1 files changed, 91 insertions, 6 deletions

diff --git a/embassy-extras/src/peripheral.rs b/embassy-extras/src/peripheral.rs
index 68972c543..92512a0f6 100644
--- a/embassy-extras/src/peripheral.rs
+++ b/embassy-extras/src/peripheral.rs
@@ -2,9 +2,19 @@ use core::cell::UnsafeCell;
 use core::marker::{PhantomData, PhantomPinned};
 use core::pin::Pin;
 
+use cortex_m::peripheral::scb::VectActive;
+use cortex_m::peripheral::{NVIC, SCB};
 use embassy::interrupt::{Interrupt, InterruptExt};
 
-pub trait PeripheralState {
+/// A type which can be used as state with `PeripheralMutex`.
+///
+/// It needs to be `Send` because `&mut` references are sent back and forth between the 'thread' which owns the `PeripheralMutex` and the interrupt,
+/// and `&mut T` is only `Send` where `T: Send`.
+///
+/// It also requires `'static` to be used safely with `PeripheralMutex::register_interrupt`,
+/// because although `Pin` guarantees that the memory of the state won't be invalidated,
+/// it doesn't guarantee that the lifetime will last.
+pub trait PeripheralState: Send {
     type Interrupt: Interrupt;
     fn on_interrupt(&mut self);
 }
@@ -19,8 +29,51 @@ pub struct PeripheralMutex<S: PeripheralState> {
     _pinned: PhantomPinned,
 }
 
+/// Whether `irq` can be preempted by the current interrupt.
+pub(crate) fn can_be_preempted(irq: &impl Interrupt) -> bool {
+    match SCB::vect_active() {
+        // Thread mode can't preempt anything.
+        VectActive::ThreadMode => false,
+        // Exceptions don't always preempt interrupts,
+        // but there isn't much of a good reason to be keeping a `PeripheralMutex` in an exception anyway.
+        VectActive::Exception(_) => true,
+        VectActive::Interrupt { irqn } => {
+            #[derive(Clone, Copy)]
+            struct NrWrap(u16);
+            unsafe impl cortex_m::interrupt::InterruptNumber for NrWrap {
+                fn number(self) -> u16 {
+                    self.0
+                }
+            }
+            NVIC::get_priority(NrWrap(irqn.into())) < irq.get_priority().into()
+        }
+    }
+}
+
+impl<S: PeripheralState + 'static> PeripheralMutex<S> {
+    /// Registers `on_interrupt` as the wrapped interrupt's interrupt handler and enables it.
+    ///
+    /// This requires this `PeripheralMutex`'s `PeripheralState` to live for `'static`,
+    /// because `Pin` only guarantees that it's memory won't be repurposed,
+    /// not that it's lifetime will last.
+    ///
+    /// To use non-`'static` `PeripheralState`, use the unsafe `register_interrupt_unchecked`.
+    ///
+    /// Note: `'static` doesn't mean it _has_ to live for the entire program, like an `&'static T`;
+    /// it just means it _can_ live for the entire program - for example, `u8` lives for `'static`.
+    pub fn register_interrupt(self: Pin<&mut Self>) {
+        // SAFETY: `S: 'static`, so there's no way it's lifetime can expire.
+        unsafe { self.register_interrupt_unchecked() }
+    }
+}
+
 impl<S: PeripheralState> PeripheralMutex<S> {
+    /// Create a new `PeripheralMutex` wrapping `irq`, with the initial state `state`.
     pub fn new(state: S, irq: S::Interrupt) -> Self {
+        if can_be_preempted(&irq) {
+            panic!("`PeripheralMutex` cannot be created in an interrupt with higher priority than the interrupt it wraps");
+        }
+
         Self {
             irq,
             irq_setup_done: false,
@@ -31,8 +84,18 @@ impl<S: PeripheralState> PeripheralMutex<S> {
         }
     }
 
-    pub fn register_interrupt(self: Pin<&mut Self>) {
-        let this = unsafe { self.get_unchecked_mut() };
+    /// Registers `on_interrupt` as the wrapped interrupt's interrupt handler and enables it.
+    ///
+    /// # Safety
+    /// The lifetime of any data in `PeripheralState` that is accessed by the interrupt handler
+    /// must not end without `Drop` being called on this `PeripheralMutex`.
+    ///
+    /// This can be accomplished by either not accessing any data with a lifetime in `on_interrupt`,
+    /// or making sure that nothing like `mem::forget` is used on the `PeripheralMutex`.
+
+    // TODO: this name isn't the best.
+    pub unsafe fn register_interrupt_unchecked(self: Pin<&mut Self>) {
+        let this = self.get_unchecked_mut();
         if this.irq_setup_done {
             return;
         }
@@ -40,7 +103,9 @@ impl<S: PeripheralState> PeripheralMutex<S> {
         this.irq.disable();
         this.irq.set_handler(|p| {
             // Safety: it's OK to get a &mut to the state, since
-            // - We're in the IRQ, no one else can't preempt us
+            // - We checked that the thread owning the `PeripheralMutex` can't preempt us in `new`.
+            //   Interrupts' priorities can only be changed with raw embassy `Interrupts`,
+            //   which can't safely store a `PeripheralMutex` across invocations.
             // - We can't have preempted a with() call because the irq is disabled during it.
             let state = unsafe { &mut *(p as *mut S) };
             state.on_interrupt();
@@ -52,19 +117,39 @@ impl<S: PeripheralState> PeripheralMutex<S> {
         this.irq_setup_done = true;
     }
 
-    pub fn with<R>(self: Pin<&mut Self>, f: impl FnOnce(&mut S, &mut S::Interrupt) -> R) -> R {
+    pub fn with<R>(self: Pin<&mut Self>, f: impl FnOnce(&mut S) -> R) -> R {
         let this = unsafe { self.get_unchecked_mut() };
 
         this.irq.disable();
 
         // Safety: it's OK to get a &mut to the state, since the irq is disabled.
         let state = unsafe { &mut *this.state.get() };
-        let r = f(state, &mut this.irq);
+        let r = f(state);
 
         this.irq.enable();
 
         r
     }
+
+    /// Returns whether the wrapped interrupt is currently in a pending state.
+    pub fn is_pending(&self) -> bool {
+        self.irq.is_pending()
+    }
+
+    /// Forces the wrapped interrupt into a pending state.
+    pub fn pend(&self) {
+        self.irq.pend()
+    }
+
+    /// Forces the wrapped interrupt out of a pending state.
+    pub fn unpend(&self) {
+        self.irq.unpend()
+    }
+
+    /// Gets the priority of the wrapped interrupt.
+    pub fn priority(&self) -> <S::Interrupt as Interrupt>::Priority {
+        self.irq.get_priority()
+    }
 }
 
 impl<S: PeripheralState> Drop for PeripheralMutex<S> {