net: split channel-based driver impl from usb cdc-ncm into a separate crate.

3 files changed, 762 insertions, 0 deletions

diff --git a/embassy-net-driver-channel/Cargo.toml b/embassy-net-driver-channel/Cargo.toml
new file mode 100644
index 000000000..700a4e8a0
--- /dev/null
+++ b/embassy-net-driver-channel/Cargo.toml
@@ -0,0 +1,12 @@
+[package]
+name = "embassy-net-driver-channel"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+defmt = { version = "0.3", optional = true }
+log = { version = "0.4.14", optional = true }
+
+embassy-sync = { version = "0.1.0", path = "../embassy-sync" }
+embassy-futures = { version = "0.1.0", path = "../embassy-futures" }
+embassy-net-driver = { version = "0.1.0", path = "../embassy-net-driver" }
diff --git a/embassy-net-driver-channel/src/fmt.rs b/embassy-net-driver-channel/src/fmt.rs
new file mode 100644
index 000000000..066970813
--- /dev/null
+++ b/embassy-net-driver-channel/src/fmt.rs
@@ -0,0 +1,225 @@
+#![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);
+            }
+        }
+    }
+}
+
+#[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
+    }
+}
diff --git a/embassy-net-driver-channel/src/lib.rs b/embassy-net-driver-channel/src/lib.rs
new file mode 100644
index 000000000..369dc5a9d
--- /dev/null
+++ b/embassy-net-driver-channel/src/lib.rs
@@ -0,0 +1,525 @@
+#![no_std]
+
+// must go first!
+mod fmt;
+
+use core::cell::RefCell;
+use core::mem::MaybeUninit;
+use core::task::{Context, Poll};
+
+pub use embassy_net_driver as driver;
+use embassy_net_driver::{Capabilities, LinkState, Medium};
+use embassy_sync::blocking_mutex::raw::NoopRawMutex;
+use embassy_sync::blocking_mutex::Mutex;
+use embassy_sync::waitqueue::WakerRegistration;
+
+pub struct State<const MTU: usize, const N_RX: usize, const N_TX: usize> {
+    rx: [PacketBuf<MTU>; N_RX],
+    tx: [PacketBuf<MTU>; N_TX],
+    inner: MaybeUninit<StateInner<'static, MTU>>,
+}
+
+impl<const MTU: usize, const N_RX: usize, const N_TX: usize> State<MTU, N_RX, N_TX> {
+    const NEW_PACKET: PacketBuf<MTU> = PacketBuf::new();
+
+    pub const fn new() -> Self {
+        Self {
+            rx: [Self::NEW_PACKET; N_RX],
+            tx: [Self::NEW_PACKET; N_TX],
+            inner: MaybeUninit::uninit(),
+        }
+    }
+}
+
+struct StateInner<'d, const MTU: usize> {
+    rx: zerocopy_channel::Channel<'d, NoopRawMutex, PacketBuf<MTU>>,
+    tx: zerocopy_channel::Channel<'d, NoopRawMutex, PacketBuf<MTU>>,
+    link_state: Mutex<NoopRawMutex, RefCell<LinkStateState>>,
+}
+
+/// State of the LinkState
+struct LinkStateState {
+    state: LinkState,
+    waker: WakerRegistration,
+}
+
+pub struct Runner<'d, const MTU: usize> {
+    tx_chan: zerocopy_channel::Receiver<'d, NoopRawMutex, PacketBuf<MTU>>,
+    rx_chan: zerocopy_channel::Sender<'d, NoopRawMutex, PacketBuf<MTU>>,
+    link_state: &'d Mutex<NoopRawMutex, RefCell<LinkStateState>>,
+}
+
+pub struct RxRunner<'d, const MTU: usize> {
+    rx_chan: zerocopy_channel::Sender<'d, NoopRawMutex, PacketBuf<MTU>>,
+    link_state: &'d Mutex<NoopRawMutex, RefCell<LinkStateState>>,
+}
+
+pub struct TxRunner<'d, const MTU: usize> {
+    tx_chan: zerocopy_channel::Receiver<'d, NoopRawMutex, PacketBuf<MTU>>,
+}
+
+impl<'d, const MTU: usize> Runner<'d, MTU> {
+    pub fn split(self) -> (RxRunner<'d, MTU>, TxRunner<'d, MTU>) {
+        (
+            RxRunner {
+                link_state: self.link_state,
+                rx_chan: self.rx_chan,
+            },
+            TxRunner { tx_chan: self.tx_chan },
+        )
+    }
+
+    pub fn set_link_state(&mut self, state: LinkState) {
+        self.link_state.lock(|s| {
+            let s = &mut *s.borrow_mut();
+            s.state = state;
+            s.waker.wake();
+        });
+    }
+
+    pub async fn rx_buf(&mut self) -> &mut [u8] {
+        let p = self.rx_chan.send().await;
+        &mut p.buf
+    }
+
+    pub fn try_rx_buf(&mut self) -> Option<&mut [u8]> {
+        let p = self.rx_chan.try_send()?;
+        Some(&mut p.buf)
+    }
+
+    pub fn poll_rx_buf(&mut self, cx: &mut Context) -> Poll<&mut [u8]> {
+        match self.rx_chan.poll_send(cx) {
+            Poll::Ready(p) => Poll::Ready(&mut p.buf),
+            Poll::Pending => Poll::Pending,
+        }
+    }
+
+    pub fn rx_done(&mut self, len: usize) {
+        let p = self.rx_chan.try_send().unwrap();
+        p.len = len;
+        self.rx_chan.send_done();
+    }
+
+    pub async fn tx_buf(&mut self) -> &mut [u8] {
+        let p = self.tx_chan.recv().await;
+        &mut p.buf[..p.len]
+    }
+
+    pub fn try_tx_buf(&mut self) -> Option<&mut [u8]> {
+        let p = self.tx_chan.try_recv()?;
+        Some(&mut p.buf[..p.len])
+    }
+
+    pub fn poll_tx_buf(&mut self, cx: &mut Context) -> Poll<&mut [u8]> {
+        match self.tx_chan.poll_recv(cx) {
+            Poll::Ready(p) => Poll::Ready(&mut p.buf[..p.len]),
+            Poll::Pending => Poll::Pending,
+        }
+    }
+
+    pub fn tx_done(&mut self) {
+        self.tx_chan.recv_done();
+    }
+}
+
+impl<'d, const MTU: usize> RxRunner<'d, MTU> {
+    pub fn set_link_state(&mut self, state: LinkState) {
+        self.link_state.lock(|s| {
+            let s = &mut *s.borrow_mut();
+            s.state = state;
+            s.waker.wake();
+        });
+    }
+
+    pub async fn rx_buf(&mut self) -> &mut [u8] {
+        let p = self.rx_chan.send().await;
+        &mut p.buf
+    }
+
+    pub fn try_rx_buf(&mut self) -> Option<&mut [u8]> {
+        let p = self.rx_chan.try_send()?;
+        Some(&mut p.buf)
+    }
+
+    pub fn poll_rx_buf(&mut self, cx: &mut Context) -> Poll<&mut [u8]> {
+        match self.rx_chan.poll_send(cx) {
+            Poll::Ready(p) => Poll::Ready(&mut p.buf),
+            Poll::Pending => Poll::Pending,
+        }
+    }
+
+    pub fn rx_done(&mut self, len: usize) {
+        let p = self.rx_chan.try_send().unwrap();
+        p.len = len;
+        self.rx_chan.send_done();
+    }
+}
+
+impl<'d, const MTU: usize> TxRunner<'d, MTU> {
+    pub async fn tx_buf(&mut self) -> &mut [u8] {
+        let p = self.tx_chan.recv().await;
+        &mut p.buf[..p.len]
+    }
+
+    pub fn try_tx_buf(&mut self) -> Option<&mut [u8]> {
+        let p = self.tx_chan.try_recv()?;
+        Some(&mut p.buf[..p.len])
+    }
+
+    pub fn poll_tx_buf(&mut self, cx: &mut Context) -> Poll<&mut [u8]> {
+        match self.tx_chan.poll_recv(cx) {
+            Poll::Ready(p) => Poll::Ready(&mut p.buf[..p.len]),
+            Poll::Pending => Poll::Pending,
+        }
+    }
+
+    pub fn tx_done(&mut self) {
+        self.tx_chan.recv_done();
+    }
+}
+
+pub fn new<'d, const MTU: usize, const N_RX: usize, const N_TX: usize>(
+    state: &'d mut State<MTU, N_RX, N_TX>,
+    ethernet_address: [u8; 6],
+) -> (Runner<'d, MTU>, Device<'d, MTU>) {
+    let mut caps = Capabilities::default();
+    caps.max_transmission_unit = MTU;
+    caps.medium = Medium::Ethernet;
+
+    // safety: this is a self-referential struct, however:
+    // - it can't move while the `'d` borrow is active.
+    // - when the borrow ends, the dangling references inside the MaybeUninit will never be used again.
+    let state_uninit: *mut MaybeUninit<StateInner<'d, MTU>> =
+        (&mut state.inner as *mut MaybeUninit<StateInner<'static, MTU>>).cast();
+    let state = unsafe { &mut *state_uninit }.write(StateInner {
+        rx: zerocopy_channel::Channel::new(&mut state.rx[..]),
+        tx: zerocopy_channel::Channel::new(&mut state.tx[..]),
+        link_state: Mutex::new(RefCell::new(LinkStateState {
+            state: LinkState::Down,
+            waker: WakerRegistration::new(),
+        })),
+    });
+
+    let (rx_sender, rx_receiver) = state.rx.split();
+    let (tx_sender, tx_receiver) = state.tx.split();
+
+    (
+        Runner {
+            tx_chan: tx_receiver,
+            rx_chan: rx_sender,
+            link_state: &state.link_state,
+        },
+        Device {
+            caps,
+            ethernet_address,
+            link_state: &state.link_state,
+            rx: rx_receiver,
+            tx: tx_sender,
+        },
+    )
+}
+
+pub struct PacketBuf<const MTU: usize> {
+    len: usize,
+    buf: [u8; MTU],
+}
+
+impl<const MTU: usize> PacketBuf<MTU> {
+    pub const fn new() -> Self {
+        Self { len: 0, buf: [0; MTU] }
+    }
+}
+
+pub struct Device<'d, const MTU: usize> {
+    rx: zerocopy_channel::Receiver<'d, NoopRawMutex, PacketBuf<MTU>>,
+    tx: zerocopy_channel::Sender<'d, NoopRawMutex, PacketBuf<MTU>>,
+    link_state: &'d Mutex<NoopRawMutex, RefCell<LinkStateState>>,
+    caps: Capabilities,
+    ethernet_address: [u8; 6],
+}
+
+impl<'d, const MTU: usize> embassy_net_driver::Driver for Device<'d, MTU> {
+    type RxToken<'a> = RxToken<'a, MTU> where Self: 'a ;
+    type TxToken<'a> = TxToken<'a, MTU> where Self: 'a ;
+
+    fn receive(&mut self, cx: &mut Context) -> Option<(Self::RxToken<'_>, Self::TxToken<'_>)> {
+        if self.rx.poll_recv(cx).is_ready() && self.tx.poll_send(cx).is_ready() {
+            Some((RxToken { rx: self.rx.borrow() }, TxToken { tx: self.tx.borrow() }))
+        } else {
+            None
+        }
+    }
+
+    /// Construct a transmit token.
+    fn transmit(&mut self, cx: &mut Context) -> Option<Self::TxToken<'_>> {
+        if self.tx.poll_send(cx).is_ready() {
+            Some(TxToken { tx: self.tx.borrow() })
+        } else {
+            None
+        }
+    }
+
+    /// Get a description of device capabilities.
+    fn capabilities(&self) -> Capabilities {
+        self.caps.clone()
+    }
+
+    fn ethernet_address(&self) -> [u8; 6] {
+        self.ethernet_address
+    }
+
+    fn link_state(&mut self, cx: &mut Context) -> LinkState {
+        self.link_state.lock(|s| {
+            let s = &mut *s.borrow_mut();
+            s.waker.register(cx.waker());
+            s.state
+        })
+    }
+}
+
+pub struct RxToken<'a, const MTU: usize> {
+    rx: zerocopy_channel::Receiver<'a, NoopRawMutex, PacketBuf<MTU>>,
+}
+
+impl<'a, const MTU: usize> embassy_net_driver::RxToken for RxToken<'a, MTU> {
+    fn consume<R, F>(mut self, f: F) -> R
+    where
+        F: FnOnce(&mut [u8]) -> R,
+    {
+        // NOTE(unwrap): we checked the queue wasn't full when creating the token.
+        let pkt = unwrap!(self.rx.try_recv());
+        let r = f(&mut pkt.buf[..pkt.len]);
+        self.rx.recv_done();
+        r
+    }
+}
+
+pub struct TxToken<'a, const MTU: usize> {
+    tx: zerocopy_channel::Sender<'a, NoopRawMutex, PacketBuf<MTU>>,
+}
+
+impl<'a, const MTU: usize> embassy_net_driver::TxToken for TxToken<'a, MTU> {
+    fn consume<R, F>(mut self, len: usize, f: F) -> R
+    where
+        F: FnOnce(&mut [u8]) -> R,
+    {
+        // NOTE(unwrap): we checked the queue wasn't full when creating the token.
+        let pkt = unwrap!(self.tx.try_send());
+        let r = f(&mut pkt.buf[..len]);
+        pkt.len = len;
+        self.tx.send_done();
+        r
+    }
+}
+
+mod zerocopy_channel {
+    use core::cell::RefCell;
+    use core::future::poll_fn;
+    use core::marker::PhantomData;
+    use core::task::{Context, Poll};
+
+    use embassy_sync::blocking_mutex::raw::RawMutex;
+    use embassy_sync::blocking_mutex::Mutex;
+    use embassy_sync::waitqueue::WakerRegistration;
+
+    pub struct Channel<'a, M: RawMutex, T> {
+        buf: *mut T,
+        phantom: PhantomData<&'a mut T>,
+        state: Mutex<M, RefCell<State>>,
+    }
+
+    impl<'a, M: RawMutex, T> Channel<'a, M, T> {
+        pub fn new(buf: &'a mut [T]) -> Self {
+            let len = buf.len();
+            assert!(len != 0);
+
+            Self {
+                buf: buf.as_mut_ptr(),
+                phantom: PhantomData,
+                state: Mutex::new(RefCell::new(State {
+                    len,
+                    front: 0,
+                    back: 0,
+                    full: false,
+                    send_waker: WakerRegistration::new(),
+                    recv_waker: WakerRegistration::new(),
+                })),
+            }
+        }
+
+        pub fn split(&mut self) -> (Sender<'_, M, T>, Receiver<'_, M, T>) {
+            (Sender { channel: self }, Receiver { channel: self })
+        }
+    }
+
+    pub struct Sender<'a, M: RawMutex, T> {
+        channel: &'a Channel<'a, M, T>,
+    }
+
+    impl<'a, M: RawMutex, T> Sender<'a, M, T> {
+        pub fn borrow(&mut self) -> Sender<'_, M, T> {
+            Sender { channel: self.channel }
+        }
+
+        pub fn try_send(&mut self) -> Option<&mut T> {
+            self.channel.state.lock(|s| {
+                let s = &mut *s.borrow_mut();
+                match s.push_index() {
+                    Some(i) => Some(unsafe { &mut *self.channel.buf.add(i) }),
+                    None => None,
+                }
+            })
+        }
+
+        pub fn poll_send(&mut self, cx: &mut Context) -> Poll<&mut T> {
+            self.channel.state.lock(|s| {
+                let s = &mut *s.borrow_mut();
+                match s.push_index() {
+                    Some(i) => Poll::Ready(unsafe { &mut *self.channel.buf.add(i) }),
+                    None => {
+                        s.recv_waker.register(cx.waker());
+                        Poll::Pending
+                    }
+                }
+            })
+        }
+
+        pub async fn send(&mut self) -> &mut T {
+            let i = poll_fn(|cx| {
+                self.channel.state.lock(|s| {
+                    let s = &mut *s.borrow_mut();
+                    match s.push_index() {
+                        Some(i) => Poll::Ready(i),
+                        None => {
+                            s.recv_waker.register(cx.waker());
+                            Poll::Pending
+                        }
+                    }
+                })
+            })
+            .await;
+            unsafe { &mut *self.channel.buf.add(i) }
+        }
+
+        pub fn send_done(&mut self) {
+            self.channel.state.lock(|s| s.borrow_mut().push_done())
+        }
+    }
+    pub struct Receiver<'a, M: RawMutex, T> {
+        channel: &'a Channel<'a, M, T>,
+    }
+
+    impl<'a, M: RawMutex, T> Receiver<'a, M, T> {
+        pub fn borrow(&mut self) -> Receiver<'_, M, T> {
+            Receiver { channel: self.channel }
+        }
+
+        pub fn try_recv(&mut self) -> Option<&mut T> {
+            self.channel.state.lock(|s| {
+                let s = &mut *s.borrow_mut();
+                match s.pop_index() {
+                    Some(i) => Some(unsafe { &mut *self.channel.buf.add(i) }),
+                    None => None,
+                }
+            })
+        }
+
+        pub fn poll_recv(&mut self, cx: &mut Context) -> Poll<&mut T> {
+            self.channel.state.lock(|s| {
+                let s = &mut *s.borrow_mut();
+                match s.pop_index() {
+                    Some(i) => Poll::Ready(unsafe { &mut *self.channel.buf.add(i) }),
+                    None => {
+                        s.send_waker.register(cx.waker());
+                        Poll::Pending
+                    }
+                }
+            })
+        }
+
+        pub async fn recv(&mut self) -> &mut T {
+            let i = poll_fn(|cx| {
+                self.channel.state.lock(|s| {
+                    let s = &mut *s.borrow_mut();
+                    match s.pop_index() {
+                        Some(i) => Poll::Ready(i),
+                        None => {
+                            s.send_waker.register(cx.waker());
+                            Poll::Pending
+                        }
+                    }
+                })
+            })
+            .await;
+            unsafe { &mut *self.channel.buf.add(i) }
+        }
+
+        pub fn recv_done(&mut self) {
+            self.channel.state.lock(|s| s.borrow_mut().pop_done())
+        }
+    }
+
+    struct State {
+        len: usize,
+
+        /// Front index. Always 0..=(N-1)
+        front: usize,
+        /// Back index. Always 0..=(N-1).
+        back: usize,
+
+        /// Used to distinguish "empty" and "full" cases when `front == back`.
+        /// May only be `true` if `front == back`, always `false` otherwise.
+        full: bool,
+
+        send_waker: WakerRegistration,
+        recv_waker: WakerRegistration,
+    }
+
+    impl State {
+        fn increment(&self, i: usize) -> usize {
+            if i + 1 == self.len {
+                0
+            } else {
+                i + 1
+            }
+        }
+
+        fn is_full(&self) -> bool {
+            self.full
+        }
+
+        fn is_empty(&self) -> bool {
+            self.front == self.back && !self.full
+        }
+
+        fn push_index(&mut self) -> Option<usize> {
+            match self.is_full() {
+                true => None,
+                false => Some(self.back),
+            }
+        }
+
+        fn push_done(&mut self) {
+            assert!(!self.is_full());
+            self.back = self.increment(self.back);
+            if self.back == self.front {
+                self.full = true;
+            }
+            self.send_waker.wake();
+        }
+
+        fn pop_index(&mut self) -> Option<usize> {
+            match self.is_empty() {
+                true => None,
+                false => Some(self.front),
+            }
+        }
+
+        fn pop_done(&mut self) {
+            assert!(!self.is_empty());
+            self.front = self.increment(self.front);
+            self.full = false;
+            self.recv_waker.wake();
+        }
+    }
+}