embassy/embassy-nrf/src/rng.rs

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use core::convert::Infallible;
use core::future::Future;
use core::marker::PhantomData;
use core::ptr;
use core::sync::atomic::AtomicPtr;
use core::sync::atomic::Ordering;
use core::task::Poll;
use embassy::interrupt::InterruptExt;
use embassy::traits;
use embassy::util::Unborrow;
use embassy::waitqueue::AtomicWaker;
use embassy_hal_common::drop::OnDrop;
use embassy_hal_common::unborrow;
use futures::future::poll_fn;
use rand_core::RngCore;
use crate::interrupt;
use crate::pac;
use crate::peripherals::RNG;
impl RNG {
fn regs() -> &'static pac::rng::RegisterBlock {
unsafe { &*pac::RNG::ptr() }
}
}
static STATE: State = State {
ptr: AtomicPtr::new(ptr::null_mut()),
end: AtomicPtr::new(ptr::null_mut()),
waker: AtomicWaker::new(),
};
struct State {
ptr: AtomicPtr<u8>,
end: AtomicPtr<u8>,
waker: AtomicWaker,
}
/// A wrapper around an nRF RNG peripheral.
///
/// It has a non-blocking API, through `embassy::traits::Rng`, and a blocking api through `rand`.
pub struct Rng<'d> {
irq: interrupt::RNG,
phantom: PhantomData<(&'d mut RNG, &'d mut interrupt::RNG)>,
}
impl<'d> Rng<'d> {
/// Creates a new RNG driver from the `RNG` peripheral and interrupt.
///
/// SAFETY: The future returned from `fill_bytes` must not have its lifetime end without running its destructor,
/// e.g. using `mem::forget`.
///
/// The synchronous API is safe.
pub unsafe fn new(
_rng: impl Unborrow<Target = RNG> + 'd,
irq: impl Unborrow<Target = interrupt::RNG> + 'd,
) -> Self {
unborrow!(irq);
let this = Self {
irq,
phantom: PhantomData,
};
this.stop();
this.disable_irq();
this.irq.set_handler(Self::on_interrupt);
this.irq.unpend();
this.irq.enable();
this
}
fn on_interrupt(_: *mut ()) {
// Clear the event.
RNG::regs().events_valrdy.reset();
// Mutate the slice within a critical section,
// so that the future isn't dropped in between us loading the pointer and actually dereferencing it.
let (ptr, end) = critical_section::with(|_| {
let ptr = STATE.ptr.load(Ordering::Relaxed);
// We need to make sure we haven't already filled the whole slice,
// in case the interrupt fired again before the executor got back to the future.
let end = STATE.end.load(Ordering::Relaxed);
if !ptr.is_null() && ptr != end {
// If the future was dropped, the pointer would have been set to null,
// so we're still good to mutate the slice.
// The safety contract of `Rng::new` means that the future can't have been dropped
// without calling its destructor.
unsafe {
*ptr = RNG::regs().value.read().value().bits();
}
}
(ptr, end)
});
if ptr.is_null() || ptr == end {
// If the future was dropped, there's nothing to do.
// If `ptr == end`, we were called by mistake, so return.
return;
}
let new_ptr = unsafe { ptr.add(1) };
match STATE
.ptr
.compare_exchange(ptr, new_ptr, Ordering::Relaxed, Ordering::Relaxed)
{
Ok(_) => {
let end = STATE.end.load(Ordering::Relaxed);
// It doesn't matter if `end` was changed under our feet, because then this will just be false.
if new_ptr == end {
STATE.waker.wake();
}
}
Err(_) => {
// If the future was dropped or finished, there's no point trying to wake it.
// It will have already stopped the RNG, so there's no need to do that either.
}
}
}
fn stop(&self) {
RNG::regs().tasks_stop.write(|w| unsafe { w.bits(1) })
}
fn start(&self) {
RNG::regs().tasks_start.write(|w| unsafe { w.bits(1) })
}
fn enable_irq(&self) {
RNG::regs().intenset.write(|w| w.valrdy().set());
}
fn disable_irq(&self) {
RNG::regs().intenclr.write(|w| w.valrdy().clear());
}
/// Enable or disable the RNG's bias correction.
///
/// Bias correction removes any bias towards a '1' or a '0' in the bits generated.
/// However, this makes the generation of numbers slower.
///
/// Defaults to disabled.
pub fn bias_correction(&self, enable: bool) {
RNG::regs().config.write(|w| w.dercen().bit(enable))
}
}
impl<'d> Drop for Rng<'d> {
fn drop(&mut self) {
self.irq.disable()
}
}
impl<'d> traits::rng::Rng for Rng<'d> {
type Error = Infallible;
#[rustfmt::skip] // For some reason rustfmt removes the where clause
type RngFuture<'a> where 'd: 'a = impl Future<Output = Result<(), Self::Error>> + 'a;
fn fill_bytes<'a>(&'a mut self, dest: &'a mut [u8]) -> Self::RngFuture<'a> {
async move {
if dest.len() == 0 {
return Ok(()); // Nothing to fill
}
let range = dest.as_mut_ptr_range();
// Even if we've preempted the interrupt, it can't preempt us again,
// so we don't need to worry about the order we write these in.
STATE.ptr.store(range.start, Ordering::Relaxed);
STATE.end.store(range.end, Ordering::Relaxed);
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self.enable_irq();
self.start();
let on_drop = OnDrop::new(|| {
self.stop();
self.disable_irq();
// The interrupt is now disabled and can't preempt us anymore, so the order doesn't matter here.
STATE.ptr.store(ptr::null_mut(), Ordering::Relaxed);
STATE.end.store(ptr::null_mut(), Ordering::Relaxed);
});
poll_fn(|cx| {
STATE.waker.register(cx.waker());
// The interrupt will never modify `end`, so load it first and then get the most up-to-date `ptr`.
let end = STATE.end.load(Ordering::Relaxed);
let ptr = STATE.ptr.load(Ordering::Relaxed);
if ptr == end {
// We're done.
Poll::Ready(())
} else {
Poll::Pending
}
})
.await;
// Trigger the teardown
drop(on_drop);
Ok(())
}
}
}
impl<'d> RngCore for Rng<'d> {
fn fill_bytes(&mut self, dest: &mut [u8]) {
self.start();
for byte in dest.iter_mut() {
let regs = RNG::regs();
while regs.events_valrdy.read().bits() == 0 {}
regs.events_valrdy.reset();
*byte = regs.value.read().value().bits();
}
self.stop();
}
fn next_u32(&mut self) -> u32 {
let mut bytes = [0; 4];
self.fill_bytes(&mut bytes);
// We don't care about the endianness, so just use the native one.
u32::from_ne_bytes(bytes)
}
fn next_u64(&mut self) -> u64 {
let mut bytes = [0; 8];
self.fill_bytes(&mut bytes);
u64::from_ne_bytes(bytes)
}
fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), rand_core::Error> {
self.fill_bytes(dest);
Ok(())
}
}
// TODO: Should `Rng` implement `CryptoRng`? It's 'suitable for cryptographic purposes' according to the specification.