From ecd08a87a9571776927499156b9931b602b2b716 Mon Sep 17 00:00:00 2001
From: Charles Thompson <chmithbiz@gmail.com>
Date: Sun, 7 May 2023 13:22:55 -0400
Subject: [PATCH] Changed busy wait to wfe

---
 rp2040-hal/src/sio.rs | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/rp2040-hal/src/sio.rs b/rp2040-hal/src/sio.rs
index 9238dd3f6..5d206d9c0 100644
--- a/rp2040-hal/src/sio.rs
+++ b/rp2040-hal/src/sio.rs
@@ -137,7 +137,13 @@ impl SioFifo {
     pub fn read(&mut self) -> Option<u32> {
         if self.is_read_ready() {
             let sio = unsafe { &(*pac::SIO::ptr()) };
-            Some(sio.fifo_rd.read().bits())
+
+            let value = sio.fifo_rd.read().bits();
+
+            // Wake up the other core that might be waiting on a read
+            cortex_m::asm::sev();
+
+            Some(value)
         } else {
             None
         }
@@ -152,17 +158,14 @@ impl SioFifo {
 
     /// Push to the FIFO, spinning if there's no space.
     pub fn write_blocking(&mut self, value: u32) {
-        // We busy-wait for the FIFO to have some space
+        // We wait for the FIFO to have some space
         while !self.is_write_ready() {
-            cortex_m::asm::nop();
+            cortex_m::asm::wfe();
         }
 
         // Write the value to the FIFO - the other core will now be able to
         // pop it off its end of the FIFO.
         self.write(value);
-
-        // Fire off an event to the other core
-        cortex_m::asm::sev();
     }
 
     /// Pop from the FIFO, spinning if there's currently no data.