Merge pull request #4688 from 9names/rp2350_msplim_stackguard

Use msplim for RP2350 stackguard

3 files changed, 76 insertions, 11 deletions

diff --git a/embassy-rp/CHANGELOG.md b/embassy-rp/CHANGELOG.md
index d1265ffc4..ea62c2387 100644
--- a/embassy-rp/CHANGELOG.md
+++ b/embassy-rp/CHANGELOG.md
@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Add PIO I2S input
 - Add PIO onewire parasite power strong pullup
 - add `wait_for_alarm` and `alarm_scheduled` methods to rtc module ([#4216](https://github.com/embassy-rs/embassy/pull/4216))
+- rp235x: use msplim for stack guard instead of MPU
 
 ## 0.8.0 - 2025-08-26
 
diff --git a/embassy-rp/src/lib.rs b/embassy-rp/src/lib.rs
index 6fb680b34..d03ba1fef 100644
--- a/embassy-rp/src/lib.rs
+++ b/embassy-rp/src/lib.rs
@@ -565,18 +565,10 @@ unsafe fn install_stack_guard(stack_bottom: *mut usize) -> Result<(), ()> {
 #[cfg(all(feature = "_rp235x", not(feature = "_test")))]
 #[inline(always)]
 unsafe fn install_stack_guard(stack_bottom: *mut usize) -> Result<(), ()> {
-    let core = unsafe { cortex_m::Peripherals::steal() };
+    // The RP2350 arm cores are cortex-m33 and can use the MSPLIM register to guard the end of stack.
+    // We'll need to do something else for the riscv cores.
+    cortex_m::register::msplim::write(stack_bottom.addr() as u32);
 
-    // Fail if MPU is already configured
-    if core.MPU.ctrl.read() != 0 {
-        return Err(());
-    }
-
-    unsafe {
-        core.MPU.ctrl.write(5); // enable mpu with background default map
-        core.MPU.rbar.write(stack_bottom as u32 & !0xff); // set address
-        core.MPU.rlar.write(((stack_bottom as usize + 255) as u32) | 1);
-    }
     Ok(())
 }
 
diff --git a/examples/rp235x/src/bin/multicore_stack_overflow.rs b/examples/rp235x/src/bin/multicore_stack_overflow.rs
new file mode 100644
index 000000000..dba44aa23
--- /dev/null
+++ b/examples/rp235x/src/bin/multicore_stack_overflow.rs
@@ -0,0 +1,72 @@
+//! This example tests stack overflow handling on core1 of the RP235x chip.
+
+#![no_std]
+#![no_main]
+
+use defmt::*;
+use embassy_executor::Executor;
+use embassy_rp::gpio::{Level, Output};
+use embassy_rp::multicore::{spawn_core1, Stack};
+use embassy_time::Timer;
+use static_cell::StaticCell;
+use {defmt_rtt as _, panic_probe as _};
+
+const CORE1_STACK_LENGTH: usize = 4096;
+
+static mut CORE1_STACK: Stack<CORE1_STACK_LENGTH> = Stack::new();
+static EXECUTOR0: StaticCell<Executor> = StaticCell::new();
+static EXECUTOR1: StaticCell<Executor> = StaticCell::new();
+
+#[cortex_m_rt::entry]
+fn main() -> ! {
+    let p = embassy_rp::init(Default::default());
+    let led = Output::new(p.PIN_25, Level::Low);
+
+    spawn_core1(
+        p.CORE1,
+        unsafe { &mut *core::ptr::addr_of_mut!(CORE1_STACK) },
+        move || {
+            let executor1 = EXECUTOR1.init(Executor::new());
+            executor1.run(|spawner| spawner.spawn(unwrap!(core1_task())));
+        },
+    );
+
+    let executor0 = EXECUTOR0.init(Executor::new());
+    executor0.run(|spawner| spawner.spawn(unwrap!(core0_task(led))));
+}
+
+#[embassy_executor::task]
+async fn core0_task(mut led: Output<'static>) {
+    info!("Hello from core 0");
+    loop {
+        info!("core 0 still alive");
+        led.set_high();
+        Timer::after_millis(500).await;
+        led.set_low();
+        Timer::after_millis(500).await;
+    }
+}
+
+fn blow_my_stack() {
+    // Allocating an array a little larger than our stack should ensure a stack overflow when it is used.
+    let t = [0u8; CORE1_STACK_LENGTH + 64];
+
+    info!("Array initialised without error");
+    // We need to use black_box to otherwise the compiler is too smart and will optimise all of this away.
+    // We shouldn't get to this code - the initialisation above will touch the stack guard.
+    for ref i in t {
+        let _data = core::hint::black_box(*i) + 1;
+    }
+}
+
+#[embassy_executor::task]
+async fn core1_task() {
+    info!("Hello from core 1");
+
+    blow_my_stack();
+
+    loop {
+        info!("core 1 still alive");
+        Timer::after_millis(1000).await;
+    }
+}