Don't wake the future for every byte in fill_bytes

embassy-nrf/src/rng.rs

@@ -1,13 +1,18 @@
+use core::cell::RefCell;
 use core::convert::Infallible;
+use core::future::Future;
 use core::marker::PhantomData;
+use core::ptr::NonNull;
+use core::task::Poll;
+use core::task::Waker;
 
 use embassy::interrupt::InterruptExt;
 use embassy::traits;
+use embassy::util::CriticalSectionMutex;
 use embassy::util::OnDrop;
-use embassy::util::Signal;
 use embassy::util::Unborrow;
 use embassy_extras::unborrow;
-use futures::Future;
+use futures::future::poll_fn;
 use rand_core::RngCore;
 
 use crate::interrupt;
@@ -20,18 +25,41 @@ impl RNG {
     }
 }
 
-static NEXT_BYTE: Signal<u8> = Signal::new();
+static STATE: CriticalSectionMutex<RefCell<State>> =
+    CriticalSectionMutex::new(RefCell::new(State {
+        buffer: None,
+        waker: None,
+        index: 0,
+    }));
+
+struct State {
+    buffer: Option<NonNull<[u8]>>,
+    waker: Option<Waker>,
+    index: usize,
+}
+
+// SAFETY: `NonNull` is `!Send` because of the possibility of it being aliased.
+// However, `buffer` is only used within `on_interrupt`,
+// and the original `&mut` passed to `fill_bytes` cannot be used because the safety contract of `Rng::new`
+// means that it must still be borrowed by `RngFuture`, and so `rustc` will not let it be accessed.
+unsafe impl Send for State {}
 
 /// A wrapper around an nRF RNG peripheral.
 ///
 /// It has a non-blocking API, through `embassy::traits::Rng`, and a blocking api through `rand`.
 pub struct Rng<'d> {
     irq: interrupt::RNG,
-    phantom: PhantomData<&'d mut RNG>,
+    phantom: PhantomData<(&'d mut RNG, &'d mut interrupt::RNG)>,
 }
 
 impl<'d> Rng<'d> {
-    pub fn new(
+    /// Creates a new RNG driver from the `RNG` peripheral and interrupt.
+    ///
+    /// SAFETY: The future returned from `fill_bytes` must not have its lifetime end without running its destructor,
+    /// e.g. using `mem::forget`.
+    ///
+    /// The synchronous API is safe.
+    pub unsafe fn new(
         _rng: impl Unborrow<Target = RNG> + 'd,
         irq: impl Unborrow<Target = interrupt::RNG> + 'd,
     ) -> Self {
@@ -42,7 +70,7 @@ impl<'d> Rng<'d> {
             phantom: PhantomData,
         };
 
-        this.stop();
+        Self::stop();
         this.disable_irq();
 
         this.irq.set_handler(Self::on_interrupt);
@@ -53,11 +81,25 @@ impl<'d> Rng<'d> {
     }
 
     fn on_interrupt(_: *mut ()) {
-        NEXT_BYTE.signal(RNG::regs().value.read().value().bits());
-        RNG::regs().events_valrdy.reset();
+        critical_section::with(|cs| {
+            let mut state = STATE.borrow(cs).borrow_mut();
+            // SAFETY: the safety requirements on `Rng::new` make sure that the original `&mut`'s lifetime is still valid,
+            // meaning it can't be aliased and is a valid pointer.
+            let buffer = unsafe { state.buffer.unwrap().as_mut() };
+            buffer[state.index] = RNG::regs().value.read().value().bits();
+            state.index += 1;
+            if state.index == buffer.len() {
+                // Stop the RNG within the interrupt so that it doesn't get triggered again on the way to waking the future.
+                Self::stop();
+                if let Some(waker) = state.waker.take() {
+                    waker.wake();
+                }
+            }
+            RNG::regs().events_valrdy.reset();
+        });
     }
 
-    fn stop(&self) {
+    fn stop() {
         RNG::regs().tasks_stop.write(|w| unsafe { w.bits(1) })
     }
 
@@ -98,17 +140,39 @@ impl<'d> traits::rng::Rng for Rng<'d> {
 
     fn fill_bytes<'a>(&'a mut self, dest: &'a mut [u8]) -> Self::RngFuture<'a> {
         async move {
+            critical_section::with(|cs| {
+                let mut state = STATE.borrow(cs).borrow_mut();
+                state.buffer = Some(dest.into());
+            });
+
             self.enable_irq();
             self.start();
 
             let on_drop = OnDrop::new(|| {
-                self.stop();
+                Self::stop();
                 self.disable_irq();
             });
 
-            for byte in dest.iter_mut() {
-                *byte = NEXT_BYTE.wait().await;
-            }
+            poll_fn(|cx| {
+                critical_section::with(|cs| {
+                    let mut state = STATE.borrow(cs).borrow_mut();
+                    state.waker = Some(cx.waker().clone());
+                    // SAFETY: see safety message in interrupt handler.
+                    // Also, both here and in the interrupt handler, we're in a critical section,
+                    // so they can't interfere with each other.
+                    let buffer = unsafe { state.buffer.unwrap().as_ref() };
+
+                    if state.index == buffer.len() {
+                        // Reset the state for next time
+                        state.buffer = None;
+                        state.index = 0;
+                        Poll::Ready(())
+                    } else {
+                        Poll::Pending
+                    }
+                })
+            })
+            .await;
 
             // Trigger the teardown
             drop(on_drop);
@@ -129,7 +193,7 @@ impl<'d> RngCore for Rng<'d> {
             *byte = regs.value.read().value().bits();
         }
 
-        self.stop();
+        Self::stop();
     }
 
     fn next_u32(&mut self) -> u32 {