10 files changed, 1064 insertions, 2 deletions

diff --git a/embassy-stm32f4-examples/Cargo.toml b/embassy-stm32f4-examples/Cargo.toml
index 5bbaecc58..e4f2aa7a1 100644
--- a/embassy-stm32f4-examples/Cargo.toml
+++ b/embassy-stm32f4-examples/Cargo.toml
@@ -46,7 +46,8 @@ cortex-m = "0.7.1"
 cortex-m-rt = "0.6.13"
 embedded-hal    = { version = "0.2.4" }
 panic-probe = "0.1.0"
-stm32f4xx-hal  = { version = "0.8.3", features = ["rt"], git = "https://github.com/stm32-rs/stm32f4xx-hal.git"}
+stm32f4xx-hal  = { version = "0.8.3", features = ["rt", "usb_fs"], git = "https://github.com/stm32-rs/stm32f4xx-hal.git"}
 futures = { version = "0.3.8", default-features = false, features = ["async-await"] }
 rtt-target = { version = "0.3", features = ["cortex-m"] }
-bxcan = "0.5.0"
\ No newline at end of file
+bxcan = "0.5.0"
+usb-device = "0.2.7"
diff --git a/embassy-stm32f4-examples/src/bin/usb_serial.rs b/embassy-stm32f4-examples/src/bin/usb_serial.rs
new file mode 100644
index 000000000..d2ccb4b21
--- /dev/null
+++ b/embassy-stm32f4-examples/src/bin/usb_serial.rs
@@ -0,0 +1,119 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+#[path = "../example_common.rs"]
+mod example_common;
+use example_common::*;
+
+use cortex_m_rt::entry;
+use defmt::panic;
+use embassy::executor::{task, Executor};
+use embassy::io::{AsyncBufReadExt, AsyncWriteExt};
+use embassy::time::{Duration, Timer};
+use embassy::util::Forever;
+use embassy_stm32f4::interrupt::OwnedInterrupt;
+use embassy_stm32f4::usb::Usb;
+use embassy_stm32f4::usb_serial::UsbSerial;
+use embassy_stm32f4::{interrupt, pac, rtc};
+use futures::future::{select, Either};
+use futures::pin_mut;
+use stm32f4xx_hal::otg_fs::{UsbBus, USB};
+use stm32f4xx_hal::prelude::*;
+use usb_device::bus::UsbBusAllocator;
+use usb_device::prelude::*;
+
+#[task]
+async fn run1(bus: &'static mut UsbBusAllocator<UsbBus<USB>>) {
+    info!("Async task");
+
+    let mut read_buf = [0u8; 128];
+    let mut write_buf = [0u8; 128];
+    let serial = UsbSerial::new(bus, &mut read_buf, &mut write_buf);
+
+    let device = UsbDeviceBuilder::new(bus, UsbVidPid(0x16c0, 0x27dd))
+        .manufacturer("Fake company")
+        .product("Serial port")
+        .serial_number("TEST")
+        .device_class(0x02)
+        .build();
+
+    let irq = interrupt::take!(OTG_FS);
+    irq.set_priority(interrupt::Priority::Level3);
+
+    let usb = Usb::new(device, serial, irq);
+    pin_mut!(usb);
+
+    let (mut read_interface, mut write_interface) = usb.as_mut().into_ref().take_serial();
+
+    let mut buf = [0u8; 5];
+    loop {
+        let recv_fut = read_interface.read(&mut buf);
+        let timeout = Timer::after(Duration::from_ticks(32768 * 3));
+
+        match select(recv_fut, timeout).await {
+            Either::Left((recv, _)) => {
+                let recv = unwrap!(recv);
+                unwrap!(write_interface.write_all(&buf[..recv]).await);
+            }
+            Either::Right(_) => {
+                unwrap!(write_interface.write_all(b"Hello\r\n").await);
+            }
+        }
+    }
+}
+
+static RTC: Forever<rtc::RTC<pac::TIM2>> = Forever::new();
+static ALARM: Forever<rtc::Alarm<pac::TIM2>> = Forever::new();
+static EXECUTOR: Forever<Executor> = Forever::new();
+static USB_BUS: Forever<UsbBusAllocator<UsbBus<USB>>> = Forever::new();
+
+#[entry]
+fn main() -> ! {
+    static mut EP_MEMORY: [u32; 1024] = [0; 1024];
+
+    info!("Hello World!");
+
+    let p = unwrap!(pac::Peripherals::take());
+
+    p.RCC.ahb1enr.modify(|_, w| w.dma1en().enabled());
+    let rcc = p.RCC.constrain();
+    let clocks = rcc
+        .cfgr
+        .use_hse(25.mhz())
+        .sysclk(48.mhz())
+        .require_pll48clk()
+        .freeze();
+
+    p.DBGMCU.cr.modify(|_, w| {
+        w.dbg_sleep().set_bit();
+        w.dbg_standby().set_bit();
+        w.dbg_stop().set_bit()
+    });
+
+    let rtc = RTC.put(rtc::RTC::new(p.TIM2, interrupt::take!(TIM2), clocks));
+    rtc.start();
+
+    unsafe { embassy::time::set_clock(rtc) };
+
+    let alarm = ALARM.put(rtc.alarm1());
+    let executor = EXECUTOR.put(Executor::new());
+    executor.set_alarm(alarm);
+
+    let gpioa = p.GPIOA.split();
+    let usb = USB {
+        usb_global: p.OTG_FS_GLOBAL,
+        usb_device: p.OTG_FS_DEVICE,
+        usb_pwrclk: p.OTG_FS_PWRCLK,
+        pin_dm: gpioa.pa11.into_alternate_af10(),
+        pin_dp: gpioa.pa12.into_alternate_af10(),
+        hclk: clocks.hclk(),
+    };
+    // Rust analyzer isn't recognizing the static ref magic `cortex-m` does
+    #[allow(unused_unsafe)]
+    let usb_bus = USB_BUS.put(UsbBus::new(usb, unsafe { EP_MEMORY }));
+
+    executor.run(move |spawner| {
+        unwrap!(spawner.spawn(run1(usb_bus)));
+    });
+}
diff --git a/embassy-stm32f4/Cargo.toml b/embassy-stm32f4/Cargo.toml
index ae3273d67..b39a141b4 100644
--- a/embassy-stm32f4/Cargo.toml
+++ b/embassy-stm32f4/Cargo.toml
@@ -41,3 +41,4 @@ embedded-dma    = { version = "0.1.2" }
 stm32f4xx-hal  = { version = "0.8.3", features = ["rt", "can"], git = "https://github.com/stm32-rs/stm32f4xx-hal.git"}
 bxcan = "0.5.0"
 nb = "*"
+usb-device = "0.2.7"
diff --git a/embassy-stm32f4/src/cdc_acm.rs b/embassy-stm32f4/src/cdc_acm.rs
new file mode 100644
index 000000000..5a85b3846
--- /dev/null
+++ b/embassy-stm32f4/src/cdc_acm.rs
@@ -0,0 +1,338 @@
+// Copied from https://github.com/mvirkkunen/usbd-serial
+#![allow(dead_code)]
+
+use core::convert::TryInto;
+use core::mem;
+use usb_device::class_prelude::*;
+use usb_device::Result;
+
+/// This should be used as `device_class` when building the `UsbDevice`.
+pub const USB_CLASS_CDC: u8 = 0x02;
+
+const USB_CLASS_CDC_DATA: u8 = 0x0a;
+const CDC_SUBCLASS_ACM: u8 = 0x02;
+const CDC_PROTOCOL_NONE: u8 = 0x00;
+
+const CS_INTERFACE: u8 = 0x24;
+const CDC_TYPE_HEADER: u8 = 0x00;
+const CDC_TYPE_CALL_MANAGEMENT: u8 = 0x01;
+const CDC_TYPE_ACM: u8 = 0x02;
+const CDC_TYPE_UNION: u8 = 0x06;
+
+const REQ_SEND_ENCAPSULATED_COMMAND: u8 = 0x00;
+#[allow(unused)]
+const REQ_GET_ENCAPSULATED_COMMAND: u8 = 0x01;
+const REQ_SET_LINE_CODING: u8 = 0x20;
+const REQ_GET_LINE_CODING: u8 = 0x21;
+const REQ_SET_CONTROL_LINE_STATE: u8 = 0x22;
+
+/// Packet level implementation of a CDC-ACM serial port.
+///
+/// This class can be used directly and it has the least overhead due to directly reading and
+/// writing USB packets with no intermediate buffers, but it will not act like a stream-like serial
+/// port. The following constraints must be followed if you use this class directly:
+///
+/// - `read_packet` must be called with a buffer large enough to hold max_packet_size bytes, and the
+///   method will return a `WouldBlock` error if there is no packet to be read.
+/// - `write_packet` must not be called with a buffer larger than max_packet_size bytes, and the
+///   method will return a `WouldBlock` error if the previous packet has not been sent yet.
+/// - If you write a packet that is exactly max_packet_size bytes long, it won't be processed by the
+///   host operating system until a subsequent shorter packet is sent. A zero-length packet (ZLP)
+///   can be sent if there is no other data to send. This is because USB bulk transactions must be
+///   terminated with a short packet, even if the bulk endpoint is used for stream-like data.
+pub struct CdcAcmClass<'a, B: UsbBus> {
+    comm_if: InterfaceNumber,
+    comm_ep: EndpointIn<'a, B>,
+    data_if: InterfaceNumber,
+    read_ep: EndpointOut<'a, B>,
+    write_ep: EndpointIn<'a, B>,
+    line_coding: LineCoding,
+    dtr: bool,
+    rts: bool,
+}
+
+impl<B: UsbBus> CdcAcmClass<'_, B> {
+    /// Creates a new CdcAcmClass with the provided UsbBus and max_packet_size in bytes. For
+    /// full-speed devices, max_packet_size has to be one of 8, 16, 32 or 64.
+    pub fn new(alloc: &UsbBusAllocator<B>, max_packet_size: u16) -> CdcAcmClass<'_, B> {
+        CdcAcmClass {
+            comm_if: alloc.interface(),
+            comm_ep: alloc.interrupt(8, 255),
+            data_if: alloc.interface(),
+            read_ep: alloc.bulk(max_packet_size),
+            write_ep: alloc.bulk(max_packet_size),
+            line_coding: LineCoding {
+                stop_bits: StopBits::One,
+                data_bits: 8,
+                parity_type: ParityType::None,
+                data_rate: 8_000,
+            },
+            dtr: false,
+            rts: false,
+        }
+    }
+
+    /// Gets the maximum packet size in bytes.
+    pub fn max_packet_size(&self) -> u16 {
+        // The size is the same for both endpoints.
+        self.read_ep.max_packet_size()
+    }
+
+    /// Gets the current line coding. The line coding contains information that's mainly relevant
+    /// for USB to UART serial port emulators, and can be ignored if not relevant.
+    pub fn line_coding(&self) -> &LineCoding {
+        &self.line_coding
+    }
+
+    /// Gets the DTR (data terminal ready) state
+    pub fn dtr(&self) -> bool {
+        self.dtr
+    }
+
+    /// Gets the RTS (request to send) state
+    pub fn rts(&self) -> bool {
+        self.rts
+    }
+
+    /// Writes a single packet into the IN endpoint.
+    pub fn write_packet(&mut self, data: &[u8]) -> Result<usize> {
+        self.write_ep.write(data)
+    }
+
+    /// Reads a single packet from the OUT endpoint.
+    pub fn read_packet(&mut self, data: &mut [u8]) -> Result<usize> {
+        self.read_ep.read(data)
+    }
+
+    /// Gets the address of the IN endpoint.
+    pub fn write_ep_address(&self) -> EndpointAddress {
+        self.write_ep.address()
+    }
+
+    /// Gets the address of the OUT endpoint.
+    pub fn read_ep_address(&self) -> EndpointAddress {
+        self.read_ep.address()
+    }
+}
+
+impl<B: UsbBus> UsbClass<B> for CdcAcmClass<'_, B> {
+    fn get_configuration_descriptors(&self, writer: &mut DescriptorWriter) -> Result<()> {
+        writer.iad(
+            self.comm_if,
+            2,
+            USB_CLASS_CDC,
+            CDC_SUBCLASS_ACM,
+            CDC_PROTOCOL_NONE,
+        )?;
+
+        writer.interface(
+            self.comm_if,
+            USB_CLASS_CDC,
+            CDC_SUBCLASS_ACM,
+            CDC_PROTOCOL_NONE,
+        )?;
+
+        writer.write(
+            CS_INTERFACE,
+            &[
+                CDC_TYPE_HEADER, // bDescriptorSubtype
+                0x10,
+                0x01, // bcdCDC (1.10)
+            ],
+        )?;
+
+        writer.write(
+            CS_INTERFACE,
+            &[
+                CDC_TYPE_ACM, // bDescriptorSubtype
+                0x00,         // bmCapabilities
+            ],
+        )?;
+
+        writer.write(
+            CS_INTERFACE,
+            &[
+                CDC_TYPE_UNION,      // bDescriptorSubtype
+                self.comm_if.into(), // bControlInterface
+                self.data_if.into(), // bSubordinateInterface
+            ],
+        )?;
+
+        writer.write(
+            CS_INTERFACE,
+            &[
+                CDC_TYPE_CALL_MANAGEMENT, // bDescriptorSubtype
+                0x00,                     // bmCapabilities
+                self.data_if.into(),      // bDataInterface
+            ],
+        )?;
+
+        writer.endpoint(&self.comm_ep)?;
+
+        writer.interface(self.data_if, USB_CLASS_CDC_DATA, 0x00, 0x00)?;
+
+        writer.endpoint(&self.write_ep)?;
+        writer.endpoint(&self.read_ep)?;
+
+        Ok(())
+    }
+
+    fn reset(&mut self) {
+        self.line_coding = LineCoding::default();
+        self.dtr = false;
+        self.rts = false;
+    }
+
+    fn control_in(&mut self, xfer: ControlIn<B>) {
+        let req = xfer.request();
+
+        if !(req.request_type == control::RequestType::Class
+            && req.recipient == control::Recipient::Interface
+            && req.index == u8::from(self.comm_if) as u16)
+        {
+            return;
+        }
+
+        match req.request {
+            // REQ_GET_ENCAPSULATED_COMMAND is not really supported - it will be rejected below.
+            REQ_GET_LINE_CODING if req.length == 7 => {
+                xfer.accept(|data| {
+                    data[0..4].copy_from_slice(&self.line_coding.data_rate.to_le_bytes());
+                    data[4] = self.line_coding.stop_bits as u8;
+                    data[5] = self.line_coding.parity_type as u8;
+                    data[6] = self.line_coding.data_bits;
+
+                    Ok(7)
+                })
+                .ok();
+            }
+            _ => {
+                xfer.reject().ok();
+            }
+        }
+    }
+
+    fn control_out(&mut self, xfer: ControlOut<B>) {
+        let req = xfer.request();
+
+        if !(req.request_type == control::RequestType::Class
+            && req.recipient == control::Recipient::Interface
+            && req.index == u8::from(self.comm_if) as u16)
+        {
+            return;
+        }
+
+        match req.request {
+            REQ_SEND_ENCAPSULATED_COMMAND => {
+                // We don't actually support encapsulated commands but pretend we do for standards
+                // compatibility.
+                xfer.accept().ok();
+            }
+            REQ_SET_LINE_CODING if xfer.data().len() >= 7 => {
+                self.line_coding.data_rate =
+                    u32::from_le_bytes(xfer.data()[0..4].try_into().unwrap());
+                self.line_coding.stop_bits = xfer.data()[4].into();
+                self.line_coding.parity_type = xfer.data()[5].into();
+                self.line_coding.data_bits = xfer.data()[6];
+
+                xfer.accept().ok();
+            }
+            REQ_SET_CONTROL_LINE_STATE => {
+                self.dtr = (req.value & 0x0001) != 0;
+                self.rts = (req.value & 0x0002) != 0;
+
+                xfer.accept().ok();
+            }
+            _ => {
+                xfer.reject().ok();
+            }
+        };
+    }
+}
+
+/// Number of stop bits for LineCoding
+#[derive(Copy, Clone, PartialEq, Eq)]
+pub enum StopBits {
+    /// 1 stop bit
+    One = 0,
+
+    /// 1.5 stop bits
+    OnePointFive = 1,
+
+    /// 2 stop bits
+    Two = 2,
+}
+
+impl From<u8> for StopBits {
+    fn from(value: u8) -> Self {
+        if value <= 2 {
+            unsafe { mem::transmute(value) }
+        } else {
+            StopBits::One
+        }
+    }
+}
+
+/// Parity for LineCoding
+#[derive(Copy, Clone, PartialEq, Eq)]
+pub enum ParityType {
+    None = 0,
+    Odd = 1,
+    Event = 2,
+    Mark = 3,
+    Space = 4,
+}
+
+impl From<u8> for ParityType {
+    fn from(value: u8) -> Self {
+        if value <= 4 {
+            unsafe { mem::transmute(value) }
+        } else {
+            ParityType::None
+        }
+    }
+}
+
+/// Line coding parameters
+///
+/// This is provided by the host for specifying the standard UART parameters such as baud rate. Can
+/// be ignored if you don't plan to interface with a physical UART.
+pub struct LineCoding {
+    stop_bits: StopBits,
+    data_bits: u8,
+    parity_type: ParityType,
+    data_rate: u32,
+}
+
+impl LineCoding {
+    /// Gets the number of stop bits for UART communication.
+    pub fn stop_bits(&self) -> StopBits {
+        self.stop_bits
+    }
+
+    /// Gets the number of data bits for UART communication.
+    pub fn data_bits(&self) -> u8 {
+        self.data_bits
+    }
+
+    /// Gets the parity type for UART communication.
+    pub fn parity_type(&self) -> ParityType {
+        self.parity_type
+    }
+
+    /// Gets the data rate in bits per second for UART communication.
+    pub fn data_rate(&self) -> u32 {
+        self.data_rate
+    }
+}
+
+impl Default for LineCoding {
+    fn default() -> Self {
+        LineCoding {
+            stop_bits: StopBits::One,
+            data_bits: 8,
+            parity_type: ParityType::None,
+            data_rate: 8_000,
+        }
+    }
+}
diff --git a/embassy-stm32f4/src/lib.rs b/embassy-stm32f4/src/lib.rs
index 0d490525c..1788f5e77 100644
--- a/embassy-stm32f4/src/lib.rs
+++ b/embassy-stm32f4/src/lib.rs
@@ -316,3 +316,10 @@ pub mod exti;
 pub mod qei;
 pub mod rtc;
 pub mod serial;
+pub mod usb;
+pub mod usb_serial;
+pub mod util;
+
+pub(crate) mod cdc_acm;
+
+pub use cortex_m_rt::interrupt;
diff --git a/embassy-stm32f4/src/usb.rs b/embassy-stm32f4/src/usb.rs
new file mode 100644
index 000000000..613b9ecb7
--- /dev/null
+++ b/embassy-stm32f4/src/usb.rs
@@ -0,0 +1,130 @@
+use core::cell::RefCell;
+use core::marker::PhantomData;
+use core::pin::Pin;
+
+use usb_device::bus::UsbBus;
+use usb_device::class::UsbClass;
+use usb_device::device::UsbDevice;
+
+use crate::interrupt;
+use crate::usb_serial::{ReadInterface, UsbSerial, WriteInterface};
+use crate::util::peripheral::{PeripheralMutex, PeripheralState};
+
+pub struct State<'bus, B: UsbBus, T: ClassSet<B>> {
+    device: UsbDevice<'bus, B>,
+    pub(crate) classes: T,
+}
+
+pub struct Usb<'bus, B: UsbBus, T: ClassSet<B>> {
+    // Don't you dare moving out `PeripheralMutex`
+    inner: RefCell<PeripheralMutex<State<'bus, B, T>>>,
+}
+
+impl<'bus, B, T> Usb<'bus, B, T>
+where
+    B: UsbBus,
+    T: ClassSet<B>,
+{
+    pub fn new<S: IntoClassSet<B, T>>(
+        device: UsbDevice<'bus, B>,
+        class_set: S,
+        irq: interrupt::OTG_FSInterrupt,
+    ) -> Self {
+        let state = State {
+            device,
+            classes: class_set.into_class_set(),
+        };
+        let mutex = PeripheralMutex::new(state, irq);
+        Self {
+            inner: RefCell::new(mutex),
+        }
+    }
+
+    pub fn start(self: Pin<&mut Self>) {
+        let this = unsafe { self.get_unchecked_mut() };
+        let mut mutex = this.inner.borrow_mut();
+        let mutex = unsafe { Pin::new_unchecked(&mut *mutex) };
+
+        // Use inner to register the irq
+        mutex.with(|_, _| {});
+    }
+}
+
+impl<'bus, 'c, B, T> Usb<'bus, B, T>
+where
+    B: UsbBus,
+    T: ClassSet<B> + SerialState<'bus, 'c, B>,
+{
+    pub fn take_serial<'a>(
+        self: Pin<&'a Self>,
+    ) -> (
+        ReadInterface<'a, 'bus, 'c, B, T>,
+        WriteInterface<'a, 'bus, 'c, B, T>,
+    ) {
+        let this = self.get_ref();
+
+        let r = ReadInterface {
+            inner: &this.inner,
+            _buf_lifetime: PhantomData,
+        };
+
+        let w = WriteInterface {
+            inner: &this.inner,
+            _buf_lifetime: PhantomData,
+        };
+        (r, w)
+    }
+}
+
+impl<'bus, B, T> PeripheralState for State<'bus, B, T>
+where
+    B: UsbBus,
+    T: ClassSet<B>,
+{
+    type Interrupt = interrupt::OTG_FSInterrupt;
+    fn on_interrupt(&mut self) {
+        self.classes.poll_all(&mut self.device);
+    }
+}
+
+pub trait ClassSet<B: UsbBus> {
+    fn poll_all(&mut self, device: &mut UsbDevice<'_, B>) -> bool;
+}
+
+pub trait IntoClassSet<B: UsbBus, C: ClassSet<B>> {
+    fn into_class_set(self) -> C;
+}
+
+pub struct ClassSet1<B: UsbBus, T: UsbClass<B>> {
+    class: T,
+    _bus: PhantomData<B>,
+}
+
+impl<B, T> ClassSet<B> for ClassSet1<B, T>
+where
+    B: UsbBus,
+    T: UsbClass<B>,
+{
+    fn poll_all(&mut self, device: &mut UsbDevice<'_, B>) -> bool {
+        device.poll(&mut [&mut self.class])
+    }
+}
+
+impl<B: UsbBus, T: UsbClass<B>> IntoClassSet<B, ClassSet1<B, T>> for T {
+    fn into_class_set(self) -> ClassSet1<B, T> {
+        ClassSet1 {
+            class: self,
+            _bus: PhantomData,
+        }
+    }
+}
+
+pub trait SerialState<'bus, 'a, B: UsbBus> {
+    fn get_serial(&mut self) -> &mut UsbSerial<'bus, 'a, B>;
+}
+
+impl<'bus, 'a, B: UsbBus> SerialState<'bus, 'a, B> for ClassSet1<B, UsbSerial<'bus, 'a, B>> {
+    fn get_serial(&mut self) -> &mut UsbSerial<'bus, 'a, B> {
+        &mut self.class
+    }
+}
diff --git a/embassy-stm32f4/src/usb_serial.rs b/embassy-stm32f4/src/usb_serial.rs
new file mode 100644
index 000000000..284d7e5f6
--- /dev/null
+++ b/embassy-stm32f4/src/usb_serial.rs
@@ -0,0 +1,290 @@
+use core::cell::RefCell;
+use core::marker::{PhantomData, PhantomPinned};
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+use embassy::io::{self, AsyncBufRead, AsyncWrite};
+use embassy::util::WakerRegistration;
+use usb_device::bus::UsbBus;
+use usb_device::class_prelude::*;
+use usb_device::UsbError;
+
+use crate::cdc_acm::CdcAcmClass;
+use crate::usb::{ClassSet, SerialState, State};
+use crate::util::peripheral::PeripheralMutex;
+use crate::util::ring_buffer::RingBuffer;
+
+pub struct ReadInterface<'a, 'bus, 'c, B: UsbBus, T: SerialState<'bus, 'c, B> + ClassSet<B>> {
+    // Don't you dare moving out `PeripheralMutex`
+    pub(crate) inner: &'a RefCell<PeripheralMutex<State<'bus, B, T>>>,
+    pub(crate) _buf_lifetime: PhantomData<&'c T>,
+}
+
+/// Write interface for USB CDC_ACM
+///
+/// This interface is buffered, meaning that after the write returns the bytes might not be fully
+/// on the wire just yet
+pub struct WriteInterface<'a, 'bus, 'c, B: UsbBus, T: SerialState<'bus, 'c, B> + ClassSet<B>> {
+    // Don't you dare moving out `PeripheralMutex`
+    pub(crate) inner: &'a RefCell<PeripheralMutex<State<'bus, B, T>>>,
+    pub(crate) _buf_lifetime: PhantomData<&'c T>,
+}
+
+impl<'a, 'bus, 'c, B, T> AsyncBufRead for ReadInterface<'a, 'bus, 'c, B, T>
+where
+    B: UsbBus,
+    T: SerialState<'bus, 'c, B> + ClassSet<B>,
+{
+    fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
+        let this = self.get_mut();
+        let mut mutex = this.inner.borrow_mut();
+        let mutex = unsafe { Pin::new_unchecked(&mut *mutex) };
+        mutex.with(|state, _irq| {
+            let serial = state.classes.get_serial();
+            let serial = Pin::new(serial);
+
+            match serial.poll_fill_buf(cx) {
+                Poll::Ready(Ok(buf)) => {
+                    let buf: &[u8] = buf;
+                    let buf: &[u8] = unsafe { core::mem::transmute(buf) };
+                    Poll::Ready(Ok(buf))
+                }
+                Poll::Ready(Err(_)) => Poll::Ready(Err(io::Error::Other)),
+                Poll::Pending => Poll::Pending,
+            }
+        })
+    }
+
+    fn consume(self: Pin<&mut Self>, amt: usize) {
+        let this = self.get_mut();
+        let mut mutex = this.inner.borrow_mut();
+        let mutex = unsafe { Pin::new_unchecked(&mut *mutex) };
+        mutex.with(|state, _irq| {
+            let serial = state.classes.get_serial();
+            let serial = Pin::new(serial);
+
+            serial.consume(amt);
+        })
+    }
+}
+
+impl<'a, 'bus, 'c, B, T> AsyncWrite for WriteInterface<'a, 'bus, 'c, B, T>
+where
+    B: UsbBus,
+    T: SerialState<'bus, 'c, B> + ClassSet<B>,
+{
+    fn poll_write(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        let this = self.get_mut();
+        let mut mutex = this.inner.borrow_mut();
+        let mutex = unsafe { Pin::new_unchecked(&mut *mutex) };
+        mutex.with(|state, _irq| {
+            let serial = state.classes.get_serial();
+            let serial = Pin::new(serial);
+
+            serial.poll_write(cx, buf)
+        })
+    }
+}
+
+pub struct UsbSerial<'bus, 'a, B: UsbBus> {
+    inner: CdcAcmClass<'bus, B>,
+    read_buf: RingBuffer<'a>,
+    write_buf: RingBuffer<'a>,
+    read_waker: WakerRegistration,
+    write_waker: WakerRegistration,
+    write_state: WriteState,
+    read_error: bool,
+    write_error: bool,
+}
+
+impl<'bus, 'a, B: UsbBus> AsyncBufRead for UsbSerial<'bus, 'a, B> {
+    fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
+        let this = self.get_mut();
+
+        if this.read_error {
+            this.read_error = false;
+            return Poll::Ready(Err(io::Error::Other));
+        }
+
+        let buf = this.read_buf.pop_buf();
+        if buf.is_empty() {
+            this.read_waker.register(cx.waker());
+            return Poll::Pending;
+        }
+        Poll::Ready(Ok(buf))
+    }
+
+    fn consume(self: Pin<&mut Self>, amt: usize) {
+        self.get_mut().read_buf.pop(amt);
+    }
+}
+
+impl<'bus, 'a, B: UsbBus> AsyncWrite for UsbSerial<'bus, 'a, B> {
+    fn poll_write(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        let this = self.get_mut();
+
+        if this.write_error {
+            this.write_error = false;
+            return Poll::Ready(Err(io::Error::Other));
+        }
+
+        let write_buf = this.write_buf.push_buf();
+        if write_buf.is_empty() {
+            this.write_waker.register(cx.waker());
+            return Poll::Pending;
+        }
+
+        let count = write_buf.len().min(buf.len());
+        write_buf[..count].copy_from_slice(&buf[..count]);
+        this.write_buf.push(count);
+
+        this.flush_write();
+        Poll::Ready(Ok(count))
+    }
+}
+
+/// Keeps track of the type of the last written packet.
+enum WriteState {
+    /// No packets in-flight
+    Idle,
+
+    /// Short packet currently in-flight
+    Short,
+
+    /// Full packet current in-flight. A full packet must be followed by a short packet for the host
+    /// OS to see the transaction. The data is the number of subsequent full packets sent so far. A
+    /// short packet is forced every SHORT_PACKET_INTERVAL packets so that the OS sees data in a
+    /// timely manner.
+    Full(usize),
+}
+
+impl<'bus, 'a, B: UsbBus> UsbSerial<'bus, 'a, B> {
+    pub fn new(
+        alloc: &'bus UsbBusAllocator<B>,
+        read_buf: &'a mut [u8],
+        write_buf: &'a mut [u8],
+    ) -> Self {
+        Self {
+            inner: CdcAcmClass::new(alloc, 64),
+            read_buf: RingBuffer::new(read_buf),
+            write_buf: RingBuffer::new(write_buf),
+            read_waker: WakerRegistration::new(),
+            write_waker: WakerRegistration::new(),
+            write_state: WriteState::Idle,
+            read_error: false,
+            write_error: false,
+        }
+    }
+
+    fn flush_write(&mut self) {
+        /// If this many full size packets have been sent in a row, a short packet will be sent so that the
+        /// host sees the data in a timely manner.
+        const SHORT_PACKET_INTERVAL: usize = 10;
+
+        let full_size_packets = match self.write_state {
+            WriteState::Full(c) => c,
+            _ => 0,
+        };
+
+        let ep_size = self.inner.max_packet_size() as usize;
+        let max_size = if full_size_packets > SHORT_PACKET_INTERVAL {
+            ep_size - 1
+        } else {
+            ep_size
+        };
+
+        let buf = {
+            let buf = self.write_buf.pop_buf();
+            if buf.len() > max_size {
+                &buf[..max_size]
+            } else {
+                buf
+            }
+        };
+
+        if !buf.is_empty() {
+            let count = match self.inner.write_packet(buf) {
+                Ok(c) => c,
+                Err(UsbError::WouldBlock) => 0,
+                Err(_) => {
+                    self.write_error = true;
+                    return;
+                }
+            };
+
+            if buf.len() == ep_size {
+                self.write_state = WriteState::Full(full_size_packets + 1);
+            } else {
+                self.write_state = WriteState::Short;
+            }
+            self.write_buf.pop(count);
+        } else if full_size_packets > 0 {
+            if let Err(e) = self.inner.write_packet(&[]) {
+                if !matches!(e, UsbError::WouldBlock) {
+                    self.write_error = true;
+                }
+                return;
+            }
+            self.write_state = WriteState::Idle;
+        }
+    }
+}
+
+impl<B> UsbClass<B> for UsbSerial<'_, '_, B>
+where
+    B: UsbBus,
+{
+    fn get_configuration_descriptors(&self, writer: &mut DescriptorWriter) -> Result<(), UsbError> {
+        self.inner.get_configuration_descriptors(writer)
+    }
+
+    fn reset(&mut self) {
+        self.inner.reset();
+        self.read_buf.clear();
+        self.write_buf.clear();
+        self.write_state = WriteState::Idle;
+    }
+
+    fn endpoint_in_complete(&mut self, addr: EndpointAddress) {
+        if addr == self.inner.write_ep_address() {
+            self.write_waker.wake();
+
+            self.flush_write();
+        }
+    }
+
+    fn endpoint_out(&mut self, addr: EndpointAddress) {
+        if addr == self.inner.read_ep_address() {
+            let buf = self.read_buf.push_buf();
+            let count = match self.inner.read_packet(buf) {
+                Ok(c) => c,
+                Err(UsbError::WouldBlock) => 0,
+                Err(_) => {
+                    self.read_error = true;
+                    return;
+                }
+            };
+
+            if count > 0 {
+                self.read_buf.push(count);
+                self.read_waker.wake();
+            }
+        }
+    }
+
+    fn control_in(&mut self, xfer: ControlIn<B>) {
+        self.inner.control_in(xfer);
+    }
+
+    fn control_out(&mut self, xfer: ControlOut<B>) {
+        self.inner.control_out(xfer);
+    }
+}
diff --git a/embassy-stm32f4/src/util/mod.rs b/embassy-stm32f4/src/util/mod.rs
new file mode 100644
index 000000000..cf3306545
--- /dev/null
+++ b/embassy-stm32f4/src/util/mod.rs
@@ -0,0 +1,12 @@
+pub mod peripheral;
+pub mod ring_buffer;
+
+/// Low power blocking wait loop using WFE/SEV.
+pub fn low_power_wait_until(mut condition: impl FnMut() -> bool) {
+    while !condition() {
+        // WFE might "eat" an event that would have otherwise woken the executor.
+        cortex_m::asm::wfe();
+    }
+    // Retrigger an event to be transparent to the executor.
+    cortex_m::asm::sev();
+}
diff --git a/embassy-stm32f4/src/util/peripheral.rs b/embassy-stm32f4/src/util/peripheral.rs
new file mode 100644
index 000000000..f2c7912ff
--- /dev/null
+++ b/embassy-stm32f4/src/util/peripheral.rs
@@ -0,0 +1,78 @@
+use core::cell::UnsafeCell;
+use core::marker::{PhantomData, PhantomPinned};
+use core::pin::Pin;
+use core::sync::atomic::{compiler_fence, Ordering};
+
+use crate::interrupt::OwnedInterrupt;
+
+pub trait PeripheralState {
+    type Interrupt: OwnedInterrupt;
+    fn on_interrupt(&mut self);
+}
+
+pub struct PeripheralMutex<S: PeripheralState> {
+    inner: Option<(UnsafeCell<S>, S::Interrupt)>,
+    _not_send: PhantomData<*mut ()>,
+    _pinned: PhantomPinned,
+}
+
+impl<S: PeripheralState> PeripheralMutex<S> {
+    pub fn new(state: S, irq: S::Interrupt) -> Self {
+        Self {
+            inner: Some((UnsafeCell::new(state), irq)),
+            _not_send: PhantomData,
+            _pinned: PhantomPinned,
+        }
+    }
+
+    pub fn with<R>(self: Pin<&mut Self>, f: impl FnOnce(&mut S, &mut S::Interrupt) -> R) -> R {
+        let this = unsafe { self.get_unchecked_mut() };
+        let (state, irq) = unwrap!(this.inner.as_mut());
+
+        irq.disable();
+        compiler_fence(Ordering::SeqCst);
+
+        irq.set_handler(
+            |p| {
+                // Safety: it's OK to get a &mut to the state, since
+                // - We're in the IRQ, no one else can't preempt us
+                // - We can't have preempted a with() call because the irq is disabled during it.
+                let state = unsafe { &mut *(p as *mut S) };
+                state.on_interrupt();
+            },
+            state.get() as *mut (),
+        );
+
+        // Safety: it's OK to get a &mut to the state, since the irq is disabled.
+        let state = unsafe { &mut *state.get() };
+
+        let r = f(state, irq);
+
+        compiler_fence(Ordering::SeqCst);
+        irq.enable();
+
+        r
+    }
+
+    pub fn try_free(self: Pin<&mut Self>) -> Option<(S, S::Interrupt)> {
+        let this = unsafe { self.get_unchecked_mut() };
+        this.inner.take().map(|(state, irq)| {
+            irq.disable();
+            irq.remove_handler();
+            (state.into_inner(), irq)
+        })
+    }
+
+    pub fn free(self: Pin<&mut Self>) -> (S, S::Interrupt) {
+        unwrap!(self.try_free())
+    }
+}
+
+impl<S: PeripheralState> Drop for PeripheralMutex<S> {
+    fn drop(&mut self) {
+        if let Some((_state, irq)) = &mut self.inner {
+            irq.disable();
+            irq.remove_handler();
+        }
+    }
+}
diff --git a/embassy-stm32f4/src/util/ring_buffer.rs b/embassy-stm32f4/src/util/ring_buffer.rs
new file mode 100644
index 000000000..0ef66f00a
--- /dev/null
+++ b/embassy-stm32f4/src/util/ring_buffer.rs
@@ -0,0 +1,86 @@
+use crate::fmt::{assert, *};
+
+pub struct RingBuffer<'a> {
+    buf: &'a mut [u8],
+    start: usize,
+    end: usize,
+    empty: bool,
+}
+
+impl<'a> RingBuffer<'a> {
+    pub fn new(buf: &'a mut [u8]) -> Self {
+        Self {
+            buf,
+            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 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
+        }
+    }
+}