From cddf7664912b43b54e034ca33cbfb269c597a9c5 Mon Sep 17 00:00:00 2001
From: Andrew Walbran <qwandor@google.com>
Date: Thu, 6 Apr 2023 15:55:29 +0100
Subject: [PATCH] WFI

---
 src/exercises/bare-metal/rtc/src/gicv3.rs | 8 ++++++++
 src/exercises/bare-metal/rtc/src/main.rs  | 4 ++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/src/exercises/bare-metal/rtc/src/gicv3.rs b/src/exercises/bare-metal/rtc/src/gicv3.rs
index 366d6320..f773c64f 100644
--- a/src/exercises/bare-metal/rtc/src/gicv3.rs
+++ b/src/exercises/bare-metal/rtc/src/gicv3.rs
@@ -553,3 +553,11 @@ pub fn irq_enable() {
         asm!("msr DAIFClr, #0xf", options(nomem, nostack));
     }
 }
+
+/// Waits for an interrupt.
+pub fn wfi() {
+    // Safe because this doesn't access memory in any way.
+    unsafe {
+        asm!("wfi", options(nomem, nostack));
+    }
+}
diff --git a/src/exercises/bare-metal/rtc/src/main.rs b/src/exercises/bare-metal/rtc/src/main.rs
index 0049f3ba..a45fe9aa 100644
--- a/src/exercises/bare-metal/rtc/src/main.rs
+++ b/src/exercises/bare-metal/rtc/src/main.rs
@@ -23,7 +23,7 @@ mod pl011;
 // ANCHOR_END: top
 mod pl031;
 
-use crate::gicv3::{irq_enable, GicV3, Trigger, SPI_START};
+use crate::gicv3::{irq_enable, wfi, GicV3, Trigger, SPI_START};
 use crate::pl031::Rtc;
 use chrono::{TimeZone, Utc};
 use core::hint::spin_loop;
@@ -113,7 +113,7 @@ extern "C" fn main(x0: u64, x1: u64, x2: u64, x3: u64) {
         rtc.interrupt_pending()
     );
     while !rtc.interrupt_pending() {
-        spin_loop();
+        wfi();
     }
     trace!(
         "matched={}, interrupt_pending={}",