Add implementation of ReadUntilIdle for nRF UART

Add type UarteWithIdle that implements Read, Write and ReadUntilIdle
traits. The type uses a timer + 2 PPI channels internally, triggered on
RTXSTARTED event.

embassy-nrf/src/uarte.rs

@@ -5,7 +5,7 @@ use core::marker::PhantomData;
 use core::sync::atomic::{compiler_fence, Ordering};
 use core::task::Poll;
 use embassy::interrupt::InterruptExt;
-use embassy::traits::uart::{Error, Read, Write};
+use embassy::traits::uart::{Error, Read, ReadUntilIdle, Write};
 use embassy::util::{AtomicWaker, OnDrop, Unborrow};
 use embassy_extras::unborrow;
 use futures::future::poll_fn;
@@ -17,7 +17,9 @@ use crate::interrupt;
 use crate::interrupt::Interrupt;
 use crate::pac;
 use crate::peripherals;
+use crate::ppi::{AnyConfigurableChannel, ConfigurableChannel, Event, Ppi, Task};
 use crate::target_constants::EASY_DMA_SIZE;
+use crate::timer::Instance as TimerInstance;
 
 // Re-export SVD variants to allow user to directly set values.
 pub use pac::uarte0::{baudrate::BAUDRATE_A as Baudrate, config::PARITY_A as Parity};
@@ -281,6 +283,161 @@ impl<'d, T: Instance> Write for Uarte<'d, T> {
     }
 }
 
+/// Interface to an UARTE peripheral that uses timers and PPI to emulate
+/// ReadUntilIdle.
+pub struct UarteWithIdle<'d, U: Instance, T: TimerInstance> {
+    uarte: Uarte<'d, U>,
+    timer: T,
+    _ppi_ch1: Ppi<'d, AnyConfigurableChannel>,
+    _ppi_ch2: Ppi<'d, AnyConfigurableChannel>,
+}
+
+impl<'d, U: Instance, T: TimerInstance> UarteWithIdle<'d, U, T> {
+    /// Creates the interface to a UARTE instance.
+    /// Sets the baud rate, parity and assigns the pins to the UARTE peripheral.
+    ///
+    /// # Safety
+    ///
+    /// The returned API is safe unless you use `mem::forget` (or similar safe mechanisms)
+    /// on stack allocated buffers which which have been passed to [`send()`](Uarte::send)
+    /// or [`receive`](Uarte::receive).
+    #[allow(unused_unsafe)]
+    pub unsafe fn new(
+        uarte: impl Unborrow<Target = U> + 'd,
+        timer: impl Unborrow<Target = T> + 'd,
+        ppi_ch1: impl Unborrow<Target = impl ConfigurableChannel> + 'd,
+        ppi_ch2: impl Unborrow<Target = impl ConfigurableChannel> + 'd,
+        irq: impl Unborrow<Target = U::Interrupt> + 'd,
+        rxd: impl Unborrow<Target = impl GpioPin> + 'd,
+        txd: impl Unborrow<Target = impl GpioPin> + 'd,
+        cts: impl Unborrow<Target = impl GpioOptionalPin> + 'd,
+        rts: impl Unborrow<Target = impl GpioOptionalPin> + 'd,
+        config: Config,
+    ) -> Self {
+        let baudrate = config.baudrate;
+        let uarte = Uarte::new(uarte, irq, rxd, txd, cts, rts, config);
+
+        unborrow!(timer, ppi_ch1, ppi_ch2);
+
+        let r = U::regs();
+        let rt = timer.regs();
+
+        // BAUDRATE register values are `baudrate * 2^32 / 16000000`
+        // source: https://devzone.nordicsemi.com/f/nordic-q-a/391/uart-baudrate-register-values
+        //
+        // We want to stop RX if line is idle for 2 bytes worth of time
+        // That is 20 bits (each byte is 1 start bit + 8 data bits + 1 stop bit)
+        // This gives us the amount of 16M ticks for 20 bits.
+        let timeout = 0x8000_0000 / (baudrate as u32 / 40);
+
+        rt.tasks_stop.write(|w| unsafe { w.bits(1) });
+        rt.bitmode.write(|w| w.bitmode()._32bit());
+        rt.prescaler.write(|w| unsafe { w.prescaler().bits(0) });
+        rt.cc[0].write(|w| unsafe { w.bits(timeout) });
+        rt.mode.write(|w| w.mode().timer());
+        rt.shorts.write(|w| {
+            w.compare0_clear().set_bit();
+            w.compare0_stop().set_bit();
+            w
+        });
+
+        let mut ppi_ch1 = Ppi::new(ppi_ch1.degrade_configurable());
+        ppi_ch1.set_event(Event::from_reg(&r.events_rxstarted));
+        ppi_ch1.set_task(Task::from_reg(&rt.tasks_clear));
+        ppi_ch1.set_fork_task(Task::from_reg(&rt.tasks_start));
+        ppi_ch1.enable();
+
+        let mut ppi_ch2 = Ppi::new(ppi_ch2.degrade_configurable());
+        ppi_ch2.set_event(Event::from_reg(&rt.events_compare[0]));
+        ppi_ch2.set_task(Task::from_reg(&r.tasks_stoprx));
+        ppi_ch2.enable();
+
+        Self {
+            uarte,
+            timer,
+            _ppi_ch1: ppi_ch1,
+            _ppi_ch2: ppi_ch2,
+        }
+    }
+}
+
+impl<'d, U: Instance, T: TimerInstance> ReadUntilIdle for UarteWithIdle<'d, U, T> {
+    #[rustfmt::skip]
+    type ReadUntilIdleFuture<'a> where Self: 'a = impl Future<Output = Result<usize, Error>> + 'a;
+    fn read_until_idle<'a>(&'a mut self, rx_buffer: &'a mut [u8]) -> Self::ReadUntilIdleFuture<'a> {
+        async move {
+            let ptr = rx_buffer.as_ptr();
+            let len = rx_buffer.len();
+            assert!(len <= EASY_DMA_SIZE);
+
+            let r = U::regs();
+            let s = U::state();
+
+            let rt = self.timer.regs();
+
+            let drop = OnDrop::new(move || {
+                info!("read drop: stopping");
+
+                rt.tasks_stop.write(|w| unsafe { w.bits(1) });
+
+                r.intenclr.write(|w| w.endrx().clear());
+                r.events_rxto.reset();
+                r.tasks_stoprx.write(|w| unsafe { w.bits(1) });
+
+                while r.events_endrx.read().bits() == 0 {}
+
+                info!("read drop: stopped");
+            });
+
+            r.rxd.ptr.write(|w| unsafe { w.ptr().bits(ptr as u32) });
+            r.rxd.maxcnt.write(|w| unsafe { w.maxcnt().bits(len as _) });
+
+            r.events_endrx.reset();
+            r.intenset.write(|w| w.endrx().set());
+
+            compiler_fence(Ordering::SeqCst);
+
+            trace!("startrx");
+            r.tasks_startrx.write(|w| unsafe { w.bits(1) });
+
+            let n: usize = poll_fn(|cx| {
+                s.endrx_waker.register(cx.waker());
+                if r.events_endrx.read().bits() != 0 {
+                    let n: usize = r.rxd.amount.read().amount().bits() as usize;
+                    return Poll::Ready(n);
+                }
+                Poll::Pending
+            })
+            .await;
+
+            compiler_fence(Ordering::SeqCst);
+            r.events_rxstarted.reset();
+            // Stop timer
+            rt.tasks_stop.write(|w| unsafe { w.bits(1) });
+            drop.defuse();
+
+            Ok(n)
+        }
+    }
+}
+
+impl<'d, U: Instance, T: TimerInstance> Read for UarteWithIdle<'d, U, T> {
+    #[rustfmt::skip]
+    type ReadFuture<'a> where Self: 'a = impl Future<Output = Result<(), Error>> + 'a;
+    fn read<'a>(&'a mut self, rx_buffer: &'a mut [u8]) -> Self::ReadFuture<'a> {
+        self.uarte.read(rx_buffer)
+    }
+}
+
+impl<'d, U: Instance, T: TimerInstance> Write for UarteWithIdle<'d, U, T> {
+    #[rustfmt::skip]
+    type WriteFuture<'a> where Self: 'a = impl Future<Output = Result<(), Error>> + 'a;
+
+    fn write<'a>(&'a mut self, tx_buffer: &'a [u8]) -> Self::WriteFuture<'a> {
+        self.uarte.write(tx_buffer)
+    }
+}
+
 mod sealed {
     use super::*;