embassy/embassy-hal-common/src/peripheral.rs

use core::marker::PhantomData;
use core::ops::{Deref, DerefMut};

/// An exclusive reference to a peripheral.
///
/// This is functionally the same as a `&'a mut T`. The reason for having a
/// dedicated struct is memory efficiency:
///
/// Peripheral singletons are typically either zero-sized (for concrete peripherals
/// like `PA9` or `Spi4`) or very small (for example `AnyPin` which is 1 byte).
/// However `&mut T` is always 4 bytes for 32-bit targets, even if T is zero-sized.
/// PeripheralRef stores a copy of `T` instead, so it's the same size.
///
/// but it is the size of `T` not the size
/// of a pointer. This is useful if T is a zero sized type.
pub struct PeripheralRef<'a, T> {
    inner: T,
    _lifetime: PhantomData<&'a mut T>,
}

impl<'a, T> PeripheralRef<'a, T> {
    #[inline]
    pub fn new(inner: T) -> Self {
        Self {
            inner,
            _lifetime: PhantomData,
        }
    }

    /// Unsafely clone (duplicate) a peripheral singleton.
    ///
    /// # Safety
    ///
    /// This returns an owned clone of the peripheral. You must manually ensure
    /// only one copy of the peripheral is in use at a time. For example, don't
    /// create two SPI drivers on `SPI1`, because they will "fight" each other.
    ///
    /// You should strongly prefer using `reborrow()` instead. It returns a
    /// `PeripheralRef` that borrows `self`, which allows the borrow checker
    /// to enforce this at compile time.
    pub unsafe fn clone_unchecked(&mut self) -> PeripheralRef<'a, T>
    where
        T: Peripheral<P = T>,
    {
        PeripheralRef::new(self.inner.clone_unchecked())
    }

    /// Reborrow into a "child" PeripheralRef.
    ///
    /// `self` will stay borrowed until the child PeripheralRef is dropped.
    pub fn reborrow(&mut self) -> PeripheralRef<'_, T>
    where
        T: Peripheral<P = T>,
    {
        // safety: we're returning the clone inside a new PeripheralRef that borrows
        // self, so user code can't use both at the same time.
        PeripheralRef::new(unsafe { self.inner.clone_unchecked() })
    }

    /// Map the inner peripheral using `Into`.
    ///
    /// This converts from `PeripheralRef<'a, T>` to `PeripheralRef<'a, U>`, using an
    /// `Into` impl to convert from `T` to `U`.
    ///
    /// For example, this can be useful to degrade GPIO pins: converting from PeripheralRef<'a, PB11>` to `PeripheralRef<'a, AnyPin>`.
    #[inline]
    pub fn map_into<U>(self) -> PeripheralRef<'a, U>
    where
        T: Into<U>,
    {
        PeripheralRef {
            inner: self.inner.into(),
            _lifetime: PhantomData,
        }
    }
}

impl<'a, T> Deref for PeripheralRef<'a, T> {
    type Target = T;

    #[inline]
    fn deref(&self) -> &Self::Target {
        &self.inner
    }
}

impl<'a, T> DerefMut for PeripheralRef<'a, T> {
    #[inline]
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.inner
    }
}

/// Trait for any type that can be used as a peripheral of type `P`.
///
/// This is used in driver constructors, to allow passing either owned peripherals (e.g. `TWISPI0`),
/// or borrowed peripherals (e.g. `&mut TWISPI0`).
///
/// For example, if you have a driver with a constructor like this:
///
/// ```ignore
/// impl<'d, T: Instance> Twim<'d, T> {
///     pub fn new(
///         twim: impl Peripheral<P = T> + 'd,
///         irq: impl Peripheral<P = T::Interrupt> + 'd,
///         sda: impl Peripheral<P = impl GpioPin> + 'd,
///         scl: impl Peripheral<P = impl GpioPin> + 'd,
///         config: Config,
///     ) -> Self { .. }
/// }
/// ```
///
/// You may call it with owned peripherals, which yields an instance that can live forever (`'static`):
///
/// ```ignore
/// let mut twi: Twim<'static, ...> = Twim::new(p.TWISPI0, irq, p.P0_03, p.P0_04, config);
/// ```
///
/// Or you may call it with borrowed peripherals, which yields an instance that can only live for as long
/// as the borrows last:
///
/// ```ignore
/// let mut twi: Twim<'_, ...> = Twim::new(&mut p.TWISPI0, &mut irq, &mut p.P0_03, &mut p.P0_04, config);
/// ```
///
/// # Implementation details, for HAL authors
///
/// When writing a HAL, the intended way to use this trait is to take `impl Peripheral<P = ..>` in
/// the HAL's public API (such as driver constructors), calling `.into_ref()` to obtain a `PeripheralRef`,
/// and storing that in the driver struct.
///
/// `.into_ref()` on an owned `T` yields a `PeripheralRef<'static, T>`.
/// `.into_ref()` on an `&'a mut T` yields a `PeripheralRef<'a, T>`.
pub trait Peripheral: Sized {
    /// Peripheral singleton type
    type P;

    /// Unsafely clone (duplicate) a peripheral singleton.
    ///
    /// # Safety
    ///
    /// This returns an owned clone of the peripheral. You must manually ensure
    /// only one copy of the peripheral is in use at a time. For example, don't
    /// create two SPI drivers on `SPI1`, because they will "fight" each other.
    ///
    /// You should strongly prefer using `into_ref()` instead. It returns a
    /// `PeripheralRef`, which allows the borrow checker to enforce this at compile time.
    unsafe fn clone_unchecked(&mut self) -> Self::P;

    /// Convert a value into a `PeripheralRef`.
    ///
    /// When called on an owned `T`, yields a `PeripheralRef<'static, T>`.
    /// When called on an `&'a mut T`, yields a `PeripheralRef<'a, T>`.
    #[inline]
    fn into_ref<'a>(mut self) -> PeripheralRef<'a, Self::P>
    where
        Self: 'a,
    {
        PeripheralRef::new(unsafe { self.clone_unchecked() })
    }
}

impl<'b, T: DerefMut> Peripheral for T
where
    T::Target: Peripheral,
{
    type P = <T::Target as Peripheral>::P;

    #[inline]
    unsafe fn clone_unchecked(&mut self) -> Self::P {
        self.deref_mut().clone_unchecked()
    }
}