split embassy-util into embassy-futures, embassy-sync.

embassy-sync/Cargo.toml

@@ -0,0 +1,34 @@
+[package]
+name = "embassy-sync"
+version = "0.1.0"
+edition = "2021"
+
+[package.metadata.embassy_docs]
+src_base = "https://github.com/embassy-rs/embassy/blob/embassy-sync-v$VERSION/embassy-sync/src/"
+src_base_git = "https://github.com/embassy-rs/embassy/blob/$COMMIT/embassy-sync/src/"
+features = ["nightly"]
+target = "thumbv7em-none-eabi"
+
+[features]
+nightly = ["embedded-io/async"]
+
+[dependencies]
+defmt = { version = "0.3", optional = true }
+log = { version = "0.4.14", optional = true }
+
+futures-util = { version = "0.3.17", default-features = false }
+atomic-polyfill = "1.0.1"
+critical-section = "1.1"
+heapless = "0.7.5"
+cfg-if = "1.0.0"
+embedded-io = "0.3.0"
+
+[dev-dependencies]
+futures-executor = { version = "0.3.17", features = [ "thread-pool" ] }
+futures-test = "0.3.17"
+futures-timer = "3.0.2"
+futures-util = { version = "0.3.17", features = [ "channel" ] }
+
+# Enable critical-section implementation for std, for tests
+critical-section = { version = "1.1", features = ["std"] }
+static_cell = "1.0"

embassy-sync/build.rs

@@ -0,0 +1,29 @@
+use std::env;
+
+fn main() {
+    let target = env::var("TARGET").unwrap();
+
+    if target.starts_with("thumbv6m-") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv6m");
+    } else if target.starts_with("thumbv7m-") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv7m");
+    } else if target.starts_with("thumbv7em-") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv7m");
+        println!("cargo:rustc-cfg=armv7em"); // (not currently used)
+    } else if target.starts_with("thumbv8m.base") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv8m");
+        println!("cargo:rustc-cfg=armv8m_base");
+    } else if target.starts_with("thumbv8m.main") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv8m");
+        println!("cargo:rustc-cfg=armv8m_main");
+    }
+
+    if target.ends_with("-eabihf") {
+        println!("cargo:rustc-cfg=has_fpu");
+    }
+}

embassy-sync/src/blocking_mutex/mod.rs

@@ -0,0 +1,189 @@
+//! Blocking mutex.
+//!
+//! This module provides a blocking mutex that can be used to synchronize data.
+pub mod raw;
+
+use core::cell::UnsafeCell;
+
+use self::raw::RawMutex;
+
+/// Blocking mutex (not async)
+///
+/// Provides a blocking mutual exclusion primitive backed by an implementation of [`raw::RawMutex`].
+///
+/// Which implementation you select depends on the context in which you're using the mutex, and you can choose which kind
+/// of interior mutability fits your use case.
+///
+/// Use [`CriticalSectionMutex`] when data can be shared between threads and interrupts.
+///
+/// Use [`NoopMutex`] when data is only shared between tasks running on the same executor.
+///
+/// Use [`ThreadModeMutex`] when data is shared between tasks running on the same executor but you want a global singleton.
+///
+/// In all cases, the blocking mutex is intended to be short lived and not held across await points.
+/// Use the async [`Mutex`](crate::mutex::Mutex) if you need a lock that is held across await points.
+pub struct Mutex<R, T: ?Sized> {
+    // NOTE: `raw` must be FIRST, so when using ThreadModeMutex the "can't drop in non-thread-mode" gets
+    // to run BEFORE dropping `data`.
+    raw: R,
+    data: UnsafeCell<T>,
+}
+
+unsafe impl<R: RawMutex + Send, T: ?Sized + Send> Send for Mutex<R, T> {}
+unsafe impl<R: RawMutex + Sync, T: ?Sized + Send> Sync for Mutex<R, T> {}
+
+impl<R: RawMutex, T> Mutex<R, T> {
+    /// Creates a new mutex in an unlocked state ready for use.
+    #[inline]
+    pub const fn new(val: T) -> Mutex<R, T> {
+        Mutex {
+            raw: R::INIT,
+            data: UnsafeCell::new(val),
+        }
+    }
+
+    /// Creates a critical section and grants temporary access to the protected data.
+    pub fn lock<U>(&self, f: impl FnOnce(&T) -> U) -> U {
+        self.raw.lock(|| {
+            let ptr = self.data.get() as *const T;
+            let inner = unsafe { &*ptr };
+            f(inner)
+        })
+    }
+}
+
+impl<R, T> Mutex<R, T> {
+    /// Creates a new mutex based on a pre-existing raw mutex.
+    ///
+    /// This allows creating a mutex in a constant context on stable Rust.
+    #[inline]
+    pub const fn const_new(raw_mutex: R, val: T) -> Mutex<R, T> {
+        Mutex {
+            raw: raw_mutex,
+            data: UnsafeCell::new(val),
+        }
+    }
+
+    /// Consumes this mutex, returning the underlying data.
+    #[inline]
+    pub fn into_inner(self) -> T {
+        self.data.into_inner()
+    }
+
+    /// Returns a mutable reference to the underlying data.
+    ///
+    /// Since this call borrows the `Mutex` mutably, no actual locking needs to
+    /// take place---the mutable borrow statically guarantees no locks exist.
+    #[inline]
+    pub fn get_mut(&mut self) -> &mut T {
+        unsafe { &mut *self.data.get() }
+    }
+}
+
+/// A mutex that allows borrowing data across executors and interrupts.
+///
+/// # Safety
+///
+/// This mutex is safe to share between different executors and interrupts.
+pub type CriticalSectionMutex<T> = Mutex<raw::CriticalSectionRawMutex, T>;
+
+/// A mutex that allows borrowing data in the context of a single executor.
+///
+/// # Safety
+///
+/// **This Mutex is only safe within a single executor.**
+pub type NoopMutex<T> = Mutex<raw::NoopRawMutex, T>;
+
+impl<T> Mutex<raw::CriticalSectionRawMutex, T> {
+    /// Borrows the data for the duration of the critical section
+    pub fn borrow<'cs>(&'cs self, _cs: critical_section::CriticalSection<'cs>) -> &'cs T {
+        let ptr = self.data.get() as *const T;
+        unsafe { &*ptr }
+    }
+}
+
+impl<T> Mutex<raw::NoopRawMutex, T> {
+    /// Borrows the data
+    pub fn borrow(&self) -> &T {
+        let ptr = self.data.get() as *const T;
+        unsafe { &*ptr }
+    }
+}
+
+// ThreadModeMutex does NOT use the generic mutex from above because it's special:
+// it's Send+Sync even if T: !Send. There's no way to do that without specialization (I think?).
+//
+// There's still a ThreadModeRawMutex for use with the generic Mutex (handy with Channel, for example),
+// but that will require T: Send even though it shouldn't be needed.
+
+#[cfg(any(cortex_m, feature = "std"))]
+pub use thread_mode_mutex::*;
+#[cfg(any(cortex_m, feature = "std"))]
+mod thread_mode_mutex {
+    use super::*;
+
+    /// A "mutex" that only allows borrowing from thread mode.
+    ///
+    /// # Safety
+    ///
+    /// **This Mutex is only safe on single-core systems.**
+    ///
+    /// On multi-core systems, a `ThreadModeMutex` **is not sufficient** to ensure exclusive access.
+    pub struct ThreadModeMutex<T: ?Sized> {
+        inner: UnsafeCell<T>,
+    }
+
+    // NOTE: ThreadModeMutex only allows borrowing from one execution context ever: thread mode.
+    // Therefore it cannot be used to send non-sendable stuff between execution contexts, so it can
+    // be Send+Sync even if T is not Send (unlike CriticalSectionMutex)
+    unsafe impl<T: ?Sized> Sync for ThreadModeMutex<T> {}
+    unsafe impl<T: ?Sized> Send for ThreadModeMutex<T> {}
+
+    impl<T> ThreadModeMutex<T> {
+        /// Creates a new mutex
+        pub const fn new(value: T) -> Self {
+            ThreadModeMutex {
+                inner: UnsafeCell::new(value),
+            }
+        }
+    }
+
+    impl<T: ?Sized> ThreadModeMutex<T> {
+        /// Lock the `ThreadModeMutex`, granting access to the data.
+        ///
+        /// # Panics
+        ///
+        /// This will panic if not currently running in thread mode.
+        pub fn lock<R>(&self, f: impl FnOnce(&T) -> R) -> R {
+            f(self.borrow())
+        }
+
+        /// Borrows the data
+        ///
+        /// # Panics
+        ///
+        /// This will panic if not currently running in thread mode.
+        pub fn borrow(&self) -> &T {
+            assert!(
+                raw::in_thread_mode(),
+                "ThreadModeMutex can only be borrowed from thread mode."
+            );
+            unsafe { &*self.inner.get() }
+        }
+    }
+
+    impl<T: ?Sized> Drop for ThreadModeMutex<T> {
+        fn drop(&mut self) {
+            // Only allow dropping from thread mode. Dropping calls drop on the inner `T`, so
+            // `drop` needs the same guarantees as `lock`. `ThreadModeMutex<T>` is Send even if
+            // T isn't, so without this check a user could create a ThreadModeMutex in thread mode,
+            // send it to interrupt context and drop it there, which would "send" a T even if T is not Send.
+            assert!(
+                raw::in_thread_mode(),
+                "ThreadModeMutex can only be dropped from thread mode."
+            );
+
+            // Drop of the inner `T` happens after this.
+        }
+    }
+}

embassy-sync/src/blocking_mutex/raw.rs

@@ -0,0 +1,149 @@
+//! Mutex primitives.
+//!
+//! This module provides a trait for mutexes that can be used in different contexts.
+use core::marker::PhantomData;
+
+/// Raw mutex trait.
+///
+/// This mutex is "raw", which means it does not actually contain the protected data, it
+/// just implements the mutex mechanism. For most uses you should use [`super::Mutex`] instead,
+/// which is generic over a RawMutex and contains the protected data.
+///
+/// Note that, unlike other mutexes, implementations only guarantee no
+/// concurrent access from other threads: concurrent access from the current
+/// thread is allwed. For example, it's possible to lock the same mutex multiple times reentrantly.
+///
+/// Therefore, locking a `RawMutex` is only enough to guarantee safe shared (`&`) access
+/// to the data, it is not enough to guarantee exclusive (`&mut`) access.
+///
+/// # Safety
+///
+/// RawMutex implementations must ensure that, while locked, no other thread can lock
+/// the RawMutex concurrently.
+///
+/// Unsafe code is allowed to rely on this fact, so incorrect implementations will cause undefined behavior.
+pub unsafe trait RawMutex {
+    /// Create a new `RawMutex` instance.
+    ///
+    /// This is a const instead of a method to allow creating instances in const context.
+    const INIT: Self;
+
+    /// Lock this `RawMutex`.
+    fn lock<R>(&self, f: impl FnOnce() -> R) -> R;
+}
+
+/// A mutex that allows borrowing data across executors and interrupts.
+///
+/// # Safety
+///
+/// This mutex is safe to share between different executors and interrupts.
+pub struct CriticalSectionRawMutex {
+    _phantom: PhantomData<()>,
+}
+unsafe impl Send for CriticalSectionRawMutex {}
+unsafe impl Sync for CriticalSectionRawMutex {}
+
+impl CriticalSectionRawMutex {
+    /// Create a new `CriticalSectionRawMutex`.
+    pub const fn new() -> Self {
+        Self { _phantom: PhantomData }
+    }
+}
+
+unsafe impl RawMutex for CriticalSectionRawMutex {
+    const INIT: Self = Self::new();
+
+    fn lock<R>(&self, f: impl FnOnce() -> R) -> R {
+        critical_section::with(|_| f())
+    }
+}
+
+// ================
+
+/// A mutex that allows borrowing data in the context of a single executor.
+///
+/// # Safety
+///
+/// **This Mutex is only safe within a single executor.**
+pub struct NoopRawMutex {
+    _phantom: PhantomData<*mut ()>,
+}
+
+unsafe impl Send for NoopRawMutex {}
+
+impl NoopRawMutex {
+    /// Create a new `NoopRawMutex`.
+    pub const fn new() -> Self {
+        Self { _phantom: PhantomData }
+    }
+}
+
+unsafe impl RawMutex for NoopRawMutex {
+    const INIT: Self = Self::new();
+    fn lock<R>(&self, f: impl FnOnce() -> R) -> R {
+        f()
+    }
+}
+
+// ================
+
+#[cfg(any(cortex_m, feature = "std"))]
+mod thread_mode {
+    use super::*;
+
+    /// A "mutex" that only allows borrowing from thread mode.
+    ///
+    /// # Safety
+    ///
+    /// **This Mutex is only safe on single-core systems.**
+    ///
+    /// On multi-core systems, a `ThreadModeRawMutex` **is not sufficient** to ensure exclusive access.
+    pub struct ThreadModeRawMutex {
+        _phantom: PhantomData<()>,
+    }
+
+    unsafe impl Send for ThreadModeRawMutex {}
+    unsafe impl Sync for ThreadModeRawMutex {}
+
+    impl ThreadModeRawMutex {
+        /// Create a new `ThreadModeRawMutex`.
+        pub const fn new() -> Self {
+            Self { _phantom: PhantomData }
+        }
+    }
+
+    unsafe impl RawMutex for ThreadModeRawMutex {
+        const INIT: Self = Self::new();
+        fn lock<R>(&self, f: impl FnOnce() -> R) -> R {
+            assert!(in_thread_mode(), "ThreadModeMutex can only be locked from thread mode.");
+
+            f()
+        }
+    }
+
+    impl Drop for ThreadModeRawMutex {
+        fn drop(&mut self) {
+            // Only allow dropping from thread mode. Dropping calls drop on the inner `T`, so
+            // `drop` needs the same guarantees as `lock`. `ThreadModeMutex<T>` is Send even if
+            // T isn't, so without this check a user could create a ThreadModeMutex in thread mode,
+            // send it to interrupt context and drop it there, which would "send" a T even if T is not Send.
+            assert!(
+                in_thread_mode(),
+                "ThreadModeMutex can only be dropped from thread mode."
+            );
+
+            // Drop of the inner `T` happens after this.
+        }
+    }
+
+    pub(crate) fn in_thread_mode() -> bool {
+        #[cfg(feature = "std")]
+        return Some("main") == std::thread::current().name();
+
+        #[cfg(not(feature = "std"))]
+        // ICSR.VECTACTIVE == 0
+        return unsafe { (0xE000ED04 as *const u32).read_volatile() } & 0x1FF == 0;
+    }
+}
+#[cfg(any(cortex_m, feature = "std"))]
+pub use thread_mode::*;

embassy-sync/src/channel/mod.rs

@@ -0,0 +1,5 @@
+//! Async channels
+
+pub mod mpmc;
+pub mod pubsub;
+pub mod signal;

embassy-sync/src/channel/mpmc.rs

@@ -0,0 +1,596 @@
+//! A queue for sending values between asynchronous tasks.
+//!
+//! It can be used concurrently by multiple producers (senders) and multiple
+//! consumers (receivers), i.e. it is an  "MPMC channel".
+//!
+//! Receivers are competing for messages. So a message that is received by
+//! one receiver is not received by any other.
+//!
+//! This queue takes a Mutex type so that various
+//! targets can be attained. For example, a ThreadModeMutex can be used
+//! for single-core Cortex-M targets where messages are only passed
+//! between tasks running in thread mode. Similarly, a CriticalSectionMutex
+//! can also be used for single-core targets where messages are to be
+//! passed from exception mode e.g. out of an interrupt handler.
+//!
+//! This module provides a bounded channel that has a limit on the number of
+//! messages that it can store, and if this limit is reached, trying to send
+//! another message will result in an error being returned.
+//!
+
+use core::cell::RefCell;
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+use heapless::Deque;
+
+use crate::blocking_mutex::raw::RawMutex;
+use crate::blocking_mutex::Mutex;
+use crate::waitqueue::WakerRegistration;
+
+/// Send-only access to a [`Channel`].
+#[derive(Copy)]
+pub struct Sender<'ch, M, T, const N: usize>
+where
+    M: RawMutex,
+{
+    channel: &'ch Channel<M, T, N>,
+}
+
+impl<'ch, M, T, const N: usize> Clone for Sender<'ch, M, T, N>
+where
+    M: RawMutex,
+{
+    fn clone(&self) -> Self {
+        Sender { channel: self.channel }
+    }
+}
+
+impl<'ch, M, T, const N: usize> Sender<'ch, M, T, N>
+where
+    M: RawMutex,
+{
+    /// Sends a value.
+    ///
+    /// See [`Channel::send()`]
+    pub fn send(&self, message: T) -> SendFuture<'ch, M, T, N> {
+        self.channel.send(message)
+    }
+
+    /// Attempt to immediately send a message.
+    ///
+    /// See [`Channel::send()`]
+    pub fn try_send(&self, message: T) -> Result<(), TrySendError<T>> {
+        self.channel.try_send(message)
+    }
+}
+
+/// Send-only access to a [`Channel`] without knowing channel size.
+#[derive(Copy)]
+pub struct DynamicSender<'ch, T> {
+    channel: &'ch dyn DynamicChannel<T>,
+}
+
+impl<'ch, T> Clone for DynamicSender<'ch, T> {
+    fn clone(&self) -> Self {
+        DynamicSender { channel: self.channel }
+    }
+}
+
+impl<'ch, M, T, const N: usize> From<Sender<'ch, M, T, N>> for DynamicSender<'ch, T>
+where
+    M: RawMutex,
+{
+    fn from(s: Sender<'ch, M, T, N>) -> Self {
+        Self { channel: s.channel }
+    }
+}
+
+impl<'ch, T> DynamicSender<'ch, T> {
+    /// Sends a value.
+    ///
+    /// See [`Channel::send()`]
+    pub fn send(&self, message: T) -> DynamicSendFuture<'ch, T> {
+        DynamicSendFuture {
+            channel: self.channel,
+            message: Some(message),
+        }
+    }
+
+    /// Attempt to immediately send a message.
+    ///
+    /// See [`Channel::send()`]
+    pub fn try_send(&self, message: T) -> Result<(), TrySendError<T>> {
+        self.channel.try_send_with_context(message, None)
+    }
+}
+
+/// Receive-only access to a [`Channel`].
+#[derive(Copy)]
+pub struct Receiver<'ch, M, T, const N: usize>
+where
+    M: RawMutex,
+{
+    channel: &'ch Channel<M, T, N>,
+}
+
+impl<'ch, M, T, const N: usize> Clone for Receiver<'ch, M, T, N>
+where
+    M: RawMutex,
+{
+    fn clone(&self) -> Self {
+        Receiver { channel: self.channel }
+    }
+}
+
+impl<'ch, M, T, const N: usize> Receiver<'ch, M, T, N>
+where
+    M: RawMutex,
+{
+    /// Receive the next value.
+    ///
+    /// See [`Channel::recv()`].
+    pub fn recv(&self) -> RecvFuture<'_, M, T, N> {
+        self.channel.recv()
+    }
+
+    /// Attempt to immediately receive the next value.
+    ///
+    /// See [`Channel::try_recv()`]
+    pub fn try_recv(&self) -> Result<T, TryRecvError> {
+        self.channel.try_recv()
+    }
+}
+
+/// Receive-only access to a [`Channel`] without knowing channel size.
+#[derive(Copy)]
+pub struct DynamicReceiver<'ch, T> {
+    channel: &'ch dyn DynamicChannel<T>,
+}
+
+impl<'ch, T> Clone for DynamicReceiver<'ch, T> {
+    fn clone(&self) -> Self {
+        DynamicReceiver { channel: self.channel }
+    }
+}
+
+impl<'ch, T> DynamicReceiver<'ch, T> {
+    /// Receive the next value.
+    ///
+    /// See [`Channel::recv()`].
+    pub fn recv(&self) -> DynamicRecvFuture<'_, T> {
+        DynamicRecvFuture { channel: self.channel }
+    }
+
+    /// Attempt to immediately receive the next value.
+    ///
+    /// See [`Channel::try_recv()`]
+    pub fn try_recv(&self) -> Result<T, TryRecvError> {
+        self.channel.try_recv_with_context(None)
+    }
+}
+
+impl<'ch, M, T, const N: usize> From<Receiver<'ch, M, T, N>> for DynamicReceiver<'ch, T>
+where
+    M: RawMutex,
+{
+    fn from(s: Receiver<'ch, M, T, N>) -> Self {
+        Self { channel: s.channel }
+    }
+}
+
+/// Future returned by [`Channel::recv`] and  [`Receiver::recv`].
+pub struct RecvFuture<'ch, M, T, const N: usize>
+where
+    M: RawMutex,
+{
+    channel: &'ch Channel<M, T, N>,
+}
+
+impl<'ch, M, T, const N: usize> Future for RecvFuture<'ch, M, T, N>
+where
+    M: RawMutex,
+{
+    type Output = T;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
+        match self.channel.try_recv_with_context(Some(cx)) {
+            Ok(v) => Poll::Ready(v),
+            Err(TryRecvError::Empty) => Poll::Pending,
+        }
+    }
+}
+
+/// Future returned by [`DynamicReceiver::recv`].
+pub struct DynamicRecvFuture<'ch, T> {
+    channel: &'ch dyn DynamicChannel<T>,
+}
+
+impl<'ch, T> Future for DynamicRecvFuture<'ch, T> {
+    type Output = T;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
+        match self.channel.try_recv_with_context(Some(cx)) {
+            Ok(v) => Poll::Ready(v),
+            Err(TryRecvError::Empty) => Poll::Pending,
+        }
+    }
+}
+
+/// Future returned by [`Channel::send`] and  [`Sender::send`].
+pub struct SendFuture<'ch, M, T, const N: usize>
+where
+    M: RawMutex,
+{
+    channel: &'ch Channel<M, T, N>,
+    message: Option<T>,
+}
+
+impl<'ch, M, T, const N: usize> Future for SendFuture<'ch, M, T, N>
+where
+    M: RawMutex,
+{
+    type Output = ();
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        match self.message.take() {
+            Some(m) => match self.channel.try_send_with_context(m, Some(cx)) {
+                Ok(..) => Poll::Ready(()),
+                Err(TrySendError::Full(m)) => {
+                    self.message = Some(m);
+                    Poll::Pending
+                }
+            },
+            None => panic!("Message cannot be None"),
+        }
+    }
+}
+
+impl<'ch, M, T, const N: usize> Unpin for SendFuture<'ch, M, T, N> where M: RawMutex {}
+
+/// Future returned by [`DynamicSender::send`].
+pub struct DynamicSendFuture<'ch, T> {
+    channel: &'ch dyn DynamicChannel<T>,
+    message: Option<T>,
+}
+
+impl<'ch, T> Future for DynamicSendFuture<'ch, T> {
+    type Output = ();
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        match self.message.take() {
+            Some(m) => match self.channel.try_send_with_context(m, Some(cx)) {
+                Ok(..) => Poll::Ready(()),
+                Err(TrySendError::Full(m)) => {
+                    self.message = Some(m);
+                    Poll::Pending
+                }
+            },
+            None => panic!("Message cannot be None"),
+        }
+    }
+}
+
+impl<'ch, T> Unpin for DynamicSendFuture<'ch, T> {}
+
+trait DynamicChannel<T> {
+    fn try_send_with_context(&self, message: T, cx: Option<&mut Context<'_>>) -> Result<(), TrySendError<T>>;
+
+    fn try_recv_with_context(&self, cx: Option<&mut Context<'_>>) -> Result<T, TryRecvError>;
+}
+
+/// Error returned by [`try_recv`](Channel::try_recv).
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub enum TryRecvError {
+    /// A message could not be received because the channel is empty.
+    Empty,
+}
+
+/// Error returned by [`try_send`](Channel::try_send).
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub enum TrySendError<T> {
+    /// The data could not be sent on the channel because the channel is
+    /// currently full and sending would require blocking.
+    Full(T),
+}
+
+struct ChannelState<T, const N: usize> {
+    queue: Deque<T, N>,
+    receiver_waker: WakerRegistration,
+    senders_waker: WakerRegistration,
+}
+
+impl<T, const N: usize> ChannelState<T, N> {
+    const fn new() -> Self {
+        ChannelState {
+            queue: Deque::new(),
+            receiver_waker: WakerRegistration::new(),
+            senders_waker: WakerRegistration::new(),
+        }
+    }
+
+    fn try_recv(&mut self) -> Result<T, TryRecvError> {
+        self.try_recv_with_context(None)
+    }
+
+    fn try_recv_with_context(&mut self, cx: Option<&mut Context<'_>>) -> Result<T, TryRecvError> {
+        if self.queue.is_full() {
+            self.senders_waker.wake();
+        }
+
+        if let Some(message) = self.queue.pop_front() {
+            Ok(message)
+        } else {
+            if let Some(cx) = cx {
+                self.receiver_waker.register(cx.waker());
+            }
+            Err(TryRecvError::Empty)
+        }
+    }
+
+    fn try_send(&mut self, message: T) -> Result<(), TrySendError<T>> {
+        self.try_send_with_context(message, None)
+    }
+
+    fn try_send_with_context(&mut self, message: T, cx: Option<&mut Context<'_>>) -> Result<(), TrySendError<T>> {
+        match self.queue.push_back(message) {
+            Ok(()) => {
+                self.receiver_waker.wake();
+                Ok(())
+            }
+            Err(message) => {
+                if let Some(cx) = cx {
+                    self.senders_waker.register(cx.waker());
+                }
+                Err(TrySendError::Full(message))
+            }
+        }
+    }
+}
+
+/// A bounded channel for communicating between asynchronous tasks
+/// with backpressure.
+///
+/// The channel will buffer up to the provided number of messages.  Once the
+/// buffer is full, attempts to `send` new messages will wait until a message is
+/// received from the channel.
+///
+/// All data sent will become available in the same order as it was sent.
+pub struct Channel<M, T, const N: usize>
+where
+    M: RawMutex,
+{
+    inner: Mutex<M, RefCell<ChannelState<T, N>>>,
+}
+
+impl<M, T, const N: usize> Channel<M, T, N>
+where
+    M: RawMutex,
+{
+    /// Establish a new bounded channel. For example, to create one with a NoopMutex:
+    ///
+    /// ```
+    /// use embassy_sync::channel::mpmc::Channel;
+    /// use embassy_sync::blocking_mutex::raw::NoopRawMutex;
+    ///
+    /// // Declare a bounded channel of 3 u32s.
+    /// let mut channel = Channel::<NoopRawMutex, u32, 3>::new();
+    /// ```
+    pub const fn new() -> Self {
+        Self {
+            inner: Mutex::new(RefCell::new(ChannelState::new())),
+        }
+    }
+
+    fn lock<R>(&self, f: impl FnOnce(&mut ChannelState<T, N>) -> R) -> R {
+        self.inner.lock(|rc| f(&mut *rc.borrow_mut()))
+    }
+
+    fn try_recv_with_context(&self, cx: Option<&mut Context<'_>>) -> Result<T, TryRecvError> {
+        self.lock(|c| c.try_recv_with_context(cx))
+    }
+
+    fn try_send_with_context(&self, m: T, cx: Option<&mut Context<'_>>) -> Result<(), TrySendError<T>> {
+        self.lock(|c| c.try_send_with_context(m, cx))
+    }
+
+    /// Get a sender for this channel.
+    pub fn sender(&self) -> Sender<'_, M, T, N> {
+        Sender { channel: self }
+    }
+
+    /// Get a receiver for this channel.
+    pub fn receiver(&self) -> Receiver<'_, M, T, N> {
+        Receiver { channel: self }
+    }
+
+    /// Send a value, waiting until there is capacity.
+    ///
+    /// Sending completes when the value has been pushed to the channel's queue.
+    /// This doesn't mean the value has been received yet.
+    pub fn send(&self, message: T) -> SendFuture<'_, M, T, N> {
+        SendFuture {
+            channel: self,
+            message: Some(message),
+        }
+    }
+
+    /// Attempt to immediately send a message.
+    ///
+    /// This method differs from [`send`](Channel::send) by returning immediately if the channel's
+    /// buffer is full, instead of waiting.
+    ///
+    /// # Errors
+    ///
+    /// If the channel capacity has been reached, i.e., the channel has `n`
+    /// buffered values where `n` is the argument passed to [`Channel`], then an
+    /// error is returned.
+    pub fn try_send(&self, message: T) -> Result<(), TrySendError<T>> {
+        self.lock(|c| c.try_send(message))
+    }
+
+    /// Receive the next value.
+    ///
+    /// If there are no messages in the channel's buffer, this method will
+    /// wait until a message is sent.
+    pub fn recv(&self) -> RecvFuture<'_, M, T, N> {
+        RecvFuture { channel: self }
+    }
+
+    /// Attempt to immediately receive a message.
+    ///
+    /// This method will either receive a message from the channel immediately or return an error
+    /// if the channel is empty.
+    pub fn try_recv(&self) -> Result<T, TryRecvError> {
+        self.lock(|c| c.try_recv())
+    }
+}
+
+/// Implements the DynamicChannel to allow creating types that are unaware of the queue size with the
+/// tradeoff cost of dynamic dispatch.
+impl<M, T, const N: usize> DynamicChannel<T> for Channel<M, T, N>
+where
+    M: RawMutex,
+{
+    fn try_send_with_context(&self, m: T, cx: Option<&mut Context<'_>>) -> Result<(), TrySendError<T>> {
+        Channel::try_send_with_context(self, m, cx)
+    }
+
+    fn try_recv_with_context(&self, cx: Option<&mut Context<'_>>) -> Result<T, TryRecvError> {
+        Channel::try_recv_with_context(self, cx)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use core::time::Duration;
+
+    use futures_executor::ThreadPool;
+    use futures_timer::Delay;
+    use futures_util::task::SpawnExt;
+    use static_cell::StaticCell;
+
+    use super::*;
+    use crate::blocking_mutex::raw::{CriticalSectionRawMutex, NoopRawMutex};
+
+    fn capacity<T, const N: usize>(c: &ChannelState<T, N>) -> usize {
+        c.queue.capacity() - c.queue.len()
+    }
+
+    #[test]
+    fn sending_once() {
+        let mut c = ChannelState::<u32, 3>::new();
+        assert!(c.try_send(1).is_ok());
+        assert_eq!(capacity(&c), 2);
+    }
+
+    #[test]
+    fn sending_when_full() {
+        let mut c = ChannelState::<u32, 3>::new();
+        let _ = c.try_send(1);
+        let _ = c.try_send(1);
+        let _ = c.try_send(1);
+        match c.try_send(2) {
+            Err(TrySendError::Full(2)) => assert!(true),
+            _ => assert!(false),
+        }
+        assert_eq!(capacity(&c), 0);
+    }
+
+    #[test]
+    fn receiving_once_with_one_send() {
+        let mut c = ChannelState::<u32, 3>::new();
+        assert!(c.try_send(1).is_ok());
+        assert_eq!(c.try_recv().unwrap(), 1);
+        assert_eq!(capacity(&c), 3);
+    }
+
+    #[test]
+    fn receiving_when_empty() {
+        let mut c = ChannelState::<u32, 3>::new();
+        match c.try_recv() {
+            Err(TryRecvError::Empty) => assert!(true),
+            _ => assert!(false),
+        }
+        assert_eq!(capacity(&c), 3);
+    }
+
+    #[test]
+    fn simple_send_and_receive() {
+        let c = Channel::<NoopRawMutex, u32, 3>::new();
+        assert!(c.try_send(1).is_ok());
+        assert_eq!(c.try_recv().unwrap(), 1);
+    }
+
+    #[test]
+    fn cloning() {
+        let c = Channel::<NoopRawMutex, u32, 3>::new();
+        let r1 = c.receiver();
+        let s1 = c.sender();
+
+        let _ = r1.clone();
+        let _ = s1.clone();
+    }
+
+    #[test]
+    fn dynamic_dispatch() {
+        let c = Channel::<NoopRawMutex, u32, 3>::new();
+        let s: DynamicSender<'_, u32> = c.sender().into();
+        let r: DynamicReceiver<'_, u32> = c.receiver().into();
+
+        assert!(s.try_send(1).is_ok());
+        assert_eq!(r.try_recv().unwrap(), 1);
+    }
+
+    #[futures_test::test]
+    async fn receiver_receives_given_try_send_async() {
+        let executor = ThreadPool::new().unwrap();
+
+        static CHANNEL: StaticCell<Channel<CriticalSectionRawMutex, u32, 3>> = StaticCell::new();
+        let c = &*CHANNEL.init(Channel::new());
+        let c2 = c;
+        assert!(executor
+            .spawn(async move {
+                assert!(c2.try_send(1).is_ok());
+            })
+            .is_ok());
+        assert_eq!(c.recv().await, 1);
+    }
+
+    #[futures_test::test]
+    async fn sender_send_completes_if_capacity() {
+        let c = Channel::<CriticalSectionRawMutex, u32, 1>::new();
+        c.send(1).await;
+        assert_eq!(c.recv().await, 1);
+    }
+
+    #[futures_test::test]
+    async fn senders_sends_wait_until_capacity() {
+        let executor = ThreadPool::new().unwrap();
+
+        static CHANNEL: StaticCell<Channel<CriticalSectionRawMutex, u32, 1>> = StaticCell::new();
+        let c = &*CHANNEL.init(Channel::new());
+        assert!(c.try_send(1).is_ok());
+
+        let c2 = c;
+        let send_task_1 = executor.spawn_with_handle(async move { c2.send(2).await });
+        let c2 = c;
+        let send_task_2 = executor.spawn_with_handle(async move { c2.send(3).await });
+        // Wish I could think of a means of determining that the async send is waiting instead.
+        // However, I've used the debugger to observe that the send does indeed wait.
+        Delay::new(Duration::from_millis(500)).await;
+        assert_eq!(c.recv().await, 1);
+        assert!(executor
+            .spawn(async move {
+                loop {
+                    c.recv().await;
+                }
+            })
+            .is_ok());
+        send_task_1.unwrap().await;
+        send_task_2.unwrap().await;
+    }
+}

embassy-sync/src/channel/pubsub/mod.rs

@@ -0,0 +1,542 @@
+//! Implementation of [PubSubChannel], a queue where published messages get received by all subscribers.
+
+#![deny(missing_docs)]
+
+use core::cell::RefCell;
+use core::fmt::Debug;
+use core::task::{Context, Poll, Waker};
+
+use heapless::Deque;
+
+use self::publisher::{ImmediatePub, Pub};
+use self::subscriber::Sub;
+use crate::blocking_mutex::raw::RawMutex;
+use crate::blocking_mutex::Mutex;
+use crate::waitqueue::MultiWakerRegistration;
+
+pub mod publisher;
+pub mod subscriber;
+
+pub use publisher::{DynImmediatePublisher, DynPublisher, ImmediatePublisher, Publisher};
+pub use subscriber::{DynSubscriber, Subscriber};
+
+/// A broadcast channel implementation where multiple publishers can send messages to multiple subscribers
+///
+/// Any published message can be read by all subscribers.
+/// A publisher can choose how it sends its message.
+///
+/// - With [Pub::publish()] the publisher has to wait until there is space in the internal message queue.
+/// - With [Pub::publish_immediate()] the publisher doesn't await and instead lets the oldest message
+/// in the queue drop if necessary. This will cause any [Subscriber] that missed the message to receive
+/// an error to indicate that it has lagged.
+///
+/// ## Example
+///
+/// ```
+/// # use embassy_sync::blocking_mutex::raw::NoopRawMutex;
+/// # use embassy_sync::channel::pubsub::WaitResult;
+/// # use embassy_sync::channel::pubsub::PubSubChannel;
+/// # use futures_executor::block_on;
+/// # let test = async {
+/// // Create the channel. This can be static as well
+/// let channel = PubSubChannel::<NoopRawMutex, u32, 4, 4, 4>::new();
+///
+/// // This is a generic subscriber with a direct reference to the channel
+/// let mut sub0 = channel.subscriber().unwrap();
+/// // This is a dynamic subscriber with a dynamic (trait object) reference to the channel
+/// let mut sub1 = channel.dyn_subscriber().unwrap();
+///
+/// let pub0 = channel.publisher().unwrap();
+///
+/// // Publish a message, but wait if the queue is full
+/// pub0.publish(42).await;
+///
+/// // Publish a message, but if the queue is full, just kick out the oldest message.
+/// // This may cause some subscribers to miss a message
+/// pub0.publish_immediate(43);
+///
+/// // Wait for a new message. If the subscriber missed a message, the WaitResult will be a Lag result
+/// assert_eq!(sub0.next_message().await, WaitResult::Message(42));
+/// assert_eq!(sub1.next_message().await, WaitResult::Message(42));
+///
+/// // Wait again, but this time ignore any Lag results
+/// assert_eq!(sub0.next_message_pure().await, 43);
+/// assert_eq!(sub1.next_message_pure().await, 43);
+///
+/// // There's also a polling interface
+/// assert_eq!(sub0.try_next_message(), None);
+/// assert_eq!(sub1.try_next_message(), None);
+/// # };
+/// #
+/// # block_on(test);
+/// ```
+///
+pub struct PubSubChannel<M: RawMutex, T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize> {
+    inner: Mutex<M, RefCell<PubSubState<T, CAP, SUBS, PUBS>>>,
+}
+
+impl<M: RawMutex, T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize>
+    PubSubChannel<M, T, CAP, SUBS, PUBS>
+{
+    /// Create a new channel
+    pub const fn new() -> Self {
+        Self {
+            inner: Mutex::const_new(M::INIT, RefCell::new(PubSubState::new())),
+        }
+    }
+
+    /// Create a new subscriber. It will only receive messages that are published after its creation.
+    ///
+    /// If there are no subscriber slots left, an error will be returned.
+    pub fn subscriber(&self) -> Result<Subscriber<M, T, CAP, SUBS, PUBS>, Error> {
+        self.inner.lock(|inner| {
+            let mut s = inner.borrow_mut();
+
+            if s.subscriber_count >= SUBS {
+                Err(Error::MaximumSubscribersReached)
+            } else {
+                s.subscriber_count += 1;
+                Ok(Subscriber(Sub::new(s.next_message_id, self)))
+            }
+        })
+    }
+
+    /// Create a new subscriber. It will only receive messages that are published after its creation.
+    ///
+    /// If there are no subscriber slots left, an error will be returned.
+    pub fn dyn_subscriber(&self) -> Result<DynSubscriber<'_, T>, Error> {
+        self.inner.lock(|inner| {
+            let mut s = inner.borrow_mut();
+
+            if s.subscriber_count >= SUBS {
+                Err(Error::MaximumSubscribersReached)
+            } else {
+                s.subscriber_count += 1;
+                Ok(DynSubscriber(Sub::new(s.next_message_id, self)))
+            }
+        })
+    }
+
+    /// Create a new publisher
+    ///
+    /// If there are no publisher slots left, an error will be returned.
+    pub fn publisher(&self) -> Result<Publisher<M, T, CAP, SUBS, PUBS>, Error> {
+        self.inner.lock(|inner| {
+            let mut s = inner.borrow_mut();
+
+            if s.publisher_count >= PUBS {
+                Err(Error::MaximumPublishersReached)
+            } else {
+                s.publisher_count += 1;
+                Ok(Publisher(Pub::new(self)))
+            }
+        })
+    }
+
+    /// Create a new publisher
+    ///
+    /// If there are no publisher slots left, an error will be returned.
+    pub fn dyn_publisher(&self) -> Result<DynPublisher<'_, T>, Error> {
+        self.inner.lock(|inner| {
+            let mut s = inner.borrow_mut();
+
+            if s.publisher_count >= PUBS {
+                Err(Error::MaximumPublishersReached)
+            } else {
+                s.publisher_count += 1;
+                Ok(DynPublisher(Pub::new(self)))
+            }
+        })
+    }
+
+    /// Create a new publisher that can only send immediate messages.
+    /// This kind of publisher does not take up a publisher slot.
+    pub fn immediate_publisher(&self) -> ImmediatePublisher<M, T, CAP, SUBS, PUBS> {
+        ImmediatePublisher(ImmediatePub::new(self))
+    }
+
+    /// Create a new publisher that can only send immediate messages.
+    /// This kind of publisher does not take up a publisher slot.
+    pub fn dyn_immediate_publisher(&self) -> DynImmediatePublisher<T> {
+        DynImmediatePublisher(ImmediatePub::new(self))
+    }
+}
+
+impl<M: RawMutex, T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize> PubSubBehavior<T>
+    for PubSubChannel<M, T, CAP, SUBS, PUBS>
+{
+    fn get_message_with_context(&self, next_message_id: &mut u64, cx: Option<&mut Context<'_>>) -> Poll<WaitResult<T>> {
+        self.inner.lock(|s| {
+            let mut s = s.borrow_mut();
+
+            // Check if we can read a message
+            match s.get_message(*next_message_id) {
+                // Yes, so we are done polling
+                Some(WaitResult::Message(message)) => {
+                    *next_message_id += 1;
+                    Poll::Ready(WaitResult::Message(message))
+                }
+                // No, so we need to reregister our waker and sleep again
+                None => {
+                    if let Some(cx) = cx {
+                        s.register_subscriber_waker(cx.waker());
+                    }
+                    Poll::Pending
+                }
+                // We missed a couple of messages. We must do our internal bookkeeping and return that we lagged
+                Some(WaitResult::Lagged(amount)) => {
+                    *next_message_id += amount;
+                    Poll::Ready(WaitResult::Lagged(amount))
+                }
+            }
+        })
+    }
+
+    fn publish_with_context(&self, message: T, cx: Option<&mut Context<'_>>) -> Result<(), T> {
+        self.inner.lock(|s| {
+            let mut s = s.borrow_mut();
+            // Try to publish the message
+            match s.try_publish(message) {
+                // We did it, we are ready
+                Ok(()) => Ok(()),
+                // The queue is full, so we need to reregister our waker and go to sleep
+                Err(message) => {
+                    if let Some(cx) = cx {
+                        s.register_publisher_waker(cx.waker());
+                    }
+                    Err(message)
+                }
+            }
+        })
+    }
+
+    fn publish_immediate(&self, message: T) {
+        self.inner.lock(|s| {
+            let mut s = s.borrow_mut();
+            s.publish_immediate(message)
+        })
+    }
+
+    fn unregister_subscriber(&self, subscriber_next_message_id: u64) {
+        self.inner.lock(|s| {
+            let mut s = s.borrow_mut();
+            s.unregister_subscriber(subscriber_next_message_id)
+        })
+    }
+
+    fn unregister_publisher(&self) {
+        self.inner.lock(|s| {
+            let mut s = s.borrow_mut();
+            s.unregister_publisher()
+        })
+    }
+}
+
+/// Internal state for the PubSub channel
+struct PubSubState<T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize> {
+    /// The queue contains the last messages that have been published and a countdown of how many subscribers are yet to read it
+    queue: Deque<(T, usize), CAP>,
+    /// Every message has an id.
+    /// Don't worry, we won't run out.
+    /// If a million messages were published every second, then the ID's would run out in about 584942 years.
+    next_message_id: u64,
+    /// Collection of wakers for Subscribers that are waiting.  
+    subscriber_wakers: MultiWakerRegistration<SUBS>,
+    /// Collection of wakers for Publishers that are waiting.  
+    publisher_wakers: MultiWakerRegistration<PUBS>,
+    /// The amount of subscribers that are active
+    subscriber_count: usize,
+    /// The amount of publishers that are active
+    publisher_count: usize,
+}
+
+impl<T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize> PubSubState<T, CAP, SUBS, PUBS> {
+    /// Create a new internal channel state
+    const fn new() -> Self {
+        Self {
+            queue: Deque::new(),
+            next_message_id: 0,
+            subscriber_wakers: MultiWakerRegistration::new(),
+            publisher_wakers: MultiWakerRegistration::new(),
+            subscriber_count: 0,
+            publisher_count: 0,
+        }
+    }
+
+    fn try_publish(&mut self, message: T) -> Result<(), T> {
+        if self.subscriber_count == 0 {
+            // We don't need to publish anything because there is no one to receive it
+            return Ok(());
+        }
+
+        if self.queue.is_full() {
+            return Err(message);
+        }
+        // We just did a check for this
+        self.queue.push_back((message, self.subscriber_count)).ok().unwrap();
+
+        self.next_message_id += 1;
+
+        // Wake all of the subscribers
+        self.subscriber_wakers.wake();
+
+        Ok(())
+    }
+
+    fn publish_immediate(&mut self, message: T) {
+        // Make space in the queue if required
+        if self.queue.is_full() {
+            self.queue.pop_front();
+        }
+
+        // This will succeed because we made sure there is space
+        self.try_publish(message).ok().unwrap();
+    }
+
+    fn get_message(&mut self, message_id: u64) -> Option<WaitResult<T>> {
+        let start_id = self.next_message_id - self.queue.len() as u64;
+
+        if message_id < start_id {
+            return Some(WaitResult::Lagged(start_id - message_id));
+        }
+
+        let current_message_index = (message_id - start_id) as usize;
+
+        if current_message_index >= self.queue.len() {
+            return None;
+        }
+
+        // We've checked that the index is valid
+        let queue_item = self.queue.iter_mut().nth(current_message_index).unwrap();
+
+        // We're reading this item, so decrement the counter
+        queue_item.1 -= 1;
+        let message = queue_item.0.clone();
+
+        if current_message_index == 0 && queue_item.1 == 0 {
+            self.queue.pop_front();
+            self.publisher_wakers.wake();
+        }
+
+        Some(WaitResult::Message(message))
+    }
+
+    fn register_subscriber_waker(&mut self, waker: &Waker) {
+        match self.subscriber_wakers.register(waker) {
+            Ok(()) => {}
+            Err(_) => {
+                // All waker slots were full. This can only happen when there was a subscriber that now has dropped.
+                // We need to throw it away. It's a bit inefficient, but we can wake everything.
+                // Any future that is still active will simply reregister.
+                // This won't happen a lot, so it's ok.
+                self.subscriber_wakers.wake();
+                self.subscriber_wakers.register(waker).unwrap();
+            }
+        }
+    }
+
+    fn register_publisher_waker(&mut self, waker: &Waker) {
+        match self.publisher_wakers.register(waker) {
+            Ok(()) => {}
+            Err(_) => {
+                // All waker slots were full. This can only happen when there was a publisher that now has dropped.
+                // We need to throw it away. It's a bit inefficient, but we can wake everything.
+                // Any future that is still active will simply reregister.
+                // This won't happen a lot, so it's ok.
+                self.publisher_wakers.wake();
+                self.publisher_wakers.register(waker).unwrap();
+            }
+        }
+    }
+
+    fn unregister_subscriber(&mut self, subscriber_next_message_id: u64) {
+        self.subscriber_count -= 1;
+
+        // All messages that haven't been read yet by this subscriber must have their counter decremented
+        let start_id = self.next_message_id - self.queue.len() as u64;
+        if subscriber_next_message_id >= start_id {
+            let current_message_index = (subscriber_next_message_id - start_id) as usize;
+            self.queue
+                .iter_mut()
+                .skip(current_message_index)
+                .for_each(|(_, counter)| *counter -= 1);
+        }
+    }
+
+    fn unregister_publisher(&mut self) {
+        self.publisher_count -= 1;
+    }
+}
+
+/// Error type for the [PubSubChannel]
+#[derive(Debug, PartialEq, Eq, Clone)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub enum Error {
+    /// All subscriber slots are used. To add another subscriber, first another subscriber must be dropped or
+    /// the capacity of the channels must be increased.
+    MaximumSubscribersReached,
+    /// All publisher slots are used. To add another publisher, first another publisher must be dropped or
+    /// the capacity of the channels must be increased.
+    MaximumPublishersReached,
+}
+
+/// 'Middle level' behaviour of the pubsub channel.
+/// This trait is used so that Sub and Pub can be generic over the channel.
+pub trait PubSubBehavior<T> {
+    /// Try to get a message from the queue with the given message id.
+    ///
+    /// If the message is not yet present and a context is given, then its waker is registered in the subsriber wakers.
+    fn get_message_with_context(&self, next_message_id: &mut u64, cx: Option<&mut Context<'_>>) -> Poll<WaitResult<T>>;
+
+    /// Try to publish a message to the queue.
+    ///
+    /// If the queue is full and a context is given, then its waker is registered in the publisher wakers.
+    fn publish_with_context(&self, message: T, cx: Option<&mut Context<'_>>) -> Result<(), T>;
+
+    /// Publish a message immediately
+    fn publish_immediate(&self, message: T);
+
+    /// Let the channel know that a subscriber has dropped
+    fn unregister_subscriber(&self, subscriber_next_message_id: u64);
+
+    /// Let the channel know that a publisher has dropped
+    fn unregister_publisher(&self);
+}
+
+/// The result of the subscriber wait procedure
+#[derive(Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub enum WaitResult<T> {
+    /// The subscriber did not receive all messages and lagged by the given amount of messages.
+    /// (This is the amount of messages that were missed)
+    Lagged(u64),
+    /// A message was received
+    Message(T),
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::blocking_mutex::raw::NoopRawMutex;
+
+    #[futures_test::test]
+    async fn dyn_pub_sub_works() {
+        let channel = PubSubChannel::<NoopRawMutex, u32, 4, 4, 4>::new();
+
+        let mut sub0 = channel.dyn_subscriber().unwrap();
+        let mut sub1 = channel.dyn_subscriber().unwrap();
+        let pub0 = channel.dyn_publisher().unwrap();
+
+        pub0.publish(42).await;
+
+        assert_eq!(sub0.next_message().await, WaitResult::Message(42));
+        assert_eq!(sub1.next_message().await, WaitResult::Message(42));
+
+        assert_eq!(sub0.try_next_message(), None);
+        assert_eq!(sub1.try_next_message(), None);
+    }
+
+    #[futures_test::test]
+    async fn all_subscribers_receive() {
+        let channel = PubSubChannel::<NoopRawMutex, u32, 4, 4, 4>::new();
+
+        let mut sub0 = channel.subscriber().unwrap();
+        let mut sub1 = channel.subscriber().unwrap();
+        let pub0 = channel.publisher().unwrap();
+
+        pub0.publish(42).await;
+
+        assert_eq!(sub0.next_message().await, WaitResult::Message(42));
+        assert_eq!(sub1.next_message().await, WaitResult::Message(42));
+
+        assert_eq!(sub0.try_next_message(), None);
+        assert_eq!(sub1.try_next_message(), None);
+    }
+
+    #[futures_test::test]
+    async fn lag_when_queue_full_on_immediate_publish() {
+        let channel = PubSubChannel::<NoopRawMutex, u32, 4, 4, 4>::new();
+
+        let mut sub0 = channel.subscriber().unwrap();
+        let pub0 = channel.publisher().unwrap();
+
+        pub0.publish_immediate(42);
+        pub0.publish_immediate(43);
+        pub0.publish_immediate(44);
+        pub0.publish_immediate(45);
+        pub0.publish_immediate(46);
+        pub0.publish_immediate(47);
+
+        assert_eq!(sub0.try_next_message(), Some(WaitResult::Lagged(2)));
+        assert_eq!(sub0.next_message().await, WaitResult::Message(44));
+        assert_eq!(sub0.next_message().await, WaitResult::Message(45));
+        assert_eq!(sub0.next_message().await, WaitResult::Message(46));
+        assert_eq!(sub0.next_message().await, WaitResult::Message(47));
+        assert_eq!(sub0.try_next_message(), None);
+    }
+
+    #[test]
+    fn limited_subs_and_pubs() {
+        let channel = PubSubChannel::<NoopRawMutex, u32, 4, 4, 4>::new();
+
+        let sub0 = channel.subscriber();
+        let sub1 = channel.subscriber();
+        let sub2 = channel.subscriber();
+        let sub3 = channel.subscriber();
+        let sub4 = channel.subscriber();
+
+        assert!(sub0.is_ok());
+        assert!(sub1.is_ok());
+        assert!(sub2.is_ok());
+        assert!(sub3.is_ok());
+        assert_eq!(sub4.err().unwrap(), Error::MaximumSubscribersReached);
+
+        drop(sub0);
+
+        let sub5 = channel.subscriber();
+        assert!(sub5.is_ok());
+
+        // publishers
+
+        let pub0 = channel.publisher();
+        let pub1 = channel.publisher();
+        let pub2 = channel.publisher();
+        let pub3 = channel.publisher();
+        let pub4 = channel.publisher();
+
+        assert!(pub0.is_ok());
+        assert!(pub1.is_ok());
+        assert!(pub2.is_ok());
+        assert!(pub3.is_ok());
+        assert_eq!(pub4.err().unwrap(), Error::MaximumPublishersReached);
+
+        drop(pub0);
+
+        let pub5 = channel.publisher();
+        assert!(pub5.is_ok());
+    }
+
+    #[test]
+    fn publisher_wait_on_full_queue() {
+        let channel = PubSubChannel::<NoopRawMutex, u32, 4, 4, 4>::new();
+
+        let pub0 = channel.publisher().unwrap();
+
+        // There are no subscribers, so the queue will never be full
+        assert_eq!(pub0.try_publish(0), Ok(()));
+        assert_eq!(pub0.try_publish(0), Ok(()));
+        assert_eq!(pub0.try_publish(0), Ok(()));
+        assert_eq!(pub0.try_publish(0), Ok(()));
+        assert_eq!(pub0.try_publish(0), Ok(()));
+
+        let sub0 = channel.subscriber().unwrap();
+
+        assert_eq!(pub0.try_publish(0), Ok(()));
+        assert_eq!(pub0.try_publish(0), Ok(()));
+        assert_eq!(pub0.try_publish(0), Ok(()));
+        assert_eq!(pub0.try_publish(0), Ok(()));
+        assert_eq!(pub0.try_publish(0), Err(0));
+
+        drop(sub0);
+    }
+}

embassy-sync/src/channel/pubsub/publisher.rs

@@ -0,0 +1,182 @@
+//! Implementation of anything directly publisher related
+
+use core::future::Future;
+use core::marker::PhantomData;
+use core::ops::{Deref, DerefMut};
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+use super::{PubSubBehavior, PubSubChannel};
+use crate::blocking_mutex::raw::RawMutex;
+
+/// A publisher to a channel
+pub struct Pub<'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> {
+    /// The channel we are a publisher for
+    channel: &'a PSB,
+    _phantom: PhantomData<T>,
+}
+
+impl<'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> Pub<'a, PSB, T> {
+    pub(super) fn new(channel: &'a PSB) -> Self {
+        Self {
+            channel,
+            _phantom: Default::default(),
+        }
+    }
+
+    /// Publish a message right now even when the queue is full.
+    /// This may cause a subscriber to miss an older message.
+    pub fn publish_immediate(&self, message: T) {
+        self.channel.publish_immediate(message)
+    }
+
+    /// Publish a message. But if the message queue is full, wait for all subscribers to have read the last message
+    pub fn publish<'s>(&'s self, message: T) -> PublisherWaitFuture<'s, 'a, PSB, T> {
+        PublisherWaitFuture {
+            message: Some(message),
+            publisher: self,
+        }
+    }
+
+    /// Publish a message if there is space in the message queue
+    pub fn try_publish(&self, message: T) -> Result<(), T> {
+        self.channel.publish_with_context(message, None)
+    }
+}
+
+impl<'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> Drop for Pub<'a, PSB, T> {
+    fn drop(&mut self) {
+        self.channel.unregister_publisher()
+    }
+}
+
+/// A publisher that holds a dynamic reference to the channel
+pub struct DynPublisher<'a, T: Clone>(pub(super) Pub<'a, dyn PubSubBehavior<T> + 'a, T>);
+
+impl<'a, T: Clone> Deref for DynPublisher<'a, T> {
+    type Target = Pub<'a, dyn PubSubBehavior<T> + 'a, T>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<'a, T: Clone> DerefMut for DynPublisher<'a, T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+/// A publisher that holds a generic reference to the channel
+pub struct Publisher<'a, M: RawMutex, T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize>(
+    pub(super) Pub<'a, PubSubChannel<M, T, CAP, SUBS, PUBS>, T>,
+);
+
+impl<'a, M: RawMutex, T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize> Deref
+    for Publisher<'a, M, T, CAP, SUBS, PUBS>
+{
+    type Target = Pub<'a, PubSubChannel<M, T, CAP, SUBS, PUBS>, T>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<'a, M: RawMutex, T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize> DerefMut
+    for Publisher<'a, M, T, CAP, SUBS, PUBS>
+{
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+/// A publisher that can only use the `publish_immediate` function, but it doesn't have to be registered with the channel.
+/// (So an infinite amount is possible)
+pub struct ImmediatePub<'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> {
+    /// The channel we are a publisher for
+    channel: &'a PSB,
+    _phantom: PhantomData<T>,
+}
+
+impl<'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> ImmediatePub<'a, PSB, T> {
+    pub(super) fn new(channel: &'a PSB) -> Self {
+        Self {
+            channel,
+            _phantom: Default::default(),
+        }
+    }
+    /// Publish the message right now even when the queue is full.
+    /// This may cause a subscriber to miss an older message.
+    pub fn publish_immediate(&self, message: T) {
+        self.channel.publish_immediate(message)
+    }
+
+    /// Publish a message if there is space in the message queue
+    pub fn try_publish(&self, message: T) -> Result<(), T> {
+        self.channel.publish_with_context(message, None)
+    }
+}
+
+/// An immediate publisher that holds a dynamic reference to the channel
+pub struct DynImmediatePublisher<'a, T: Clone>(pub(super) ImmediatePub<'a, dyn PubSubBehavior<T> + 'a, T>);
+
+impl<'a, T: Clone> Deref for DynImmediatePublisher<'a, T> {
+    type Target = ImmediatePub<'a, dyn PubSubBehavior<T> + 'a, T>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<'a, T: Clone> DerefMut for DynImmediatePublisher<'a, T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+/// An immediate publisher that holds a generic reference to the channel
+pub struct ImmediatePublisher<'a, M: RawMutex, T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize>(
+    pub(super) ImmediatePub<'a, PubSubChannel<M, T, CAP, SUBS, PUBS>, T>,
+);
+
+impl<'a, M: RawMutex, T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize> Deref
+    for ImmediatePublisher<'a, M, T, CAP, SUBS, PUBS>
+{
+    type Target = ImmediatePub<'a, PubSubChannel<M, T, CAP, SUBS, PUBS>, T>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<'a, M: RawMutex, T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize> DerefMut
+    for ImmediatePublisher<'a, M, T, CAP, SUBS, PUBS>
+{
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+/// Future for the publisher wait action
+pub struct PublisherWaitFuture<'s, 'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> {
+    /// The message we need to publish
+    message: Option<T>,
+    publisher: &'s Pub<'a, PSB, T>,
+}
+
+impl<'s, 'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> Future for PublisherWaitFuture<'s, 'a, PSB, T> {
+    type Output = ();
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let message = self.message.take().unwrap();
+        match self.publisher.channel.publish_with_context(message, Some(cx)) {
+            Ok(()) => Poll::Ready(()),
+            Err(message) => {
+                self.message = Some(message);
+                Poll::Pending
+            }
+        }
+    }
+}
+
+impl<'s, 'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> Unpin for PublisherWaitFuture<'s, 'a, PSB, T> {}

embassy-sync/src/channel/pubsub/subscriber.rs

@@ -0,0 +1,152 @@
+//! Implementation of anything directly subscriber related
+
+use core::future::Future;
+use core::marker::PhantomData;
+use core::ops::{Deref, DerefMut};
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+use super::{PubSubBehavior, PubSubChannel, WaitResult};
+use crate::blocking_mutex::raw::RawMutex;
+
+/// A subscriber to a channel
+pub struct Sub<'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> {
+    /// The message id of the next message we are yet to receive
+    next_message_id: u64,
+    /// The channel we are a subscriber to
+    channel: &'a PSB,
+    _phantom: PhantomData<T>,
+}
+
+impl<'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> Sub<'a, PSB, T> {
+    pub(super) fn new(next_message_id: u64, channel: &'a PSB) -> Self {
+        Self {
+            next_message_id,
+            channel,
+            _phantom: Default::default(),
+        }
+    }
+
+    /// Wait for a published message
+    pub fn next_message<'s>(&'s mut self) -> SubscriberWaitFuture<'s, 'a, PSB, T> {
+        SubscriberWaitFuture { subscriber: self }
+    }
+
+    /// Wait for a published message (ignoring lag results)
+    pub async fn next_message_pure(&mut self) -> T {
+        loop {
+            match self.next_message().await {
+                WaitResult::Lagged(_) => continue,
+                WaitResult::Message(message) => break message,
+            }
+        }
+    }
+
+    /// Try to see if there's a published message we haven't received yet.
+    ///
+    /// This function does not peek. The message is received if there is one.
+    pub fn try_next_message(&mut self) -> Option<WaitResult<T>> {
+        match self.channel.get_message_with_context(&mut self.next_message_id, None) {
+            Poll::Ready(result) => Some(result),
+            Poll::Pending => None,
+        }
+    }
+
+    /// Try to see if there's a published message we haven't received yet (ignoring lag results).
+    ///
+    /// This function does not peek. The message is received if there is one.
+    pub fn try_next_message_pure(&mut self) -> Option<T> {
+        loop {
+            match self.try_next_message() {
+                Some(WaitResult::Lagged(_)) => continue,
+                Some(WaitResult::Message(message)) => break Some(message),
+                None => break None,
+            }
+        }
+    }
+}
+
+impl<'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> Drop for Sub<'a, PSB, T> {
+    fn drop(&mut self) {
+        self.channel.unregister_subscriber(self.next_message_id)
+    }
+}
+
+impl<'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> Unpin for Sub<'a, PSB, T> {}
+
+/// Warning: The stream implementation ignores lag results and returns all messages.
+/// This might miss some messages without you knowing it.
+impl<'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> futures_util::Stream for Sub<'a, PSB, T> {
+    type Item = T;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        match self
+            .channel
+            .get_message_with_context(&mut self.next_message_id, Some(cx))
+        {
+            Poll::Ready(WaitResult::Message(message)) => Poll::Ready(Some(message)),
+            Poll::Ready(WaitResult::Lagged(_)) => {
+                cx.waker().wake_by_ref();
+                Poll::Pending
+            }
+            Poll::Pending => Poll::Pending,
+        }
+    }
+}
+
+/// A subscriber that holds a dynamic reference to the channel
+pub struct DynSubscriber<'a, T: Clone>(pub(super) Sub<'a, dyn PubSubBehavior<T> + 'a, T>);
+
+impl<'a, T: Clone> Deref for DynSubscriber<'a, T> {
+    type Target = Sub<'a, dyn PubSubBehavior<T> + 'a, T>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<'a, T: Clone> DerefMut for DynSubscriber<'a, T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+/// A subscriber that holds a generic reference to the channel
+pub struct Subscriber<'a, M: RawMutex, T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize>(
+    pub(super) Sub<'a, PubSubChannel<M, T, CAP, SUBS, PUBS>, T>,
+);
+
+impl<'a, M: RawMutex, T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize> Deref
+    for Subscriber<'a, M, T, CAP, SUBS, PUBS>
+{
+    type Target = Sub<'a, PubSubChannel<M, T, CAP, SUBS, PUBS>, T>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<'a, M: RawMutex, T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize> DerefMut
+    for Subscriber<'a, M, T, CAP, SUBS, PUBS>
+{
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+/// Future for the subscriber wait action
+pub struct SubscriberWaitFuture<'s, 'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> {
+    subscriber: &'s mut Sub<'a, PSB, T>,
+}
+
+impl<'s, 'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> Future for SubscriberWaitFuture<'s, 'a, PSB, T> {
+    type Output = WaitResult<T>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        self.subscriber
+            .channel
+            .get_message_with_context(&mut self.subscriber.next_message_id, Some(cx))
+    }
+}
+
+impl<'s, 'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> Unpin for SubscriberWaitFuture<'s, 'a, PSB, T> {}

embassy-sync/src/channel/signal.rs

@@ -0,0 +1,100 @@
+//! A synchronization primitive for passing the latest value to a task.
+use core::cell::UnsafeCell;
+use core::future::Future;
+use core::mem;
+use core::task::{Context, Poll, Waker};
+
+/// Single-slot signaling primitive.
+///
+/// This is similar to a [`Channel`](crate::channel::mpmc::Channel) with a buffer size of 1, except
+/// "sending" to it (calling [`Signal::signal`]) when full will overwrite the previous value instead
+/// of waiting for the receiver to pop the previous value.
+///
+/// It is useful for sending data between tasks when the receiver only cares about
+/// the latest data, and therefore it's fine to "lose" messages. This is often the case for "state"
+/// updates.
+///
+/// For more advanced use cases, you might want to use [`Channel`](crate::channel::mpmc::Channel) instead.
+///
+/// Signals are generally declared as `static`s and then borrowed as required.
+///
+/// ```
+/// use embassy_sync::channel::signal::Signal;
+///
+/// enum SomeCommand {
+///   On,
+///   Off,
+/// }
+///
+/// static SOME_SIGNAL: Signal<SomeCommand> = Signal::new();
+/// ```
+pub struct Signal<T> {
+    state: UnsafeCell<State<T>>,
+}
+
+enum State<T> {
+    None,
+    Waiting(Waker),
+    Signaled(T),
+}
+
+unsafe impl<T: Send> Send for Signal<T> {}
+unsafe impl<T: Send> Sync for Signal<T> {}
+
+impl<T> Signal<T> {
+    /// Create a new `Signal`.
+    pub const fn new() -> Self {
+        Self {
+            state: UnsafeCell::new(State::None),
+        }
+    }
+}
+
+impl<T: Send> Signal<T> {
+    /// Mark this Signal as signaled.
+    pub fn signal(&self, val: T) {
+        critical_section::with(|_| unsafe {
+            let state = &mut *self.state.get();
+            if let State::Waiting(waker) = mem::replace(state, State::Signaled(val)) {
+                waker.wake();
+            }
+        })
+    }
+
+    /// Remove the queued value in this `Signal`, if any.
+    pub fn reset(&self) {
+        critical_section::with(|_| unsafe {
+            let state = &mut *self.state.get();
+            *state = State::None
+        })
+    }
+
+    /// Manually poll the Signal future.
+    pub fn poll_wait(&self, cx: &mut Context<'_>) -> Poll<T> {
+        critical_section::with(|_| unsafe {
+            let state = &mut *self.state.get();
+            match state {
+                State::None => {
+                    *state = State::Waiting(cx.waker().clone());
+                    Poll::Pending
+                }
+                State::Waiting(w) if w.will_wake(cx.waker()) => Poll::Pending,
+                State::Waiting(_) => panic!("waker overflow"),
+                State::Signaled(_) => match mem::replace(state, State::None) {
+                    State::Signaled(res) => Poll::Ready(res),
+                    _ => unreachable!(),
+                },
+            }
+        })
+    }
+
+    /// Future that completes when this Signal has been signaled.
+    pub fn wait(&self) -> impl Future<Output = T> + '_ {
+        futures_util::future::poll_fn(move |cx| self.poll_wait(cx))
+    }
+
+    /// non-blocking method to check whether this signal has been signaled.
+    pub fn signaled(&self) -> bool {
+        critical_section::with(|_| matches!(unsafe { &*self.state.get() }, State::Signaled(_)))
+    }
+}

embassy-sync/src/fmt.rs

@@ -0,0 +1,228 @@
+#![macro_use]
+#![allow(unused_macros)]
+
+#[cfg(all(feature = "defmt", feature = "log"))]
+compile_error!("You may not enable both `defmt` and `log` features.");
+
+macro_rules! assert {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::assert!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::assert!($($x)*);
+        }
+    };
+}
+
+macro_rules! assert_eq {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::assert_eq!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::assert_eq!($($x)*);
+        }
+    };
+}
+
+macro_rules! assert_ne {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::assert_ne!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::assert_ne!($($x)*);
+        }
+    };
+}
+
+macro_rules! debug_assert {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::debug_assert!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::debug_assert!($($x)*);
+        }
+    };
+}
+
+macro_rules! debug_assert_eq {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::debug_assert_eq!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::debug_assert_eq!($($x)*);
+        }
+    };
+}
+
+macro_rules! debug_assert_ne {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::debug_assert_ne!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::debug_assert_ne!($($x)*);
+        }
+    };
+}
+
+macro_rules! todo {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::todo!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::todo!($($x)*);
+        }
+    };
+}
+
+macro_rules! unreachable {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::unreachable!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::unreachable!($($x)*);
+        }
+    };
+}
+
+macro_rules! panic {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::panic!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::panic!($($x)*);
+        }
+    };
+}
+
+macro_rules! trace {
+    ($s:literal $(, $x:expr)* $(,)?) => {
+        {
+            #[cfg(feature = "log")]
+            ::log::trace!($s $(, $x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::trace!($s $(, $x)*);
+            #[cfg(not(any(feature = "log", feature="defmt")))]
+            let _ = ($( & $x ),*);
+        }
+    };
+}
+
+macro_rules! debug {
+    ($s:literal $(, $x:expr)* $(,)?) => {
+        {
+            #[cfg(feature = "log")]
+            ::log::debug!($s $(, $x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::debug!($s $(, $x)*);
+            #[cfg(not(any(feature = "log", feature="defmt")))]
+            let _ = ($( & $x ),*);
+        }
+    };
+}
+
+macro_rules! info {
+    ($s:literal $(, $x:expr)* $(,)?) => {
+        {
+            #[cfg(feature = "log")]
+            ::log::info!($s $(, $x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::info!($s $(, $x)*);
+            #[cfg(not(any(feature = "log", feature="defmt")))]
+            let _ = ($( & $x ),*);
+        }
+    };
+}
+
+macro_rules! warn {
+    ($s:literal $(, $x:expr)* $(,)?) => {
+        {
+            #[cfg(feature = "log")]
+            ::log::warn!($s $(, $x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::warn!($s $(, $x)*);
+            #[cfg(not(any(feature = "log", feature="defmt")))]
+            let _ = ($( & $x ),*);
+        }
+    };
+}
+
+macro_rules! error {
+    ($s:literal $(, $x:expr)* $(,)?) => {
+        {
+            #[cfg(feature = "log")]
+            ::log::error!($s $(, $x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::error!($s $(, $x)*);
+            #[cfg(not(any(feature = "log", feature="defmt")))]
+            let _ = ($( & $x ),*);
+        }
+    };
+}
+
+#[cfg(feature = "defmt")]
+macro_rules! unwrap {
+    ($($x:tt)*) => {
+        ::defmt::unwrap!($($x)*)
+    };
+}
+
+#[cfg(not(feature = "defmt"))]
+macro_rules! unwrap {
+    ($arg:expr) => {
+        match $crate::fmt::Try::into_result($arg) {
+            ::core::result::Result::Ok(t) => t,
+            ::core::result::Result::Err(e) => {
+                ::core::panic!("unwrap of `{}` failed: {:?}", ::core::stringify!($arg), e);
+            }
+        }
+    };
+    ($arg:expr, $($msg:expr),+ $(,)? ) => {
+        match $crate::fmt::Try::into_result($arg) {
+            ::core::result::Result::Ok(t) => t,
+            ::core::result::Result::Err(e) => {
+                ::core::panic!("unwrap of `{}` failed: {}: {:?}", ::core::stringify!($arg), ::core::format_args!($($msg,)*), e);
+            }
+        }
+    }
+}
+
+#[cfg(feature = "defmt-timestamp-uptime")]
+defmt::timestamp! {"{=u64:us}", crate::time::Instant::now().as_micros() }
+
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+pub struct NoneError;
+
+pub trait Try {
+    type Ok;
+    type Error;
+    fn into_result(self) -> Result<Self::Ok, Self::Error>;
+}
+
+impl<T> Try for Option<T> {
+    type Ok = T;
+    type Error = NoneError;
+
+    #[inline]
+    fn into_result(self) -> Result<T, NoneError> {
+        self.ok_or(NoneError)
+    }
+}
+
+impl<T, E> Try for Result<T, E> {
+    type Ok = T;
+    type Error = E;
+
+    #[inline]
+    fn into_result(self) -> Self {
+        self
+    }
+}

embassy-sync/src/lib.rs

@@ -0,0 +1,17 @@
+#![cfg_attr(not(any(feature = "std", feature = "wasm")), no_std)]
+#![cfg_attr(feature = "nightly", feature(generic_associated_types, type_alias_impl_trait))]
+#![allow(clippy::new_without_default)]
+#![doc = include_str!("../../README.md")]
+#![warn(missing_docs)]
+
+// This mod MUST go first, so that the others see its macros.
+pub(crate) mod fmt;
+
+// internal use
+mod ring_buffer;
+
+pub mod blocking_mutex;
+pub mod channel;
+pub mod mutex;
+pub mod pipe;
+pub mod waitqueue;

embassy-sync/src/mutex.rs

@@ -0,0 +1,167 @@
+//! Async mutex.
+//!
+//! This module provides a mutex that can be used to synchronize data between asynchronous tasks.
+use core::cell::{RefCell, UnsafeCell};
+use core::ops::{Deref, DerefMut};
+use core::task::Poll;
+
+use futures_util::future::poll_fn;
+
+use crate::blocking_mutex::raw::RawMutex;
+use crate::blocking_mutex::Mutex as BlockingMutex;
+use crate::waitqueue::WakerRegistration;
+
+/// Error returned by [`Mutex::try_lock`]
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub struct TryLockError;
+
+struct State {
+    locked: bool,
+    waker: WakerRegistration,
+}
+
+/// Async mutex.
+///
+/// The mutex is generic over a blocking [`RawMutex`](crate::blocking_mutex::raw::RawMutex).
+/// The raw mutex is used to guard access to the internal "is locked" flag. It
+/// is held for very short periods only, while locking and unlocking. It is *not* held
+/// for the entire time the async Mutex is locked.
+///
+/// Which implementation you select depends on the context in which you're using the mutex.
+///
+/// Use [`CriticalSectionRawMutex`](crate::blocking_mutex::raw::CriticalSectionRawMutex) when data can be shared between threads and interrupts.
+///
+/// Use [`NoopRawMutex`](crate::blocking_mutex::raw::NoopRawMutex) when data is only shared between tasks running on the same executor.
+///
+/// Use [`ThreadModeRawMutex`](crate::blocking_mutex::raw::ThreadModeRawMutex) when data is shared between tasks running on the same executor but you want a singleton.
+///
+pub struct Mutex<M, T>
+where
+    M: RawMutex,
+    T: ?Sized,
+{
+    state: BlockingMutex<M, RefCell<State>>,
+    inner: UnsafeCell<T>,
+}
+
+unsafe impl<M: RawMutex + Send, T: ?Sized + Send> Send for Mutex<M, T> {}
+unsafe impl<M: RawMutex + Sync, T: ?Sized + Send> Sync for Mutex<M, T> {}
+
+/// Async mutex.
+impl<M, T> Mutex<M, T>
+where
+    M: RawMutex,
+{
+    /// Create a new mutex with the given value.
+    pub const fn new(value: T) -> Self {
+        Self {
+            inner: UnsafeCell::new(value),
+            state: BlockingMutex::new(RefCell::new(State {
+                locked: false,
+                waker: WakerRegistration::new(),
+            })),
+        }
+    }
+}
+
+impl<M, T> Mutex<M, T>
+where
+    M: RawMutex,
+    T: ?Sized,
+{
+    /// Lock the mutex.
+    ///
+    /// This will wait for the mutex to be unlocked if it's already locked.
+    pub async fn lock(&self) -> MutexGuard<'_, M, T> {
+        poll_fn(|cx| {
+            let ready = self.state.lock(|s| {
+                let mut s = s.borrow_mut();
+                if s.locked {
+                    s.waker.register(cx.waker());
+                    false
+                } else {
+                    s.locked = true;
+                    true
+                }
+            });
+
+            if ready {
+                Poll::Ready(MutexGuard { mutex: self })
+            } else {
+                Poll::Pending
+            }
+        })
+        .await
+    }
+
+    /// Attempt to immediately lock the mutex.
+    ///
+    /// If the mutex is already locked, this will return an error instead of waiting.
+    pub fn try_lock(&self) -> Result<MutexGuard<'_, M, T>, TryLockError> {
+        self.state.lock(|s| {
+            let mut s = s.borrow_mut();
+            if s.locked {
+                Err(TryLockError)
+            } else {
+                s.locked = true;
+                Ok(())
+            }
+        })?;
+
+        Ok(MutexGuard { mutex: self })
+    }
+}
+
+/// Async mutex guard.
+///
+/// Owning an instance of this type indicates having
+/// successfully locked the mutex, and grants access to the contents.
+///
+/// Dropping it unlocks the mutex.
+pub struct MutexGuard<'a, M, T>
+where
+    M: RawMutex,
+    T: ?Sized,
+{
+    mutex: &'a Mutex<M, T>,
+}
+
+impl<'a, M, T> Drop for MutexGuard<'a, M, T>
+where
+    M: RawMutex,
+    T: ?Sized,
+{
+    fn drop(&mut self) {
+        self.mutex.state.lock(|s| {
+            let mut s = s.borrow_mut();
+            s.locked = false;
+            s.waker.wake();
+        })
+    }
+}
+
+impl<'a, M, T> Deref for MutexGuard<'a, M, T>
+where
+    M: RawMutex,
+    T: ?Sized,
+{
+    type Target = T;
+    fn deref(&self) -> &Self::Target {
+        // Safety: the MutexGuard represents exclusive access to the contents
+        // of the mutex, so it's OK to get it.
+        unsafe { &*(self.mutex.inner.get() as *const T) }
+    }
+}
+
+impl<'a, M, T> DerefMut for MutexGuard<'a, M, T>
+where
+    M: RawMutex,
+    T: ?Sized,
+{
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        // Safety: the MutexGuard represents exclusive access to the contents
+        // of the mutex, so it's OK to get it.
+        unsafe { &mut *(self.mutex.inner.get()) }
+    }
+}

embassy-sync/src/pipe.rs

@@ -0,0 +1,551 @@
+//! Async byte stream pipe.
+
+use core::cell::RefCell;
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+use crate::blocking_mutex::raw::RawMutex;
+use crate::blocking_mutex::Mutex;
+use crate::ring_buffer::RingBuffer;
+use crate::waitqueue::WakerRegistration;
+
+/// Write-only access to a [`Pipe`].
+#[derive(Copy)]
+pub struct Writer<'p, M, const N: usize>
+where
+    M: RawMutex,
+{
+    pipe: &'p Pipe<M, N>,
+}
+
+impl<'p, M, const N: usize> Clone for Writer<'p, M, N>
+where
+    M: RawMutex,
+{
+    fn clone(&self) -> Self {
+        Writer { pipe: self.pipe }
+    }
+}
+
+impl<'p, M, const N: usize> Writer<'p, M, N>
+where
+    M: RawMutex,
+{
+    /// Writes a value.
+    ///
+    /// See [`Pipe::write()`]
+    pub fn write<'a>(&'a self, buf: &'a [u8]) -> WriteFuture<'a, M, N> {
+        self.pipe.write(buf)
+    }
+
+    /// Attempt to immediately write a message.
+    ///
+    /// See [`Pipe::write()`]
+    pub fn try_write(&self, buf: &[u8]) -> Result<usize, TryWriteError> {
+        self.pipe.try_write(buf)
+    }
+}
+
+/// Future returned by [`Pipe::write`] and  [`Writer::write`].
+pub struct WriteFuture<'p, M, const N: usize>
+where
+    M: RawMutex,
+{
+    pipe: &'p Pipe<M, N>,
+    buf: &'p [u8],
+}
+
+impl<'p, M, const N: usize> Future for WriteFuture<'p, M, N>
+where
+    M: RawMutex,
+{
+    type Output = usize;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        match self.pipe.try_write_with_context(Some(cx), self.buf) {
+            Ok(n) => Poll::Ready(n),
+            Err(TryWriteError::Full) => Poll::Pending,
+        }
+    }
+}
+
+impl<'p, M, const N: usize> Unpin for WriteFuture<'p, M, N> where M: RawMutex {}
+
+/// Read-only access to a [`Pipe`].
+#[derive(Copy)]
+pub struct Reader<'p, M, const N: usize>
+where
+    M: RawMutex,
+{
+    pipe: &'p Pipe<M, N>,
+}
+
+impl<'p, M, const N: usize> Clone for Reader<'p, M, N>
+where
+    M: RawMutex,
+{
+    fn clone(&self) -> Self {
+        Reader { pipe: self.pipe }
+    }
+}
+
+impl<'p, M, const N: usize> Reader<'p, M, N>
+where
+    M: RawMutex,
+{
+    /// Reads a value.
+    ///
+    /// See [`Pipe::read()`]
+    pub fn read<'a>(&'a self, buf: &'a mut [u8]) -> ReadFuture<'a, M, N> {
+        self.pipe.read(buf)
+    }
+
+    /// Attempt to immediately read a message.
+    ///
+    /// See [`Pipe::read()`]
+    pub fn try_read(&self, buf: &mut [u8]) -> Result<usize, TryReadError> {
+        self.pipe.try_read(buf)
+    }
+}
+
+/// Future returned by [`Pipe::read`] and  [`Reader::read`].
+pub struct ReadFuture<'p, M, const N: usize>
+where
+    M: RawMutex,
+{
+    pipe: &'p Pipe<M, N>,
+    buf: &'p mut [u8],
+}
+
+impl<'p, M, const N: usize> Future for ReadFuture<'p, M, N>
+where
+    M: RawMutex,
+{
+    type Output = usize;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        match self.pipe.try_read_with_context(Some(cx), self.buf) {
+            Ok(n) => Poll::Ready(n),
+            Err(TryReadError::Empty) => Poll::Pending,
+        }
+    }
+}
+
+impl<'p, M, const N: usize> Unpin for ReadFuture<'p, M, N> where M: RawMutex {}
+
+/// Error returned by [`try_read`](Pipe::try_read).
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub enum TryReadError {
+    /// No data could be read from the pipe because it is currently
+    /// empty, and reading would require blocking.
+    Empty,
+}
+
+/// Error returned by [`try_write`](Pipe::try_write).
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub enum TryWriteError {
+    /// No data could be written to the pipe because it is
+    /// currently full, and writing would require blocking.
+    Full,
+}
+
+struct PipeState<const N: usize> {
+    buffer: RingBuffer<N>,
+    read_waker: WakerRegistration,
+    write_waker: WakerRegistration,
+}
+
+impl<const N: usize> PipeState<N> {
+    const fn new() -> Self {
+        PipeState {
+            buffer: RingBuffer::new(),
+            read_waker: WakerRegistration::new(),
+            write_waker: WakerRegistration::new(),
+        }
+    }
+
+    fn clear(&mut self) {
+        self.buffer.clear();
+        self.write_waker.wake();
+    }
+
+    fn try_read(&mut self, buf: &mut [u8]) -> Result<usize, TryReadError> {
+        self.try_read_with_context(None, buf)
+    }
+
+    fn try_read_with_context(&mut self, cx: Option<&mut Context<'_>>, buf: &mut [u8]) -> Result<usize, TryReadError> {
+        if self.buffer.is_full() {
+            self.write_waker.wake();
+        }
+
+        let available = self.buffer.pop_buf();
+        if available.is_empty() {
+            if let Some(cx) = cx {
+                self.read_waker.register(cx.waker());
+            }
+            return Err(TryReadError::Empty);
+        }
+
+        let n = available.len().min(buf.len());
+        buf[..n].copy_from_slice(&available[..n]);
+        self.buffer.pop(n);
+        Ok(n)
+    }
+
+    fn try_write(&mut self, buf: &[u8]) -> Result<usize, TryWriteError> {
+        self.try_write_with_context(None, buf)
+    }
+
+    fn try_write_with_context(&mut self, cx: Option<&mut Context<'_>>, buf: &[u8]) -> Result<usize, TryWriteError> {
+        if self.buffer.is_empty() {
+            self.read_waker.wake();
+        }
+
+        let available = self.buffer.push_buf();
+        if available.is_empty() {
+            if let Some(cx) = cx {
+                self.write_waker.register(cx.waker());
+            }
+            return Err(TryWriteError::Full);
+        }
+
+        let n = available.len().min(buf.len());
+        available[..n].copy_from_slice(&buf[..n]);
+        self.buffer.push(n);
+        Ok(n)
+    }
+}
+
+/// A bounded pipe for communicating between asynchronous tasks
+/// with backpressure.
+///
+/// The pipe will buffer up to the provided number of messages.  Once the
+/// buffer is full, attempts to `write` new messages will wait until a message is
+/// read from the pipe.
+///
+/// All data written will become available in the same order as it was written.
+pub struct Pipe<M, const N: usize>
+where
+    M: RawMutex,
+{
+    inner: Mutex<M, RefCell<PipeState<N>>>,
+}
+
+impl<M, const N: usize> Pipe<M, N>
+where
+    M: RawMutex,
+{
+    /// Establish a new bounded pipe. For example, to create one with a NoopMutex:
+    ///
+    /// ```
+    /// use embassy_sync::pipe::Pipe;
+    /// use embassy_sync::blocking_mutex::raw::NoopRawMutex;
+    ///
+    /// // Declare a bounded pipe, with a buffer of 256 bytes.
+    /// let mut pipe = Pipe::<NoopRawMutex, 256>::new();
+    /// ```
+    pub const fn new() -> Self {
+        Self {
+            inner: Mutex::new(RefCell::new(PipeState::new())),
+        }
+    }
+
+    fn lock<R>(&self, f: impl FnOnce(&mut PipeState<N>) -> R) -> R {
+        self.inner.lock(|rc| f(&mut *rc.borrow_mut()))
+    }
+
+    fn try_read_with_context(&self, cx: Option<&mut Context<'_>>, buf: &mut [u8]) -> Result<usize, TryReadError> {
+        self.lock(|c| c.try_read_with_context(cx, buf))
+    }
+
+    fn try_write_with_context(&self, cx: Option<&mut Context<'_>>, buf: &[u8]) -> Result<usize, TryWriteError> {
+        self.lock(|c| c.try_write_with_context(cx, buf))
+    }
+
+    /// Get a writer for this pipe.
+    pub fn writer(&self) -> Writer<'_, M, N> {
+        Writer { pipe: self }
+    }
+
+    /// Get a reader for this pipe.
+    pub fn reader(&self) -> Reader<'_, M, N> {
+        Reader { pipe: self }
+    }
+
+    /// Write a value, waiting until there is capacity.
+    ///
+    /// Writeing completes when the value has been pushed to the pipe's queue.
+    /// This doesn't mean the value has been read yet.
+    pub fn write<'a>(&'a self, buf: &'a [u8]) -> WriteFuture<'a, M, N> {
+        WriteFuture { pipe: self, buf }
+    }
+
+    /// Attempt to immediately write a message.
+    ///
+    /// This method differs from [`write`](Pipe::write) by returning immediately if the pipe's
+    /// buffer is full, instead of waiting.
+    ///
+    /// # Errors
+    ///
+    /// If the pipe capacity has been reached, i.e., the pipe has `n`
+    /// buffered values where `n` is the argument passed to [`Pipe`], then an
+    /// error is returned.
+    pub fn try_write(&self, buf: &[u8]) -> Result<usize, TryWriteError> {
+        self.lock(|c| c.try_write(buf))
+    }
+
+    /// Receive the next value.
+    ///
+    /// If there are no messages in the pipe's buffer, this method will
+    /// wait until a message is written.
+    pub fn read<'a>(&'a self, buf: &'a mut [u8]) -> ReadFuture<'a, M, N> {
+        ReadFuture { pipe: self, buf }
+    }
+
+    /// Attempt to immediately read a message.
+    ///
+    /// This method will either read a message from the pipe immediately or return an error
+    /// if the pipe is empty.
+    pub fn try_read(&self, buf: &mut [u8]) -> Result<usize, TryReadError> {
+        self.lock(|c| c.try_read(buf))
+    }
+
+    /// Clear the data in the pipe's buffer.
+    pub fn clear(&self) {
+        self.lock(|c| c.clear())
+    }
+
+    /// Return whether the pipe is full (no free space in the buffer)
+    pub fn is_full(&self) -> bool {
+        self.len() == N
+    }
+
+    /// Return whether the pipe is empty (no data buffered)
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// Total byte capacity.
+    ///
+    /// This is the same as the `N` generic param.
+    pub fn capacity(&self) -> usize {
+        N
+    }
+
+    /// Used byte capacity.
+    pub fn len(&self) -> usize {
+        self.lock(|c| c.buffer.len())
+    }
+
+    /// Free byte capacity.
+    ///
+    /// This is equivalent to `capacity() - len()`
+    pub fn free_capacity(&self) -> usize {
+        N - self.len()
+    }
+}
+
+#[cfg(feature = "nightly")]
+mod io_impls {
+    use core::convert::Infallible;
+
+    use futures_util::FutureExt;
+
+    use super::*;
+
+    impl<M: RawMutex, const N: usize> embedded_io::Io for Pipe<M, N> {
+        type Error = Infallible;
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::asynch::Read for Pipe<M, N> {
+        type ReadFuture<'a> = impl Future<Output = Result<usize, Self::Error>>
+        where
+            Self: 'a;
+
+        fn read<'a>(&'a mut self, buf: &'a mut [u8]) -> Self::ReadFuture<'a> {
+            Pipe::read(self, buf).map(Ok)
+        }
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::asynch::Write for Pipe<M, N> {
+        type WriteFuture<'a> = impl Future<Output = Result<usize, Self::Error>>
+        where
+            Self: 'a;
+
+        fn write<'a>(&'a mut self, buf: &'a [u8]) -> Self::WriteFuture<'a> {
+            Pipe::write(self, buf).map(Ok)
+        }
+
+        type FlushFuture<'a> = impl Future<Output = Result<(), Self::Error>>
+        where
+            Self: 'a;
+
+        fn flush<'a>(&'a mut self) -> Self::FlushFuture<'a> {
+            futures_util::future::ready(Ok(()))
+        }
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::Io for &Pipe<M, N> {
+        type Error = Infallible;
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::asynch::Read for &Pipe<M, N> {
+        type ReadFuture<'a> = impl Future<Output = Result<usize, Self::Error>>
+        where
+            Self: 'a;
+
+        fn read<'a>(&'a mut self, buf: &'a mut [u8]) -> Self::ReadFuture<'a> {
+            Pipe::read(self, buf).map(Ok)
+        }
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::asynch::Write for &Pipe<M, N> {
+        type WriteFuture<'a> = impl Future<Output = Result<usize, Self::Error>>
+        where
+            Self: 'a;
+
+        fn write<'a>(&'a mut self, buf: &'a [u8]) -> Self::WriteFuture<'a> {
+            Pipe::write(self, buf).map(Ok)
+        }
+
+        type FlushFuture<'a> = impl Future<Output = Result<(), Self::Error>>
+        where
+            Self: 'a;
+
+        fn flush<'a>(&'a mut self) -> Self::FlushFuture<'a> {
+            futures_util::future::ready(Ok(()))
+        }
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::Io for Reader<'_, M, N> {
+        type Error = Infallible;
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::asynch::Read for Reader<'_, M, N> {
+        type ReadFuture<'a> = impl Future<Output = Result<usize, Self::Error>>
+        where
+            Self: 'a;
+
+        fn read<'a>(&'a mut self, buf: &'a mut [u8]) -> Self::ReadFuture<'a> {
+            Reader::read(self, buf).map(Ok)
+        }
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::Io for Writer<'_, M, N> {
+        type Error = Infallible;
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::asynch::Write for Writer<'_, M, N> {
+        type WriteFuture<'a> = impl Future<Output = Result<usize, Self::Error>>
+        where
+            Self: 'a;
+
+        fn write<'a>(&'a mut self, buf: &'a [u8]) -> Self::WriteFuture<'a> {
+            Writer::write(self, buf).map(Ok)
+        }
+
+        type FlushFuture<'a> = impl Future<Output = Result<(), Self::Error>>
+        where
+            Self: 'a;
+
+        fn flush<'a>(&'a mut self) -> Self::FlushFuture<'a> {
+            futures_util::future::ready(Ok(()))
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use futures_executor::ThreadPool;
+    use futures_util::task::SpawnExt;
+    use static_cell::StaticCell;
+
+    use super::*;
+    use crate::blocking_mutex::raw::{CriticalSectionRawMutex, NoopRawMutex};
+
+    fn capacity<const N: usize>(c: &PipeState<N>) -> usize {
+        N - c.buffer.len()
+    }
+
+    #[test]
+    fn writing_once() {
+        let mut c = PipeState::<3>::new();
+        assert!(c.try_write(&[1]).is_ok());
+        assert_eq!(capacity(&c), 2);
+    }
+
+    #[test]
+    fn writing_when_full() {
+        let mut c = PipeState::<3>::new();
+        assert_eq!(c.try_write(&[42]), Ok(1));
+        assert_eq!(c.try_write(&[43]), Ok(1));
+        assert_eq!(c.try_write(&[44]), Ok(1));
+        assert_eq!(c.try_write(&[45]), Err(TryWriteError::Full));
+        assert_eq!(capacity(&c), 0);
+    }
+
+    #[test]
+    fn receiving_once_with_one_send() {
+        let mut c = PipeState::<3>::new();
+        assert!(c.try_write(&[42]).is_ok());
+        let mut buf = [0; 16];
+        assert_eq!(c.try_read(&mut buf), Ok(1));
+        assert_eq!(buf[0], 42);
+        assert_eq!(capacity(&c), 3);
+    }
+
+    #[test]
+    fn receiving_when_empty() {
+        let mut c = PipeState::<3>::new();
+        let mut buf = [0; 16];
+        assert_eq!(c.try_read(&mut buf), Err(TryReadError::Empty));
+        assert_eq!(capacity(&c), 3);
+    }
+
+    #[test]
+    fn simple_send_and_receive() {
+        let c = Pipe::<NoopRawMutex, 3>::new();
+        assert!(c.try_write(&[42]).is_ok());
+        let mut buf = [0; 16];
+        assert_eq!(c.try_read(&mut buf), Ok(1));
+        assert_eq!(buf[0], 42);
+    }
+
+    #[test]
+    fn cloning() {
+        let c = Pipe::<NoopRawMutex, 3>::new();
+        let r1 = c.reader();
+        let w1 = c.writer();
+
+        let _ = r1.clone();
+        let _ = w1.clone();
+    }
+
+    #[futures_test::test]
+    async fn receiver_receives_given_try_write_async() {
+        let executor = ThreadPool::new().unwrap();
+
+        static CHANNEL: StaticCell<Pipe<CriticalSectionRawMutex, 3>> = StaticCell::new();
+        let c = &*CHANNEL.init(Pipe::new());
+        let c2 = c;
+        let f = async move {
+            assert_eq!(c2.try_write(&[42]), Ok(1));
+        };
+        executor.spawn(f).unwrap();
+        let mut buf = [0; 16];
+        assert_eq!(c.read(&mut buf).await, 1);
+        assert_eq!(buf[0], 42);
+    }
+
+    #[futures_test::test]
+    async fn sender_send_completes_if_capacity() {
+        let c = Pipe::<CriticalSectionRawMutex, 1>::new();
+        c.write(&[42]).await;
+        let mut buf = [0; 16];
+        assert_eq!(c.read(&mut buf).await, 1);
+        assert_eq!(buf[0], 42);
+    }
+}

embassy-sync/src/ring_buffer.rs

@@ -0,0 +1,146 @@
+pub struct RingBuffer<const N: usize> {
+    buf: [u8; N],
+    start: usize,
+    end: usize,
+    empty: bool,
+}
+
+impl<const N: usize> RingBuffer<N> {
+    pub const fn new() -> Self {
+        Self {
+            buf: [0; N],
+            start: 0,
+            end: 0,
+            empty: true,
+        }
+    }
+
+    pub fn push_buf(&mut self) -> &mut [u8] {
+        if self.start == self.end && !self.empty {
+            trace!("  ringbuf: push_buf empty");
+            return &mut self.buf[..0];
+        }
+
+        let n = if self.start <= self.end {
+            self.buf.len() - self.end
+        } else {
+            self.start - self.end
+        };
+
+        trace!("  ringbuf: push_buf {:?}..{:?}", self.end, self.end + n);
+        &mut self.buf[self.end..self.end + n]
+    }
+
+    pub fn push(&mut self, n: usize) {
+        trace!("  ringbuf: push {:?}", n);
+        if n == 0 {
+            return;
+        }
+
+        self.end = self.wrap(self.end + n);
+        self.empty = false;
+    }
+
+    pub fn pop_buf(&mut self) -> &mut [u8] {
+        if self.empty {
+            trace!("  ringbuf: pop_buf empty");
+            return &mut self.buf[..0];
+        }
+
+        let n = if self.end <= self.start {
+            self.buf.len() - self.start
+        } else {
+            self.end - self.start
+        };
+
+        trace!("  ringbuf: pop_buf {:?}..{:?}", self.start, self.start + n);
+        &mut self.buf[self.start..self.start + n]
+    }
+
+    pub fn pop(&mut self, n: usize) {
+        trace!("  ringbuf: pop {:?}", n);
+        if n == 0 {
+            return;
+        }
+
+        self.start = self.wrap(self.start + n);
+        self.empty = self.start == self.end;
+    }
+
+    pub fn is_full(&self) -> bool {
+        self.start == self.end && !self.empty
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.empty
+    }
+
+    #[allow(unused)]
+    pub fn len(&self) -> usize {
+        if self.empty {
+            0
+        } else if self.start < self.end {
+            self.end - self.start
+        } else {
+            N + self.end - self.start
+        }
+    }
+
+    pub fn clear(&mut self) {
+        self.start = 0;
+        self.end = 0;
+        self.empty = true;
+    }
+
+    fn wrap(&self, n: usize) -> usize {
+        assert!(n <= self.buf.len());
+        if n == self.buf.len() {
+            0
+        } else {
+            n
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn push_pop() {
+        let mut rb: RingBuffer<4> = RingBuffer::new();
+        let buf = rb.push_buf();
+        assert_eq!(4, buf.len());
+        buf[0] = 1;
+        buf[1] = 2;
+        buf[2] = 3;
+        buf[3] = 4;
+        rb.push(4);
+
+        let buf = rb.pop_buf();
+        assert_eq!(4, buf.len());
+        assert_eq!(1, buf[0]);
+        rb.pop(1);
+
+        let buf = rb.pop_buf();
+        assert_eq!(3, buf.len());
+        assert_eq!(2, buf[0]);
+        rb.pop(1);
+
+        let buf = rb.pop_buf();
+        assert_eq!(2, buf.len());
+        assert_eq!(3, buf[0]);
+        rb.pop(1);
+
+        let buf = rb.pop_buf();
+        assert_eq!(1, buf.len());
+        assert_eq!(4, buf[0]);
+        rb.pop(1);
+
+        let buf = rb.pop_buf();
+        assert_eq!(0, buf.len());
+
+        let buf = rb.push_buf();
+        assert_eq!(4, buf.len());
+    }
+}

embassy-sync/src/waitqueue/mod.rs

@@ -0,0 +1,7 @@
+//! Async low-level wait queues
+
+mod waker;
+pub use waker::*;
+
+mod multi_waker;
+pub use multi_waker::*;

embassy-sync/src/waitqueue/multi_waker.rs

@@ -0,0 +1,33 @@
+use core::task::Waker;
+
+use super::WakerRegistration;
+
+/// Utility struct to register and wake multiple wakers.
+pub struct MultiWakerRegistration<const N: usize> {
+    wakers: [WakerRegistration; N],
+}
+
+impl<const N: usize> MultiWakerRegistration<N> {
+    /// Create a new empty instance
+    pub const fn new() -> Self {
+        const WAKER: WakerRegistration = WakerRegistration::new();
+        Self { wakers: [WAKER; N] }
+    }
+
+    /// Register a waker. If the buffer is full the function returns it in the error
+    pub fn register<'a>(&mut self, w: &'a Waker) -> Result<(), &'a Waker> {
+        if let Some(waker_slot) = self.wakers.iter_mut().find(|waker_slot| !waker_slot.occupied()) {
+            waker_slot.register(w);
+            Ok(())
+        } else {
+            Err(w)
+        }
+    }
+
+    /// Wake all registered wakers. This clears the buffer
+    pub fn wake(&mut self) {
+        for waker_slot in self.wakers.iter_mut() {
+            waker_slot.wake()
+        }
+    }
+}

embassy-sync/src/waitqueue/waker.rs

@@ -0,0 +1,92 @@
+use core::cell::Cell;
+use core::mem;
+use core::task::Waker;
+
+use crate::blocking_mutex::raw::CriticalSectionRawMutex;
+use crate::blocking_mutex::Mutex;
+
+/// Utility struct to register and wake a waker.
+#[derive(Debug)]
+pub struct WakerRegistration {
+    waker: Option<Waker>,
+}
+
+impl WakerRegistration {
+    /// Create a new `WakerRegistration`.
+    pub const fn new() -> Self {
+        Self { waker: None }
+    }
+
+    /// Register a waker. Overwrites the previous waker, if any.
+    pub fn register(&mut self, w: &Waker) {
+        match self.waker {
+            // Optimization: If both the old and new Wakers wake the same task, we can simply
+            // keep the old waker, skipping the clone. (In most executor implementations,
+            // cloning a waker is somewhat expensive, comparable to cloning an Arc).
+            Some(ref w2) if (w2.will_wake(w)) => {}
+            _ => {
+                // clone the new waker and store it
+                if let Some(old_waker) = mem::replace(&mut self.waker, Some(w.clone())) {
+                    // We had a waker registered for another task. Wake it, so the other task can
+                    // reregister itself if it's still interested.
+                    //
+                    // If two tasks are waiting on the same thing concurrently, this will cause them
+                    // to wake each other in a loop fighting over this WakerRegistration. This wastes
+                    // CPU but things will still work.
+                    //
+                    // If the user wants to have two tasks waiting on the same thing they should use
+                    // a more appropriate primitive that can store multiple wakers.
+                    old_waker.wake()
+                }
+            }
+        }
+    }
+
+    /// Wake the registered waker, if any.
+    pub fn wake(&mut self) {
+        if let Some(w) = self.waker.take() {
+            w.wake()
+        }
+    }
+
+    /// Returns true if a waker is currently registered
+    pub fn occupied(&self) -> bool {
+        self.waker.is_some()
+    }
+}
+
+/// Utility struct to register and wake a waker.
+pub struct AtomicWaker {
+    waker: Mutex<CriticalSectionRawMutex, Cell<Option<Waker>>>,
+}
+
+impl AtomicWaker {
+    /// Create a new `AtomicWaker`.
+    pub const fn new() -> Self {
+        Self {
+            waker: Mutex::const_new(CriticalSectionRawMutex::new(), Cell::new(None)),
+        }
+    }
+
+    /// Register a waker. Overwrites the previous waker, if any.
+    pub fn register(&self, w: &Waker) {
+        critical_section::with(|cs| {
+            let cell = self.waker.borrow(cs);
+            cell.set(match cell.replace(None) {
+                Some(w2) if (w2.will_wake(w)) => Some(w2),
+                _ => Some(w.clone()),
+            })
+        })
+    }
+
+    /// Wake the registered waker, if any.
+    pub fn wake(&self) {
+        critical_section::with(|cs| {
+            let cell = self.waker.borrow(cs);
+            if let Some(w) = cell.replace(None) {
+                w.wake_by_ref();
+                cell.set(Some(w));
+            }
+        })
+    }
+}