSet;
@@ -136,6 +137,7 @@ impl<T> Channel<T> {
                 self.read.set(read.wrapping_add(1));
                 // notify a sender
                 self.send_wakers.notify_one();
+                asm::sev();
                 Some(val)
             } else {
                 // empty
@@ -160,6 +162,7 @@ impl<T> Channel<T> {
                 self.write.set(write.wrapping_add(1));
                 // notify a receiver
                 self.recv_wakers.notify_one();
+                asm::sev();
                 Ok(())
             } else {
                 // full

async-cortex-m/src/unsync/mutex.rs

@@ -8,6 +8,8 @@ use core::{
     task::{Context, Poll},
 };
 
+use cortex_m::asm;
+
 use super::waker_set::WakerSet;
 
 /// A mutual exclusion primitive for protecting shared data
@@ -92,6 +94,7 @@ impl<T> Drop for MutexGuard<'_, T> {
     fn drop(&mut self) {
         self.0.locked.set(false);
         self.0.wakers.notify_any();
+        asm::wfe();
     }
 }
 

nrf52/examples/7-echo.rs

@@ -35,7 +35,7 @@ fn main() -> ! {
 
     let (mut tx, mut rx) = serial::take();
     task::block_on(async {
-        let mut buf = [0; 16];
+        let mut buf = [0; 1];
         loop {
             rx.read(&mut buf).await;
             tx.write(&buf).await;