integrate static-executor, cleanup time module.

18 files changed, 842 insertions, 387 deletions

diff --git a/Cargo.toml b/Cargo.toml
index abce83ec6..17fbf0a8d 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -14,8 +14,6 @@ exclude = [
 panic-probe = { git = "https://github.com/knurling-rs/probe-run", branch="main" }
 defmt-rtt = { git = "https://github.com/knurling-rs/defmt", branch="cursed-symbol-names-linkers-must-repent-for-their-sins" }
 defmt = { git = "https://github.com/knurling-rs/defmt", branch="cursed-symbol-names-linkers-must-repent-for-their-sins" }
-static-executor = { git = "https://github.com/Dirbaio/static-executor", branch="multi"}
-futures-intrusive = { git = "https://github.com/Dirbaio/futures-intrusive", branch="master"}
 
 [profile.dev]
 codegen-units = 1
diff --git a/embassy-macros/Cargo.toml b/embassy-macros/Cargo.toml
new file mode 100644
index 000000000..1f3e20aa8
--- /dev/null
+++ b/embassy-macros/Cargo.toml
@@ -0,0 +1,13 @@
+[package]
+name = "embassy-macros"
+version = "0.1.0"
+authors = ["Dario Nieuwenhuis <[email protected]>"]
+edition = "2018"
+
+[dependencies]
+syn = { version = "1.0.39", features = ["full", "extra-traits"] }
+quote = "1.0.7"
+darling = "0.10.2"
+
+[lib]
+proc-macro = true
diff --git a/embassy-macros/src/lib.rs b/embassy-macros/src/lib.rs
new file mode 100644
index 000000000..1745311ba
--- /dev/null
+++ b/embassy-macros/src/lib.rs
@@ -0,0 +1,114 @@
+#![feature(proc_macro_diagnostic)]
+
+extern crate proc_macro;
+
+use darling::FromMeta;
+use proc_macro::{Diagnostic, Level, Span, TokenStream};
+use quote::{format_ident, quote};
+use syn::spanned::Spanned;
+
+#[derive(Debug, FromMeta)]
+struct MacroArgs {
+    #[darling(default)]
+    pool_size: Option<usize>,
+}
+
+#[proc_macro_attribute]
+pub fn task(args: TokenStream, item: TokenStream) -> TokenStream {
+    let args = syn::parse_macro_input!(args as syn::AttributeArgs);
+    let mut task_fn = syn::parse_macro_input!(item as syn::ItemFn);
+
+    let args = match MacroArgs::from_list(&args) {
+        Ok(v) => v,
+        Err(e) => {
+            return TokenStream::from(e.write_errors());
+        }
+    };
+
+    let pool_size: usize = args.pool_size.unwrap_or(1);
+
+    let mut fail = false;
+    if task_fn.sig.asyncness.is_none() {
+        task_fn
+            .sig
+            .span()
+            .unwrap()
+            .error("task functions must be async")
+            .emit();
+        fail = true;
+    }
+    if task_fn.sig.generics.params.len() != 0 {
+        task_fn
+            .sig
+            .span()
+            .unwrap()
+            .error("task functions must not be generic")
+            .emit();
+        fail = true;
+    }
+    if pool_size < 1 {
+        Span::call_site()
+            .error("pool_size must be 1 or greater")
+            .emit();
+        fail = true
+    }
+
+    let mut arg_names: syn::punctuated::Punctuated<syn::Ident, syn::Token![,]> =
+        syn::punctuated::Punctuated::new();
+    let args = &task_fn.sig.inputs;
+
+    for arg in args.iter() {
+        match arg {
+            syn::FnArg::Receiver(_) => {
+                arg.span()
+                    .unwrap()
+                    .error("task functions must not have receiver arguments")
+                    .emit();
+                fail = true;
+            }
+            syn::FnArg::Typed(t) => match t.pat.as_ref() {
+                syn::Pat::Ident(i) => arg_names.push(i.ident.clone()),
+                _ => {
+                    arg.span()
+                        .unwrap()
+                        .error("pattern matching in task arguments is not yet supporteds")
+                        .emit();
+                    fail = true;
+                }
+            },
+        }
+    }
+
+    if fail {
+        return TokenStream::new();
+    }
+
+    let name = task_fn.sig.ident.clone();
+
+    let type_name = format_ident!("__embassy_executor_type_{}", name);
+    let pool_name = format_ident!("__embassy_executor_pool_{}", name);
+    let task_fn_name = format_ident!("__embassy_executor_task_{}", name);
+    let create_fn_name = format_ident!("__embassy_executor_create_{}", name);
+
+    let visibility = &task_fn.vis;
+
+    task_fn.sig.ident = task_fn_name.clone();
+
+    let result = quote! {
+        #task_fn
+        #[allow(non_camel_case_types)]
+        type #type_name = impl ::core::future::Future + 'static;
+
+        fn #create_fn_name(#args) -> #type_name {
+            #task_fn_name(#arg_names)
+        }
+
+        #[allow(non_upper_case_globals)]
+        static #pool_name: [::embassy::executor::Task<#type_name>; #pool_size] = [::embassy::executor::Task::new(); #pool_size];
+
+        #visibility fn #name(#args) -> ::embassy::executor::SpawnToken {
+            unsafe { ::embassy::executor::Task::spawn(&#pool_name, || #create_fn_name(#arg_names)) }
+        }
+    };
+    result.into()
+}
diff --git a/embassy/Cargo.toml b/embassy/Cargo.toml
index 7b8e6cfad..4abcda99e 100644
--- a/embassy/Cargo.toml
+++ b/embassy/Cargo.toml
@@ -13,4 +13,4 @@ cortex-m = "0.6.3"
 futures     = { version = "0.3.5", default-features = false, features = [ "async-await" ] }
 pin-project = { version = "0.4.23", default-features = false }
 futures-intrusive = { version = "0.3.1", default-features = false }
-static-executor  = { version = "0.1.0", features=["defmt"]}
+embassy-macros  = { version = "0.1.0", path = "../embassy-macros"}
diff --git a/embassy/src/executor.rs b/embassy/src/executor.rs
deleted file mode 100644
index d42a19b9a..000000000
--- a/embassy/src/executor.rs
+++ /dev/null
@@ -1,48 +0,0 @@
-use core::marker::PhantomData;
-use static_executor as se;
-
-use crate::time;
-use crate::time::Alarm;
-
-pub use se::{task, SpawnError, SpawnToken};
-
-pub struct Executor<A: Alarm> {
-    inner: se::Executor,
-    alarm: A,
-    timer: time::TimerService,
-}
-
-impl<A: Alarm> Executor<A> {
-    pub fn new(alarm: A, signal_fn: fn()) -> Self {
-        alarm.set_callback(signal_fn);
-        Self {
-            inner: se::Executor::new(signal_fn),
-            alarm,
-            timer: time::TimerService::new(time::IntrusiveClock),
-        }
-    }
-
-    /// Spawn a future on this executor.
-    ///
-    /// safety: can only be called from the executor thread
-    pub unsafe fn spawn(&'static self, token: SpawnToken) -> Result<(), SpawnError> {
-        self.inner.spawn(token)
-    }
-
-    /// Runs the executor until the queue is empty.
-    ///
-    /// safety: can only be called from the executor thread
-    pub unsafe fn run(&'static self) {
-        time::with_timer_service(&self.timer, || {
-            self.timer.check_expirations();
-            self.inner.run();
-
-            match self.timer.next_expiration() {
-                // If this is in the past, set_alarm will immediately trigger the alarm,
-                // which will make the wfe immediately return so we do another loop iteration.
-                Some(at) => self.alarm.set(at),
-                None => self.alarm.clear(),
-            }
-        })
-    }
-}
diff --git a/embassy/src/executor/executor.rs b/embassy/src/executor/executor.rs
new file mode 100644
index 000000000..7ef9230e4
--- /dev/null
+++ b/embassy/src/executor/executor.rs
@@ -0,0 +1,305 @@
+#![no_std]
+#![feature(const_fn)]
+
+use core::cell::Cell;
+use core::cell::UnsafeCell;
+use core::future::Future;
+use core::mem;
+use core::mem::MaybeUninit;
+use core::pin::Pin;
+use core::ptr;
+use core::ptr::NonNull;
+use core::sync::atomic::{AtomicPtr, AtomicU32, Ordering};
+use core::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
+
+//=============
+// UninitCell
+
+struct UninitCell<T>(MaybeUninit<UnsafeCell<T>>);
+impl<T> UninitCell<T> {
+    const fn uninit() -> Self {
+        Self(MaybeUninit::uninit())
+    }
+
+    unsafe fn as_mut_ptr(&self) -> *mut T {
+        (*self.0.as_ptr()).get()
+    }
+
+    unsafe fn as_mut(&self) -> &mut T {
+        &mut *self.as_mut_ptr()
+    }
+
+    unsafe fn write(&self, val: T) {
+        ptr::write(self.as_mut_ptr(), val)
+    }
+
+    unsafe fn drop_in_place(&self) {
+        ptr::drop_in_place(self.as_mut_ptr())
+    }
+}
+
+impl<T: Copy> UninitCell<T> {
+    unsafe fn read(&self) -> T {
+        ptr::read(self.as_mut_ptr())
+    }
+}
+
+//=============
+// Data structures
+
+const STATE_RUNNING: u32 = 1 << 0;
+const STATE_QUEUED: u32 = 1 << 1;
+
+struct Header {
+    state: AtomicU32,
+    next: AtomicPtr<Header>,
+    executor: Cell<*const Executor>,
+    poll_fn: UninitCell<unsafe fn(*mut Header)>, // Valid if STATE_RUNNING
+}
+
+// repr(C) is needed to guarantee that header is located at offset 0
+// This makes it safe to cast between Header and Task pointers.
+#[repr(C)]
+pub struct Task<F: Future + 'static> {
+    header: Header,
+    future: UninitCell<F>, // Valid if STATE_RUNNING
+}
+
+#[derive(Copy, Clone, Debug)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub enum SpawnError {
+    Busy,
+}
+
+//=============
+// Atomic task queue using a very, very simple lock-free linked-list queue:
+//
+// To enqueue a task, task.next is set to the old head, and head is atomically set to task.
+//
+// Dequeuing is done in batches: the queue is emptied by atomically replacing head with
+// null. Then the batch is iterated following the next pointers until null is reached.
+//
+// Note that batches will be iterated in the opposite order as they were enqueued. This should
+// be OK for our use case. Hopefully it doesn't create executor fairness problems.
+
+struct Queue {
+    head: AtomicPtr<Header>,
+}
+
+impl Queue {
+    const fn new() -> Self {
+        Self {
+            head: AtomicPtr::new(ptr::null_mut()),
+        }
+    }
+
+    /// Enqueues an item. Returns true if the queue was empty.
+    unsafe fn enqueue(&self, item: *mut Header) -> bool {
+        let mut prev = self.head.load(Ordering::Acquire);
+        loop {
+            (*item).next.store(prev, Ordering::Relaxed);
+            match self
+                .head
+                .compare_exchange_weak(prev, item, Ordering::AcqRel, Ordering::Acquire)
+            {
+                Ok(_) => break,
+                Err(next_prev) => prev = next_prev,
+            }
+        }
+
+        prev.is_null()
+    }
+
+    unsafe fn dequeue_all(&self, on_task: impl Fn(*mut Header)) {
+        loop {
+            let mut task = self.head.swap(ptr::null_mut(), Ordering::AcqRel);
+
+            if task.is_null() {
+                // Queue is empty, we're done
+                return;
+            }
+
+            while !task.is_null() {
+                on_task(task);
+                task = (*task).next.load(Ordering::Relaxed);
+            }
+        }
+    }
+}
+
+//=============
+// Waker
+
+static WAKER_VTABLE: RawWakerVTable =
+    RawWakerVTable::new(waker_clone, waker_wake, waker_wake, waker_drop);
+
+unsafe fn waker_clone(p: *const ()) -> RawWaker {
+    RawWaker::new(p, &WAKER_VTABLE)
+}
+
+unsafe fn waker_wake(p: *const ()) {
+    let header = &*(p as *const Header);
+
+    let mut current = header.state.load(Ordering::Acquire);
+    loop {
+        // If already scheduled, or if not started,
+        if (current & STATE_QUEUED != 0) || (current & STATE_RUNNING == 0) {
+            return;
+        }
+
+        // Mark it as scheduled
+        let new = current | STATE_QUEUED;
+
+        match header
+            .state
+            .compare_exchange_weak(current, new, Ordering::AcqRel, Ordering::Acquire)
+        {
+            Ok(_) => break,
+            Err(next_current) => current = next_current,
+        }
+    }
+
+    // We have just marked the task as scheduled, so enqueue it.
+    let executor = &*header.executor.get();
+    executor.enqueue(p as *mut Header);
+}
+
+unsafe fn waker_drop(_: *const ()) {
+    // nop
+}
+
+//=============
+// Task
+
+impl<F: Future + 'static> Task<F> {
+    pub const fn new() -> Self {
+        Self {
+            header: Header {
+                state: AtomicU32::new(0),
+                next: AtomicPtr::new(ptr::null_mut()),
+                executor: Cell::new(ptr::null()),
+                poll_fn: UninitCell::uninit(),
+            },
+            future: UninitCell::uninit(),
+        }
+    }
+
+    pub unsafe fn spawn(pool: &'static [Self], future: impl FnOnce() -> F) -> SpawnToken {
+        for task in pool {
+            let state = STATE_RUNNING | STATE_QUEUED;
+            if task
+                .header
+                .state
+                .compare_and_swap(0, state, Ordering::AcqRel)
+                == 0
+            {
+                // Initialize the task
+                task.header.poll_fn.write(Self::poll);
+                task.future.write(future());
+
+                return SpawnToken {
+                    header: Some(NonNull::new_unchecked(&task.header as *const Header as _)),
+                };
+            }
+        }
+
+        return SpawnToken { header: None };
+    }
+
+    unsafe fn poll(p: *mut Header) {
+        let this = &*(p as *const Task<F>);
+
+        let future = Pin::new_unchecked(this.future.as_mut());
+        let waker = Waker::from_raw(RawWaker::new(p as _, &WAKER_VTABLE));
+        let mut cx = Context::from_waker(&waker);
+        match future.poll(&mut cx) {
+            Poll::Ready(_) => {
+                this.future.drop_in_place();
+                this.header
+                    .state
+                    .fetch_and(!STATE_RUNNING, Ordering::AcqRel);
+            }
+            Poll::Pending => {}
+        }
+    }
+}
+
+unsafe impl<F: Future + 'static> Sync for Task<F> {}
+
+//=============
+// Spawn token
+
+#[must_use = "Calling a task function does nothing on its own. To spawn a task, pass the result to Executor::spawn()"]
+pub struct SpawnToken {
+    header: Option<NonNull<Header>>,
+}
+
+impl Drop for SpawnToken {
+    fn drop(&mut self) {
+        // TODO maybe we can deallocate the task instead.
+        panic!("Please do not drop SpawnToken instances")
+    }
+}
+
+//=============
+// Executor
+
+pub struct Executor {
+    queue: Queue,
+    signal_fn: fn(),
+}
+
+impl Executor {
+    pub const fn new(signal_fn: fn()) -> Self {
+        Self {
+            queue: Queue::new(),
+            signal_fn: signal_fn,
+        }
+    }
+
+    unsafe fn enqueue(&self, item: *mut Header) {
+        if self.queue.enqueue(item) {
+            (self.signal_fn)()
+        }
+    }
+
+    /// Spawn a future on this executor.
+    ///
+    /// safety: can only be called from the executor thread
+    pub unsafe fn spawn(&'static self, token: SpawnToken) -> Result<(), SpawnError> {
+        let header = token.header;
+        mem::forget(token);
+
+        match header {
+            Some(header) => {
+                let header = header.as_ref();
+                header.executor.set(self);
+                self.enqueue(header as *const _ as _);
+                Ok(())
+            }
+            None => Err(SpawnError::Busy),
+        }
+    }
+
+    /// Runs the executor until the queue is empty.
+    ///
+    /// safety: can only be called from the executor thread
+    pub unsafe fn run(&self) {
+        self.queue.dequeue_all(|p| {
+            let header = &*p;
+
+            let state = header.state.fetch_and(!STATE_QUEUED, Ordering::AcqRel);
+            if state & STATE_RUNNING == 0 {
+                // If task is not running, ignore it. This can happen in the following scenario:
+                //   - Task gets dequeued, poll starts
+                //   - While task is being polled, it gets woken. It gets placed in the queue.
+                //   - Task poll finishes, returning done=true
+                //   - RUNNING bit is cleared, but the task is already in the queue.
+                return;
+            }
+
+            // Run the task
+            header.poll_fn.read()(p as _);
+        });
+    }
+}
diff --git a/embassy/src/executor/mod.rs b/embassy/src/executor/mod.rs
new file mode 100644
index 000000000..1a68bdfde
--- /dev/null
+++ b/embassy/src/executor/mod.rs
@@ -0,0 +1,9 @@
+mod executor;
+mod timer_executor;
+
+// for time::Timer
+pub(crate) use timer_executor::current_timer_queue;
+
+pub use embassy_macros::task;
+pub use executor::{Executor, SpawnError, SpawnToken, Task};
+pub use timer_executor::TimerExecutor;
diff --git a/embassy/src/executor/timer_executor.rs b/embassy/src/executor/timer_executor.rs
new file mode 100644
index 000000000..21a81383a
--- /dev/null
+++ b/embassy/src/executor/timer_executor.rs
@@ -0,0 +1,77 @@
+use super::executor::{Executor, SpawnError, SpawnToken};
+use core::ptr;
+use core::sync::atomic::{AtomicPtr, Ordering};
+use futures_intrusive::timer as fi;
+
+use crate::time::Alarm;
+
+pub(crate) struct IntrusiveClock;
+
+impl fi::Clock for IntrusiveClock {
+    fn now(&self) -> u64 {
+        crate::time::now()
+    }
+}
+
+pub(crate) type TimerQueue = fi::LocalTimerService;
+
+pub struct TimerExecutor<A: Alarm> {
+    inner: Executor,
+    alarm: A,
+    timer_queue: TimerQueue,
+}
+
+impl<A: Alarm> TimerExecutor<A> {
+    pub fn new(alarm: A, signal_fn: fn()) -> Self {
+        alarm.set_callback(signal_fn);
+        Self {
+            inner: Executor::new(signal_fn),
+            alarm,
+            timer_queue: TimerQueue::new(&IntrusiveClock),
+        }
+    }
+
+    /// Spawn a future on this executor.
+    ///
+    /// safety: can only be called from the executor thread
+    pub unsafe fn spawn(&'static self, token: SpawnToken) -> Result<(), SpawnError> {
+        self.inner.spawn(token)
+    }
+
+    /// Runs the executor until the queue is empty.
+    ///
+    /// safety: can only be called from the executor thread
+    pub unsafe fn run(&'static self) {
+        with_timer_queue(&self.timer_queue, || {
+            self.timer_queue.check_expirations();
+            self.inner.run();
+
+            match self.timer_queue.next_expiration() {
+                // If this is in the past, set_alarm will immediately trigger the alarm,
+                // which will make the wfe immediately return so we do another loop iteration.
+                Some(at) => self.alarm.set(at),
+                None => self.alarm.clear(),
+            }
+        })
+    }
+}
+
+static CURRENT_TIMER_QUEUE: AtomicPtr<TimerQueue> = AtomicPtr::new(ptr::null_mut());
+
+fn with_timer_queue<R>(svc: &'static TimerQueue, f: impl FnOnce() -> R) -> R {
+    let svc = svc as *const _ as *mut _;
+    let prev_svc = CURRENT_TIMER_QUEUE.swap(svc, Ordering::Relaxed);
+    let r = f();
+    let svc2 = CURRENT_TIMER_QUEUE.swap(prev_svc, Ordering::Relaxed);
+    assert_eq!(svc, svc2);
+    r
+}
+
+pub(crate) fn current_timer_queue() -> &'static TimerQueue {
+    unsafe {
+        CURRENT_TIMER_QUEUE
+            .load(Ordering::Relaxed)
+            .as_ref()
+            .unwrap()
+    }
+}
diff --git a/embassy/src/time.rs b/embassy/src/time.rs
deleted file mode 100644
index 532a53a38..000000000
--- a/embassy/src/time.rs
+++ /dev/null
@@ -1,331 +0,0 @@
-use core::cell::Cell;
-use core::convert::TryInto;
-use core::future::Future;
-use core::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Sub, SubAssign};
-use core::pin::Pin;
-use core::ptr;
-use core::sync::atomic::{AtomicPtr, Ordering};
-use core::task::{Context, Poll};
-
-use crate::util::*;
-use fi::LocalTimer;
-use futures_intrusive::timer as fi;
-static mut CLOCK: Option<&'static dyn Clock> = None;
-
-pub unsafe fn set_clock(clock: &'static dyn Clock) {
-    CLOCK = Some(clock);
-}
-
-fn now() -> u64 {
-    unsafe { CLOCK.dexpect(defmt::intern!("No clock set")).now() }
-}
-
-#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
-pub struct Instant {
-    ticks: u64,
-}
-
-// TODO allow customizing, probably via Cargo features `tick-hz-32768` or something.
-pub const TICKS_PER_SECOND: u32 = 32768;
-
-impl Instant {
-    pub fn now() -> Instant {
-        Instant { ticks: now() }
-    }
-
-    pub fn into_ticks(&self) -> u64 {
-        self.ticks
-    }
-
-    pub fn duration_since(&self, earlier: Instant) -> Duration {
-        Duration {
-            ticks: (self.ticks - earlier.ticks).try_into().unwrap(),
-        }
-    }
-
-    pub fn checked_duration_since(&self, earlier: Instant) -> Option<Duration> {
-        if self.ticks < earlier.ticks {
-            None
-        } else {
-            Some(Duration {
-                ticks: (self.ticks - earlier.ticks).try_into().unwrap(),
-            })
-        }
-    }
-
-    pub fn saturating_duration_since(&self, earlier: Instant) -> Duration {
-        Duration {
-            ticks: if self.ticks < earlier.ticks {
-                0
-            } else {
-                (self.ticks - earlier.ticks).try_into().unwrap()
-            },
-        }
-    }
-
-    pub fn elapsed(&self) -> Duration {
-        Instant::now() - *self
-    }
-
-    pub fn checked_add(&self, duration: Duration) -> Option<Instant> {
-        self.ticks
-            .checked_add(duration.ticks.into())
-            .map(|ticks| Instant { ticks })
-    }
-    pub fn checked_sub(&self, duration: Duration) -> Option<Instant> {
-        self.ticks
-            .checked_sub(duration.ticks.into())
-            .map(|ticks| Instant { ticks })
-    }
-}
-
-impl Add<Duration> for Instant {
-    type Output = Instant;
-
-    fn add(self, other: Duration) -> Instant {
-        self.checked_add(other)
-            .expect("overflow when adding duration to instant")
-    }
-}
-
-impl AddAssign<Duration> for Instant {
-    fn add_assign(&mut self, other: Duration) {
-        *self = *self + other;
-    }
-}
-
-impl Sub<Duration> for Instant {
-    type Output = Instant;
-
-    fn sub(self, other: Duration) -> Instant {
-        self.checked_sub(other)
-            .expect("overflow when subtracting duration from instant")
-    }
-}
-
-impl SubAssign<Duration> for Instant {
-    fn sub_assign(&mut self, other: Duration) {
-        *self = *self - other;
-    }
-}
-
-impl Sub<Instant> for Instant {
-    type Output = Duration;
-
-    fn sub(self, other: Instant) -> Duration {
-        self.duration_since(other)
-    }
-}
-
-#[derive(defmt::Format, Debug, Default, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
-pub struct Duration {
-    ticks: u32,
-}
-
-impl Duration {
-    pub fn into_ticks(&self) -> u32 {
-        self.ticks
-    }
-
-    pub const fn from_ticks(ticks: u32) -> Duration {
-        Duration { ticks }
-    }
-
-    pub const fn from_secs(secs: u32) -> Duration {
-        Duration {
-            ticks: secs * TICKS_PER_SECOND,
-        }
-    }
-
-    pub const fn from_millis(millis: u32) -> Duration {
-        Duration {
-            ticks: millis * TICKS_PER_SECOND / 1000,
-        }
-    }
-
-    pub fn checked_add(self, rhs: Duration) -> Option<Duration> {
-        self.ticks
-            .checked_add(rhs.ticks)
-            .map(|ticks| Duration { ticks })
-    }
-
-    pub fn checked_sub(self, rhs: Duration) -> Option<Duration> {
-        self.ticks
-            .checked_sub(rhs.ticks)
-            .map(|ticks| Duration { ticks })
-    }
-
-    pub fn checked_mul(self, rhs: u32) -> Option<Duration> {
-        self.ticks.checked_mul(rhs).map(|ticks| Duration { ticks })
-    }
-
-    pub fn checked_div(self, rhs: u32) -> Option<Duration> {
-        self.ticks.checked_div(rhs).map(|ticks| Duration { ticks })
-    }
-}
-
-impl Add for Duration {
-    type Output = Duration;
-
-    fn add(self, rhs: Duration) -> Duration {
-        self.checked_add(rhs)
-            .expect("overflow when adding durations")
-    }
-}
-
-impl AddAssign for Duration {
-    fn add_assign(&mut self, rhs: Duration) {
-        *self = *self + rhs;
-    }
-}
-
-impl Sub for Duration {
-    type Output = Duration;
-
-    fn sub(self, rhs: Duration) -> Duration {
-        self.checked_sub(rhs)
-            .expect("overflow when subtracting durations")
-    }
-}
-
-impl SubAssign for Duration {
-    fn sub_assign(&mut self, rhs: Duration) {
-        *self = *self - rhs;
-    }
-}
-
-impl Mul<u32> for Duration {
-    type Output = Duration;
-
-    fn mul(self, rhs: u32) -> Duration {
-        self.checked_mul(rhs)
-            .expect("overflow when multiplying duration by scalar")
-    }
-}
-
-impl Mul<Duration> for u32 {
-    type Output = Duration;
-
-    fn mul(self, rhs: Duration) -> Duration {
-        rhs * self
-    }
-}
-
-impl MulAssign<u32> for Duration {
-    fn mul_assign(&mut self, rhs: u32) {
-        *self = *self * rhs;
-    }
-}
-
-impl Div<u32> for Duration {
-    type Output = Duration;
-
-    fn div(self, rhs: u32) -> Duration {
-        self.checked_div(rhs)
-            .expect("divide by zero error when dividing duration by scalar")
-    }
-}
-
-impl DivAssign<u32> for Duration {
-    fn div_assign(&mut self, rhs: u32) {
-        *self = *self / rhs;
-    }
-}
-
-pub(crate) struct IntrusiveClock;
-
-impl fi::Clock for IntrusiveClock {
-    fn now(&self) -> u64 {
-        now()
-    }
-}
-
-pub(crate) type TimerService = fi::LocalTimerService<IntrusiveClock>;
-
-static CURRENT_TIMER_SERVICE: AtomicPtr<TimerService> = AtomicPtr::new(ptr::null_mut());
-
-pub(crate) fn with_timer_service<R>(svc: &'static TimerService, f: impl FnOnce() -> R) -> R {
-    let svc = svc as *const _ as *mut _;
-    let prev_svc = CURRENT_TIMER_SERVICE.swap(svc, Ordering::Relaxed);
-    let r = f();
-    let svc2 = CURRENT_TIMER_SERVICE.swap(prev_svc, Ordering::Relaxed);
-    assert_eq!(svc, svc2);
-    r
-}
-
-fn current_timer_service() -> &'static TimerService {
-    unsafe {
-        CURRENT_TIMER_SERVICE
-            .load(Ordering::Relaxed)
-            .as_ref()
-            .unwrap()
-    }
-}
-
-pub struct Timer {
-    inner: fi::LocalTimerFuture<'static>,
-}
-
-impl Timer {
-    pub fn at(when: Instant) -> Self {
-        let svc: &TimerService = current_timer_service();
-        Self {
-            inner: svc.deadline(when.into_ticks()),
-        }
-    }
-
-    pub fn after(dur: Duration) -> Self {
-        Self::at(Instant::now() + dur)
-    }
-}
-
-impl Future for Timer {
-    type Output = ();
-    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        unsafe { Pin::new_unchecked(&mut self.get_unchecked_mut().inner) }.poll(cx)
-    }
-}
-/// Monotonic clock
-pub trait Clock {
-    /// Return the current timestamp in ticks.
-    /// This is guaranteed to be monotonic, i.e. a call to now() will always return
-    /// a greater or equal value than earler calls.
-    fn now(&self) -> u64;
-}
-
-impl<T: Clock + ?Sized> Clock for &T {
-    fn now(&self) -> u64 {
-        T::now(self)
-    }
-}
-
-/// Trait to register a callback at a given timestamp.
-pub trait Alarm {
-    /// Sets the callback function to be called when the alarm triggers.
-    /// The callback may be called from any context (interrupt or thread mode).
-    fn set_callback(&self, callback: fn());
-
-    /// Sets an alarm at the given timestamp. When the clock reaches that
-    /// timestamp, the provided callback funcion will be called.
-    ///
-    /// When callback is called, it is guaranteed that now() will return a value greater or equal than timestamp.
-    ///
-    /// Only one alarm can be active at a time. This overwrites any previously-set alarm if any.
-    fn set(&self, timestamp: u64);
-
-    /// Clears the previously-set alarm.
-    /// If no alarm was set, this is a noop.
-    fn clear(&self);
-}
-
-impl<T: Alarm + ?Sized> Alarm for &T {
-    fn set_callback(&self, callback: fn()) {
-        T::set_callback(self, callback);
-    }
-    fn set(&self, timestamp: u64) {
-        T::set(self, timestamp);
-    }
-    fn clear(&self) {
-        T::clear(self)
-    }
-}
diff --git a/embassy/src/time/duration.rs b/embassy/src/time/duration.rs
new file mode 100644
index 000000000..1d5ad7542
--- /dev/null
+++ b/embassy/src/time/duration.rs
@@ -0,0 +1,118 @@
+use core::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Sub, SubAssign};
+
+use super::TICKS_PER_SECOND;
+
+#[derive(defmt::Format, Debug, Default, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Duration {
+    pub(crate) ticks: u32,
+}
+
+impl Duration {
+    pub fn into_ticks(&self) -> u32 {
+        self.ticks
+    }
+
+    pub const fn from_ticks(ticks: u32) -> Duration {
+        Duration { ticks }
+    }
+
+    pub const fn from_secs(secs: u32) -> Duration {
+        Duration {
+            ticks: secs * TICKS_PER_SECOND,
+        }
+    }
+
+    pub const fn from_millis(millis: u32) -> Duration {
+        Duration {
+            ticks: millis * TICKS_PER_SECOND / 1000,
+        }
+    }
+
+    pub fn checked_add(self, rhs: Duration) -> Option<Duration> {
+        self.ticks
+            .checked_add(rhs.ticks)
+            .map(|ticks| Duration { ticks })
+    }
+
+    pub fn checked_sub(self, rhs: Duration) -> Option<Duration> {
+        self.ticks
+            .checked_sub(rhs.ticks)
+            .map(|ticks| Duration { ticks })
+    }
+
+    pub fn checked_mul(self, rhs: u32) -> Option<Duration> {
+        self.ticks.checked_mul(rhs).map(|ticks| Duration { ticks })
+    }
+
+    pub fn checked_div(self, rhs: u32) -> Option<Duration> {
+        self.ticks.checked_div(rhs).map(|ticks| Duration { ticks })
+    }
+}
+
+impl Add for Duration {
+    type Output = Duration;
+
+    fn add(self, rhs: Duration) -> Duration {
+        self.checked_add(rhs)
+            .expect("overflow when adding durations")
+    }
+}
+
+impl AddAssign for Duration {
+    fn add_assign(&mut self, rhs: Duration) {
+        *self = *self + rhs;
+    }
+}
+
+impl Sub for Duration {
+    type Output = Duration;
+
+    fn sub(self, rhs: Duration) -> Duration {
+        self.checked_sub(rhs)
+            .expect("overflow when subtracting durations")
+    }
+}
+
+impl SubAssign for Duration {
+    fn sub_assign(&mut self, rhs: Duration) {
+        *self = *self - rhs;
+    }
+}
+
+impl Mul<u32> for Duration {
+    type Output = Duration;
+
+    fn mul(self, rhs: u32) -> Duration {
+        self.checked_mul(rhs)
+            .expect("overflow when multiplying duration by scalar")
+    }
+}
+
+impl Mul<Duration> for u32 {
+    type Output = Duration;
+
+    fn mul(self, rhs: Duration) -> Duration {
+        rhs * self
+    }
+}
+
+impl MulAssign<u32> for Duration {
+    fn mul_assign(&mut self, rhs: u32) {
+        *self = *self * rhs;
+    }
+}
+
+impl Div<u32> for Duration {
+    type Output = Duration;
+
+    fn div(self, rhs: u32) -> Duration {
+        self.checked_div(rhs)
+            .expect("divide by zero error when dividing duration by scalar")
+    }
+}
+
+impl DivAssign<u32> for Duration {
+    fn div_assign(&mut self, rhs: u32) {
+        *self = *self / rhs;
+    }
+}
diff --git a/embassy/src/time/instant.rs b/embassy/src/time/instant.rs
new file mode 100644
index 000000000..9c23d088b
--- /dev/null
+++ b/embassy/src/time/instant.rs
@@ -0,0 +1,102 @@
+use core::convert::TryInto;
+use core::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Sub, SubAssign};
+use core::pin::Pin;
+use core::ptr;
+use core::sync::atomic::{AtomicPtr, Ordering};
+use core::task::{Context, Poll};
+
+use super::{now, Duration};
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Instant {
+    ticks: u64,
+}
+
+impl Instant {
+    pub fn now() -> Instant {
+        Instant { ticks: now() }
+    }
+
+    pub fn into_ticks(&self) -> u64 {
+        self.ticks
+    }
+
+    pub fn duration_since(&self, earlier: Instant) -> Duration {
+        Duration {
+            ticks: (self.ticks - earlier.ticks).try_into().unwrap(),
+        }
+    }
+
+    pub fn checked_duration_since(&self, earlier: Instant) -> Option<Duration> {
+        if self.ticks < earlier.ticks {
+            None
+        } else {
+            Some(Duration {
+                ticks: (self.ticks - earlier.ticks).try_into().unwrap(),
+            })
+        }
+    }
+
+    pub fn saturating_duration_since(&self, earlier: Instant) -> Duration {
+        Duration {
+            ticks: if self.ticks < earlier.ticks {
+                0
+            } else {
+                (self.ticks - earlier.ticks).try_into().unwrap()
+            },
+        }
+    }
+
+    pub fn elapsed(&self) -> Duration {
+        Instant::now() - *self
+    }
+
+    pub fn checked_add(&self, duration: Duration) -> Option<Instant> {
+        self.ticks
+            .checked_add(duration.ticks.into())
+            .map(|ticks| Instant { ticks })
+    }
+    pub fn checked_sub(&self, duration: Duration) -> Option<Instant> {
+        self.ticks
+            .checked_sub(duration.ticks.into())
+            .map(|ticks| Instant { ticks })
+    }
+}
+
+impl Add<Duration> for Instant {
+    type Output = Instant;
+
+    fn add(self, other: Duration) -> Instant {
+        self.checked_add(other)
+            .expect("overflow when adding duration to instant")
+    }
+}
+
+impl AddAssign<Duration> for Instant {
+    fn add_assign(&mut self, other: Duration) {
+        *self = *self + other;
+    }
+}
+
+impl Sub<Duration> for Instant {
+    type Output = Instant;
+
+    fn sub(self, other: Duration) -> Instant {
+        self.checked_sub(other)
+            .expect("overflow when subtracting duration from instant")
+    }
+}
+
+impl SubAssign<Duration> for Instant {
+    fn sub_assign(&mut self, other: Duration) {
+        *self = *self - other;
+    }
+}
+
+impl Sub<Instant> for Instant {
+    type Output = Duration;
+
+    fn sub(self, other: Instant) -> Duration {
+        self.duration_since(other)
+    }
+}
diff --git a/embassy/src/time/mod.rs b/embassy/src/time/mod.rs
new file mode 100644
index 000000000..b3ae10e72
--- /dev/null
+++ b/embassy/src/time/mod.rs
@@ -0,0 +1,24 @@
+mod duration;
+mod instant;
+mod timer;
+mod traits;
+
+pub use duration::Duration;
+pub use instant::Instant;
+pub use timer::Timer;
+pub use traits::*;
+
+use crate::util::Dewrap;
+
+// TODO allow customizing, probably via Cargo features `tick-hz-32768` or something.
+pub const TICKS_PER_SECOND: u32 = 32768;
+
+static mut CLOCK: Option<&'static dyn Clock> = None;
+
+pub unsafe fn set_clock(clock: &'static dyn Clock) {
+    CLOCK = Some(clock);
+}
+
+pub(crate) fn now() -> u64 {
+    unsafe { CLOCK.dexpect(defmt::intern!("No clock set")).now() }
+}
diff --git a/embassy/src/time/timer.rs b/embassy/src/time/timer.rs
new file mode 100644
index 000000000..0315d9fba
--- /dev/null
+++ b/embassy/src/time/timer.rs
@@ -0,0 +1,30 @@
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+use futures_intrusive::timer::{LocalTimer, LocalTimerFuture};
+
+use super::{Duration, Instant};
+use crate::executor::current_timer_queue;
+
+pub struct Timer {
+    inner: LocalTimerFuture<'static>,
+}
+
+impl Timer {
+    pub fn at(when: Instant) -> Self {
+        Self {
+            inner: current_timer_queue().deadline(when.into_ticks()),
+        }
+    }
+
+    pub fn after(dur: Duration) -> Self {
+        Self::at(Instant::now() + dur)
+    }
+}
+
+impl Future for Timer {
+    type Output = ();
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        unsafe { Pin::new_unchecked(&mut self.get_unchecked_mut().inner) }.poll(cx)
+    }
+}
diff --git a/embassy/src/time/traits.rs b/embassy/src/time/traits.rs
new file mode 100644
index 000000000..7faa27cdb
--- /dev/null
+++ b/embassy/src/time/traits.rs
@@ -0,0 +1,44 @@
+/// Monotonic clock
+pub trait Clock {
+    /// Return the current timestamp in ticks.
+    /// This is guaranteed to be monotonic, i.e. a call to now() will always return
+    /// a greater or equal value than earler calls.
+    fn now(&self) -> u64;
+}
+
+impl<T: Clock + ?Sized> Clock for &T {
+    fn now(&self) -> u64 {
+        T::now(self)
+    }
+}
+
+/// Trait to register a callback at a given timestamp.
+pub trait Alarm {
+    /// Sets the callback function to be called when the alarm triggers.
+    /// The callback may be called from any context (interrupt or thread mode).
+    fn set_callback(&self, callback: fn());
+
+    /// Sets an alarm at the given timestamp. When the clock reaches that
+    /// timestamp, the provided callback funcion will be called.
+    ///
+    /// When callback is called, it is guaranteed that now() will return a value greater or equal than timestamp.
+    ///
+    /// Only one alarm can be active at a time. This overwrites any previously-set alarm if any.
+    fn set(&self, timestamp: u64);
+
+    /// Clears the previously-set alarm.
+    /// If no alarm was set, this is a noop.
+    fn clear(&self);
+}
+
+impl<T: Alarm + ?Sized> Alarm for &T {
+    fn set_callback(&self, callback: fn()) {
+        T::set_callback(self, callback);
+    }
+    fn set(&self, timestamp: u64) {
+        T::set(self, timestamp);
+    }
+    fn clear(&self) {
+        T::clear(self)
+    }
+}
diff --git a/examples/Cargo.toml b/examples/Cargo.toml
index 7b49ffb1f..7c44b07a4 100644
--- a/examples/Cargo.toml
+++ b/examples/Cargo.toml
@@ -26,5 +26,5 @@ panic-probe = "0.1.0"
 nrf52840-hal    = { version = "0.11.0" }
 embassy = { version = "0.1.0", path = "../embassy" }
 embassy-nrf = { version = "0.1.0", path = "../embassy-nrf", features = ["defmt-trace", "52840"] }
-static-executor  = { version = "0.1.0", features=["defmt"]}
-futures = { version = "0.3.5", default-features = false }
\ No newline at end of file
+futures = { version = "0.3.5", default-features = false }
+cortex-m-rtic = { git = "https://github.com/rtic-rs/cortex-m-rtic", branch = "master"}
\ No newline at end of file
diff --git a/examples/src/bin/gpiote.rs b/examples/src/bin/gpiote.rs
index 5578062fa..5a6ae9333 100644
--- a/examples/src/bin/gpiote.rs
+++ b/examples/src/bin/gpiote.rs
@@ -13,7 +13,7 @@ use embassy_nrf::gpiote;
 use futures::pin_mut;
 use nrf52840_hal::gpio;
 
-use static_executor::{task, Executor};
+use embassy::executor::{task, Executor};
 static EXECUTOR: Executor = Executor::new(|| cortex_m::asm::sev());
 
 #[task]
diff --git a/examples/src/bin/qspi.rs b/examples/src/bin/qspi.rs
index 4e6ee53ea..c60a666ec 100644
--- a/examples/src/bin/qspi.rs
+++ b/examples/src/bin/qspi.rs
@@ -11,7 +11,7 @@ use embassy::flash::Flash;
 use embassy_nrf::qspi;
 use nrf52840_hal::gpio;
 
-use static_executor::{task, Executor};
+use embassy::executor::{task, Executor};
 static EXECUTOR: Executor = Executor::new(|| cortex_m::asm::sev());
 
 const PAGE_SIZE: usize = 4096;
diff --git a/examples/src/bin/uart.rs b/examples/src/bin/uart.rs
index 6b9df380a..0eec2cd8d 100644
--- a/examples/src/bin/uart.rs
+++ b/examples/src/bin/uart.rs
@@ -12,7 +12,7 @@ use embassy_nrf::uarte;
 use futures::pin_mut;
 use nrf52840_hal::gpio;
 
-use static_executor::{task, Executor};
+use embassy::executor::{task, Executor};
 static EXECUTOR: Executor = Executor::new(|| cortex_m::asm::sev());
 
 #[task]