stm32/uart: refactor rx ringbuffer

- remove some race conditions
- allow full use of rx buffer

7 files changed, 426 insertions, 428 deletions

diff --git a/.vscode/.gitignore b/.vscode/.gitignore
index 9fbb9ec95..8c3dd8a31 100644
--- a/.vscode/.gitignore
+++ b/.vscode/.gitignore
@@ -1,3 +1,4 @@
 *.cortex-debug.*.json
 launch.json
-tasks.json
\ No newline at end of file
+tasks.json
+*.cfg
diff --git a/embassy-stm32/src/dma/bdma.rs b/embassy-stm32/src/dma/bdma.rs
index 0202ec379..9dafa26d0 100644
--- a/embassy-stm32/src/dma/bdma.rs
+++ b/embassy-stm32/src/dma/bdma.rs
@@ -111,24 +111,18 @@ pub(crate) unsafe fn on_irq_inner(dma: pac::bdma::Dma, channel_num: usize, index
         panic!("DMA: error on BDMA@{:08x} channel {}", dma.0 as u32, channel_num);
     }
 
-    let mut wake = false;
-
     if isr.htif(channel_num) && cr.read().htie() {
         // Acknowledge half transfer complete interrupt
         dma.ifcr().write(|w| w.set_htif(channel_num, true));
-        wake = true;
-    }
-
-    if isr.tcif(channel_num) && cr.read().tcie() {
+    } else if isr.tcif(channel_num) && cr.read().tcie() {
         // Acknowledge transfer complete interrupt
         dma.ifcr().write(|w| w.set_tcif(channel_num, true));
         STATE.complete_count[index].fetch_add(1, Ordering::Release);
-        wake = true;
+    } else {
+        return;
     }
 
-    if wake {
-        STATE.ch_wakers[index].wake();
-    }
+    STATE.ch_wakers[index].wake();
 }
 
 #[cfg(any(bdma_v2, dmamux))]
@@ -371,7 +365,7 @@ impl<'a, C: Channel> Future for Transfer<'a, C> {
 struct DmaCtrlImpl<'a, C: Channel>(PeripheralRef<'a, C>);
 
 impl<'a, C: Channel> DmaCtrl for DmaCtrlImpl<'a, C> {
-    fn ndtr(&self) -> usize {
+    fn get_remaining_transfers(&self) -> usize {
         let ch = self.0.regs().ch(self.0.num());
         unsafe { ch.ndtr().read() }.ndt() as usize
     }
@@ -457,21 +451,17 @@ impl<'a, C: Channel, W: Word> RingBuffer<'a, C, W> {
     }
 
     /// Read bytes from the ring buffer
+    /// Return a tuple of the length read and the length remaining in the buffer
+    /// If not all of the bytes were read, then there will be some bytes in the buffer remaining
+    /// The length remaining is the capacity, ring_buf.len(), less the bytes remaining after the read
     /// OverrunError is returned if the portion to be read was overwritten by the DMA controller.
-    pub fn read(&mut self, buf: &mut [W]) -> Result<usize, OverrunError> {
+    pub fn read(&mut self, buf: &mut [W]) -> Result<(usize, usize), OverrunError> {
         self.ringbuf.read(DmaCtrlImpl(self.channel.reborrow()), buf)
     }
 
-    pub fn is_empty(&self) -> bool {
-        self.ringbuf.is_empty()
-    }
-
-    pub fn len(&self) -> usize {
-        self.ringbuf.len()
-    }
-
-    pub fn capacity(&self) -> usize {
-        self.ringbuf.dma_buf.len()
+    /// The capacity of the ringbuffer
+    pub fn cap(&self) -> usize {
+        self.ringbuf.cap()
     }
 
     pub fn set_waker(&mut self, waker: &Waker) {
@@ -506,12 +496,6 @@ impl<'a, C: Channel, W: Word> RingBuffer<'a, C, W> {
         let ch = self.channel.regs().ch(self.channel.num());
         unsafe { ch.cr().read() }.en()
     }
-
-    /// Synchronize the position of the ring buffer to the actual DMA controller position
-    pub fn reload_position(&mut self) {
-        let ch = self.channel.regs().ch(self.channel.num());
-        self.ringbuf.ndtr = unsafe { ch.ndtr().read() }.ndt() as usize;
-    }
 }
 
 impl<'a, C: Channel, W: Word> Drop for RingBuffer<'a, C, W> {
diff --git a/embassy-stm32/src/dma/dma.rs b/embassy-stm32/src/dma/dma.rs
index 7b17d9e49..47b749ece 100644
--- a/embassy-stm32/src/dma/dma.rs
+++ b/embassy-stm32/src/dma/dma.rs
@@ -187,24 +187,18 @@ pub(crate) unsafe fn on_irq_inner(dma: pac::dma::Dma, channel_num: usize, index:
         panic!("DMA: error on DMA@{:08x} channel {}", dma.0 as u32, channel_num);
     }
 
-    let mut wake = false;
-
     if isr.htif(channel_num % 4) && cr.read().htie() {
         // Acknowledge half transfer complete interrupt
         dma.ifcr(channel_num / 4).write(|w| w.set_htif(channel_num % 4, true));
-        wake = true;
-    }
-
-    if isr.tcif(channel_num % 4) && cr.read().tcie() {
+    } else if isr.tcif(channel_num % 4) && cr.read().tcie() {
         // Acknowledge  transfer complete interrupt
         dma.ifcr(channel_num / 4).write(|w| w.set_tcif(channel_num % 4, true));
         STATE.complete_count[index].fetch_add(1, Ordering::Release);
-        wake = true;
+    } else {
+        return;
     }
 
-    if wake {
-        STATE.ch_wakers[index].wake();
-    }
+    STATE.ch_wakers[index].wake();
 }
 
 #[cfg(any(dma_v2, dmamux))]
@@ -612,7 +606,7 @@ impl<'a, C: Channel, W: Word> Drop for DoubleBuffered<'a, C, W> {
 struct DmaCtrlImpl<'a, C: Channel>(PeripheralRef<'a, C>);
 
 impl<'a, C: Channel> DmaCtrl for DmaCtrlImpl<'a, C> {
-    fn ndtr(&self) -> usize {
+    fn get_remaining_transfers(&self) -> usize {
         let ch = self.0.regs().st(self.0.num());
         unsafe { ch.ndtr().read() }.ndt() as usize
     }
@@ -713,21 +707,17 @@ impl<'a, C: Channel, W: Word> RingBuffer<'a, C, W> {
     }
 
     /// Read bytes from the ring buffer
+    /// Return a tuple of the length read and the length remaining in the buffer
+    /// If not all of the bytes were read, then there will be some bytes in the buffer remaining
+    /// The length remaining is the capacity, ring_buf.len(), less the bytes remaining after the read
     /// OverrunError is returned if the portion to be read was overwritten by the DMA controller.
-    pub fn read(&mut self, buf: &mut [W]) -> Result<usize, OverrunError> {
+    pub fn read(&mut self, buf: &mut [W]) -> Result<(usize, usize), OverrunError> {
         self.ringbuf.read(DmaCtrlImpl(self.channel.reborrow()), buf)
     }
 
-    pub fn is_empty(&self) -> bool {
-        self.ringbuf.is_empty()
-    }
-
-    pub fn len(&self) -> usize {
-        self.ringbuf.len()
-    }
-
-    pub fn capacity(&self) -> usize {
-        self.ringbuf.dma_buf.len()
+    // The capacity of the ringbuffer
+    pub fn cap(&self) -> usize {
+        self.ringbuf.cap()
     }
 
     pub fn set_waker(&mut self, waker: &Waker) {
@@ -766,12 +756,6 @@ impl<'a, C: Channel, W: Word> RingBuffer<'a, C, W> {
         let ch = self.channel.regs().st(self.channel.num());
         unsafe { ch.cr().read() }.en()
     }
-
-    /// Synchronize the position of the ring buffer to the actual DMA controller position
-    pub fn reload_position(&mut self) {
-        let ch = self.channel.regs().st(self.channel.num());
-        self.ringbuf.ndtr = unsafe { ch.ndtr().read() }.ndt() as usize;
-    }
 }
 
 impl<'a, C: Channel, W: Word> Drop for RingBuffer<'a, C, W> {
diff --git a/embassy-stm32/src/dma/ringbuffer.rs b/embassy-stm32/src/dma/ringbuffer.rs
index 38cc87ae9..72a84a57f 100644
--- a/embassy-stm32/src/dma/ringbuffer.rs
+++ b/embassy-stm32/src/dma/ringbuffer.rs
@@ -25,14 +25,13 @@ use super::word::Word;
 /// +-----------------------------------------+     +-----------------------------------------+
 ///  ^          ^               ^                    ^            ^           ^
 ///  |          |               |                    |            |           |
-///  +- first --+               |                    +- end ------+           |
+///  +- start --+               |                    +- end ------+           |
 ///  |                          |                    |                        |
-///  +- end --------------------+                    +- first ----------------+
+///  +- end --------------------+                    +- start ----------------+
 /// ```
 pub struct DmaRingBuffer<'a, W: Word> {
     pub(crate) dma_buf: &'a mut [W],
-    first: usize,
-    pub ndtr: usize,
+    start: usize,
 }
 
 #[derive(Debug, PartialEq)]
@@ -41,7 +40,7 @@ pub struct OverrunError;
 pub trait DmaCtrl {
     /// Get the NDTR register value, i.e. the space left in the underlying
     /// buffer until the dma writer wraps.
-    fn ndtr(&self) -> usize;
+    fn get_remaining_transfers(&self) -> usize;
 
     /// Get the transfer completed counter.
     /// This counter is incremented by the dma controller when NDTR is reloaded,
@@ -54,151 +53,131 @@ pub trait DmaCtrl {
 
 impl<'a, W: Word> DmaRingBuffer<'a, W> {
     pub fn new(dma_buf: &'a mut [W]) -> Self {
-        let ndtr = dma_buf.len();
-        Self {
-            dma_buf,
-            first: 0,
-            ndtr,
-        }
+        Self { dma_buf, start: 0 }
     }
 
     /// Reset the ring buffer to its initial state
     pub fn clear(&mut self, mut dma: impl DmaCtrl) {
-        self.first = 0;
-        self.ndtr = self.dma_buf.len();
+        self.start = 0;
         dma.reset_complete_count();
     }
 
-    /// The buffer end position
-    fn end(&self) -> usize {
-        self.dma_buf.len() - self.ndtr
-    }
-
-    /// Returns whether the buffer is empty
-    pub fn is_empty(&self) -> bool {
-        self.first == self.end()
+    /// The capacity of the ringbuffer
+    pub const fn cap(&self) -> usize {
+        self.dma_buf.len()
     }
 
-    /// The current number of bytes in the buffer
-    /// This may change at any time if dma is currently active
-    pub fn len(&self) -> usize {
-        // Read out a stable end (the dma periheral can change it at anytime)
-        let end = self.end();
-        if self.first <= end {
-            // No wrap
-            end - self.first
-        } else {
-            self.dma_buf.len() - self.first + end
-        }
+    /// The current position of the ringbuffer
+    fn pos(&self, remaining_transfers: usize) -> usize {
+        self.cap() - remaining_transfers
     }
 
     /// Read bytes from the ring buffer
+    /// Return a tuple of the length read and the length remaining in the buffer
+    /// If not all of the bytes were read, then there will be some bytes in the buffer remaining
+    /// The length remaining is the capacity, ring_buf.len(), less the bytes remaining after the read
     /// OverrunError is returned if the portion to be read was overwritten by the DMA controller.
-    pub fn read(&mut self, mut dma: impl DmaCtrl, buf: &mut [W]) -> Result<usize, OverrunError> {
-        let end = self.end();
+    pub fn read(&mut self, mut dma: impl DmaCtrl, buf: &mut [W]) -> Result<(usize, usize), OverrunError> {
+        /*
+            This algorithm is optimistic: we assume we haven't overrun more than a full buffer and then check
+            after we've done our work to see we have. This is because on stm32, an interrupt is not guaranteed
+            to fire in the same clock cycle that a register is read, so checking get_complete_count early does
+            not yield relevant information.
+
+            Therefore, the only variable we really need to know is ndtr. If the dma has overrun by more than a full
+            buffer, we will do a bit more work than we have to, but algorithms should not be optimized for error
+            conditions.
+
+            After we've done our work, we confirm that we haven't overrun more than a full buffer, and also that
+            the dma has not overrun within the data we could have copied. We check the data we could have copied
+            rather than the data we actually copied because it costs nothing and confirms an error condition
+            earlier.
+        */
+        let end = self.pos(dma.get_remaining_transfers());
+        if self.start == end && dma.get_complete_count() == 0 {
+            // No bytes are available in the buffer
+            Ok((0, self.cap()))
+        } else if self.start < end {
+            // The available, unread portion in the ring buffer DOES NOT wrap
+            // Copy out the bytes from the dma buffer
+            let len = self.copy_to(buf, self.start..end);
 
-        compiler_fence(Ordering::SeqCst);
+            compiler_fence(Ordering::SeqCst);
 
-        if self.first == end {
-            // The buffer is currently empty
+            /*
+                first, check if the dma has wrapped at all if it's after end
+                or more than once if it's before start
 
-            if dma.get_complete_count() > 0 {
-                // The DMA has written such that the ring buffer wraps at least once
-                self.ndtr = dma.ndtr();
-                if self.end() > self.first || dma.get_complete_count() > 1 {
-                    return Err(OverrunError);
-                }
-            }
+                this is in a critical section to try to reduce mushy behavior.
+                it's not ideal but it's the best we can do
 
-            Ok(0)
-        } else if self.first < end {
-            // The available, unread portion in the ring buffer DOES NOT wrap
+                then, get the current position of of the dma write and check
+                if it's inside data we could have copied
+            */
+            let (pos, complete_count) =
+                critical_section::with(|_| (self.pos(dma.get_remaining_transfers()), dma.get_complete_count()));
+            if (pos >= self.start && pos < end) || (complete_count > 0 && pos >= end) || complete_count > 1 {
+                Err(OverrunError)
+            } else {
+                self.start = (self.start + len) % self.cap();
 
-            if dma.get_complete_count() > 1 {
-                return Err(OverrunError);
+                Ok((len, self.cap() - self.start))
             }
+        } else if self.start + buf.len() < self.cap() {
+            // The available, unread portion in the ring buffer DOES wrap
+            // The DMA writer has wrapped since we last read and is currently
+            // writing (or the next byte added will be) in the beginning of the ring buffer.
 
-            // Copy out the bytes from the dma buffer
-            let len = self.copy_to(buf, self.first..end);
+            // The provided read buffer is not large enough to include all bytes from the tail of the dma buffer.
+
+            // Copy out from the dma buffer
+            let len = self.copy_to(buf, self.start..self.cap());
 
             compiler_fence(Ordering::SeqCst);
 
-            match dma.get_complete_count() {
-                0 => {
-                    // The DMA writer has not wrapped before nor after the copy
-                }
-                1 => {
-                    // The DMA writer has written such that the ring buffer now wraps
-                    self.ndtr = dma.ndtr();
-                    if self.end() > self.first || dma.get_complete_count() > 1 {
-                        // The bytes that we have copied out have overflowed
-                        // as the writer has now both wrapped and is currently writing
-                        // within the region that we have just copied out
-                        return Err(OverrunError);
-                    }
-                }
-                _ => {
-                    return Err(OverrunError);
-                }
-            }
+            /*
+                first, check if the dma has wrapped around more than once
 
-            self.first = (self.first + len) % self.dma_buf.len();
-            Ok(len)
+                then, get the current position of of the dma write and check
+                if it's inside data we could have copied
+            */
+            let pos = self.pos(dma.get_remaining_transfers());
+            if pos > self.start || pos < end || dma.get_complete_count() > 1 {
+                Err(OverrunError)
+            } else {
+                self.start = (self.start + len) % self.cap();
+
+                Ok((len, self.start + end))
+            }
         } else {
             // The available, unread portion in the ring buffer DOES wrap
             // The DMA writer has wrapped since we last read and is currently
             // writing (or the next byte added will be) in the beginning of the ring buffer.
 
-            let complete_count = dma.get_complete_count();
-            if complete_count > 1 {
-                return Err(OverrunError);
-            }
-
-            // If the unread portion wraps then the writer must also have wrapped
-            assert!(complete_count == 1);
-
-            if self.first + buf.len() < self.dma_buf.len() {
-                // The provided read buffer is not large enough to include all bytes from the tail of the dma buffer.
+            // The provided read buffer is large enough to include all bytes from the tail of the dma buffer,
+            // so the next read will not have any unread tail bytes in the ring buffer.
 
-                // Copy out from the dma buffer
-                let len = self.copy_to(buf, self.first..self.dma_buf.len());
+            // Copy out from the dma buffer
+            let tail = self.copy_to(buf, self.start..self.cap());
+            let head = self.copy_to(&mut buf[tail..], 0..end);
 
-                compiler_fence(Ordering::SeqCst);
+            compiler_fence(Ordering::SeqCst);
 
-                // We have now copied out the data from dma_buf
-                // Make sure that the just read part was not overwritten during the copy
-                self.ndtr = dma.ndtr();
-                if self.end() > self.first || dma.get_complete_count() > 1 {
-                    // The writer has entered the data that we have just read since we read out `end` in the beginning and until now.
-                    return Err(OverrunError);
-                }
+            /*
+                first, check if the dma has wrapped around more than once
 
-                self.first = (self.first + len) % self.dma_buf.len();
-                Ok(len)
+                then, get the current position of of the dma write and check
+                if it's inside data we could have copied
+            */
+            let pos = self.pos(dma.get_remaining_transfers());
+            if pos > self.start || pos < end || dma.reset_complete_count() > 1 {
+                Err(OverrunError)
             } else {
-                // The provided read buffer is large enough to include all bytes from the tail of the dma buffer,
-                // so the next read will not have any unread tail bytes in the ring buffer.
-
-                // Copy out from the dma buffer
-                let tail = self.copy_to(buf, self.first..self.dma_buf.len());
-                let head = self.copy_to(&mut buf[tail..], 0..end);
-
-                compiler_fence(Ordering::SeqCst);
-
-                // We have now copied out the data from dma_buf
-                // Reset complete counter and make sure that the just read part was not overwritten during the copy
-                self.ndtr = dma.ndtr();
-                let complete_count = dma.reset_complete_count();
-                if self.end() > self.first || complete_count > 1 {
-                    return Err(OverrunError);
-                }
-
-                self.first = head;
-                Ok(tail + head)
+                self.start = head;
+                Ok((tail + head, self.cap() - self.start))
             }
         }
     }
-
     /// Copy from the dma buffer at `data_range` into `buf`
     fn copy_to(&mut self, buf: &mut [W], data_range: Range<usize>) -> usize {
         // Limit the number of bytes that can be copied
@@ -218,203 +197,289 @@ impl<'a, W: Word> DmaRingBuffer<'a, W> {
         length
     }
 }
-
 #[cfg(test)]
 mod tests {
     use core::array;
-    use core::cell::RefCell;
+    use std::{cell, vec};
 
     use super::*;
 
-    struct TestCtrl {
-        next_ndtr: RefCell<Option<usize>>,
-        complete_count: usize,
+    #[allow(dead_code)]
+    #[derive(PartialEq, Debug)]
+    enum TestCircularTransferRequest {
+        GetCompleteCount(usize),
+        ResetCompleteCount(usize),
+        PositionRequest(usize),
     }
 
-    impl TestCtrl {
-        pub const fn new() -> Self {
-            Self {
-                next_ndtr: RefCell::new(None),
-                complete_count: 0,
+    struct TestCircularTransfer {
+        len: usize,
+        requests: cell::RefCell<vec::Vec<TestCircularTransferRequest>>,
+    }
+
+    impl DmaCtrl for &mut TestCircularTransfer {
+        fn get_remaining_transfers(&self) -> usize {
+            match self.requests.borrow_mut().pop().unwrap() {
+                TestCircularTransferRequest::PositionRequest(pos) => {
+                    let len = self.len;
+
+                    assert!(len >= pos);
+
+                    len - pos
+                }
+                _ => unreachable!(),
             }
         }
 
-        pub fn set_next_ndtr(&mut self, ndtr: usize) {
-            self.next_ndtr.borrow_mut().replace(ndtr);
+        fn get_complete_count(&self) -> usize {
+            match self.requests.borrow_mut().pop().unwrap() {
+                TestCircularTransferRequest::GetCompleteCount(complete_count) => complete_count,
+                _ => unreachable!(),
+            }
         }
-    }
 
-    impl DmaCtrl for &mut TestCtrl {
-        fn ndtr(&self) -> usize {
-            self.next_ndtr.borrow_mut().unwrap()
+        fn reset_complete_count(&mut self) -> usize {
+            match self.requests.get_mut().pop().unwrap() {
+                TestCircularTransferRequest::ResetCompleteCount(complete_count) => complete_count,
+                _ => unreachable!(),
+            }
         }
+    }
 
-        fn get_complete_count(&self) -> usize {
-            self.complete_count
+    impl TestCircularTransfer {
+        pub fn new(len: usize) -> Self {
+            Self {
+                requests: cell::RefCell::new(vec![]),
+                len: len,
+            }
         }
 
-        fn reset_complete_count(&mut self) -> usize {
-            let old = self.complete_count;
-            self.complete_count = 0;
-            old
+        pub fn setup(&self, mut requests: vec::Vec<TestCircularTransferRequest>) {
+            requests.reverse();
+            self.requests.replace(requests);
         }
     }
 
     #[test]
-    fn empty() {
+    fn empty_and_read_not_started() {
         let mut dma_buf = [0u8; 16];
         let ringbuf = DmaRingBuffer::new(&mut dma_buf);
 
-        assert!(ringbuf.is_empty());
-        assert_eq!(0, ringbuf.len());
+        assert_eq!(0, ringbuf.start);
     }
 
     #[test]
     fn can_read() {
+        let mut dma = TestCircularTransfer::new(16);
+
         let mut dma_buf: [u8; 16] = array::from_fn(|idx| idx as u8); // 0, 1, ..., 15
-        let mut ctrl = TestCtrl::new();
         let mut ringbuf = DmaRingBuffer::new(&mut dma_buf);
-        ringbuf.ndtr = 6;
 
-        assert!(!ringbuf.is_empty());
-        assert_eq!(10, ringbuf.len());
+        assert_eq!(0, ringbuf.start);
+        assert_eq!(16, ringbuf.len());
 
+        dma.setup(vec![
+            TestCircularTransferRequest::PositionRequest(8),
+            TestCircularTransferRequest::PositionRequest(10),
+            TestCircularTransferRequest::GetCompleteCount(0),
+        ]);
         let mut buf = [0; 2];
-        assert_eq!(2, ringbuf.read(&mut ctrl, &mut buf).unwrap());
+        assert_eq!(2, ringbuf.read(&mut dma, &mut buf).unwrap().0);
         assert_eq!([0, 1], buf);
-        assert_eq!(8, ringbuf.len());
+        assert_eq!(2, ringbuf.start);
 
+        dma.setup(vec![
+            TestCircularTransferRequest::PositionRequest(10),
+            TestCircularTransferRequest::PositionRequest(12),
+            TestCircularTransferRequest::GetCompleteCount(0),
+        ]);
         let mut buf = [0; 2];
-        assert_eq!(2, ringbuf.read(&mut ctrl, &mut buf).unwrap());
+        assert_eq!(2, ringbuf.read(&mut dma, &mut buf).unwrap().0);
         assert_eq!([2, 3], buf);
-        assert_eq!(6, ringbuf.len());
+        assert_eq!(4, ringbuf.start);
 
+        dma.setup(vec![
+            TestCircularTransferRequest::PositionRequest(12),
+            TestCircularTransferRequest::PositionRequest(14),
+            TestCircularTransferRequest::GetCompleteCount(0),
+        ]);
         let mut buf = [0; 8];
-        assert_eq!(6, ringbuf.read(&mut ctrl, &mut buf).unwrap());
+        assert_eq!(8, ringbuf.read(&mut dma, &mut buf).unwrap().0);
         assert_eq!([4, 5, 6, 7, 8, 9], buf[..6]);
-        assert_eq!(0, ringbuf.len());
-
-        let mut buf = [0; 2];
-        assert_eq!(0, ringbuf.read(&mut ctrl, &mut buf).unwrap());
+        assert_eq!(12, ringbuf.start);
     }
 
     #[test]
     fn can_read_with_wrap() {
+        let mut dma = TestCircularTransfer::new(16);
+
         let mut dma_buf: [u8; 16] = array::from_fn(|idx| idx as u8); // 0, 1, ..., 15
-        let mut ctrl = TestCtrl::new();
         let mut ringbuf = DmaRingBuffer::new(&mut dma_buf);
-        ringbuf.first = 12;
-        ringbuf.ndtr = 10;
-
-        // The dma controller has written 4 + 6 bytes and has reloaded NDTR
-        ctrl.complete_count = 1;
-        ctrl.set_next_ndtr(10);
-
-        assert!(!ringbuf.is_empty());
-        assert_eq!(6 + 4, ringbuf.len());
 
-        let mut buf = [0; 2];
-        assert_eq!(2, ringbuf.read(&mut ctrl, &mut buf).unwrap());
-        assert_eq!([12, 13], buf);
-        assert_eq!(6 + 2, ringbuf.len());
-
-        let mut buf = [0; 4];
-        assert_eq!(4, ringbuf.read(&mut ctrl, &mut buf).unwrap());
-        assert_eq!([14, 15, 0, 1], buf);
-        assert_eq!(4, ringbuf.len());
+        assert_eq!(0, ringbuf.start);
+        assert_eq!(16, ringbuf.len());
+
+        /*
+            Read to close to the end of the buffer
+        */
+        dma.setup(vec![
+            TestCircularTransferRequest::PositionRequest(14),
+            TestCircularTransferRequest::PositionRequest(16),
+            TestCircularTransferRequest::GetCompleteCount(0),
+        ]);
+        let mut buf = [0; 14];
+        assert_eq!(14, ringbuf.read(&mut dma, &mut buf).unwrap().0);
+        assert_eq!(14, ringbuf.start);
+
+        /*
+            Now, read around the buffer
+        */
+        dma.setup(vec![
+            TestCircularTransferRequest::PositionRequest(6),
+            TestCircularTransferRequest::PositionRequest(8),
+            TestCircularTransferRequest::ResetCompleteCount(1),
+        ]);
+        let mut buf = [0; 6];
+        assert_eq!(6, ringbuf.read(&mut dma, &mut buf).unwrap().0);
+        assert_eq!(4, ringbuf.start);
     }
 
     #[test]
     fn can_read_when_dma_writer_is_wrapped_and_read_does_not_wrap() {
+        let mut dma = TestCircularTransfer::new(16);
+
         let mut dma_buf: [u8; 16] = array::from_fn(|idx| idx as u8); // 0, 1, ..., 15
-        let mut ctrl = TestCtrl::new();
         let mut ringbuf = DmaRingBuffer::new(&mut dma_buf);
-        ringbuf.first = 2;
-        ringbuf.ndtr = 6;
-
-        // The dma controller has written 6 + 2 bytes and has reloaded NDTR
-        ctrl.complete_count = 1;
-        ctrl.set_next_ndtr(14);
 
+        assert_eq!(0, ringbuf.start);
+        assert_eq!(16, ringbuf.len());
+
+        /*
+            Read to close to the end of the buffer
+        */
+        dma.setup(vec![
+            TestCircularTransferRequest::PositionRequest(14),
+            TestCircularTransferRequest::PositionRequest(16),
+            TestCircularTransferRequest::GetCompleteCount(0),
+        ]);
+        let mut buf = [0; 14];
+        assert_eq!(14, ringbuf.read(&mut dma, &mut buf).unwrap().0);
+        assert_eq!(14, ringbuf.start);
+
+        /*
+            Now, read to the end of the buffer
+        */
+        dma.setup(vec![
+            TestCircularTransferRequest::PositionRequest(6),
+            TestCircularTransferRequest::PositionRequest(8),
+            TestCircularTransferRequest::ResetCompleteCount(1),
+        ]);
         let mut buf = [0; 2];
-        assert_eq!(2, ringbuf.read(&mut ctrl, &mut buf).unwrap());
-        assert_eq!([2, 3], buf);
-
-        assert_eq!(1, ctrl.complete_count); // The interrupt flag IS NOT cleared
+        assert_eq!(2, ringbuf.read(&mut dma, &mut buf).unwrap().0);
+        assert_eq!(0, ringbuf.start);
     }
 
     #[test]
-    fn can_read_when_dma_writer_is_wrapped_and_read_wraps() {
+    fn can_read_when_dma_writer_wraps_once_with_same_ndtr() {
+        let mut dma = TestCircularTransfer::new(16);
+
         let mut dma_buf: [u8; 16] = array::from_fn(|idx| idx as u8); // 0, 1, ..., 15
-        let mut ctrl = TestCtrl::new();
         let mut ringbuf = DmaRingBuffer::new(&mut dma_buf);
-        ringbuf.first = 12;
-        ringbuf.ndtr = 10;
-
-        // The dma controller has written 6 + 2 bytes and has reloaded NDTR
-        ctrl.complete_count = 1;
-        ctrl.set_next_ndtr(14);
-
-        let mut buf = [0; 10];
-        assert_eq!(10, ringbuf.read(&mut ctrl, &mut buf).unwrap());
-        assert_eq!([12, 13, 14, 15, 0, 1, 2, 3, 4, 5], buf);
 
-        assert_eq!(0, ctrl.complete_count); // The interrupt flag IS cleared
-    }
-
-    #[test]
-    fn cannot_read_when_dma_writer_wraps_with_same_ndtr() {
-        let mut dma_buf = [0u8; 16];
-        let mut ctrl = TestCtrl::new();
-        let mut ringbuf = DmaRingBuffer::new(&mut dma_buf);
-        ringbuf.first = 6;
-        ringbuf.ndtr = 10;
-        ctrl.set_next_ndtr(9);
-
-        assert!(ringbuf.is_empty()); // The ring buffer thinks that it is empty
-
-        // The dma controller has written exactly 16 bytes
-        ctrl.complete_count = 1;
-
-        let mut buf = [0; 2];
-        assert_eq!(Err(OverrunError), ringbuf.read(&mut ctrl, &mut buf));
-
-        assert_eq!(1, ctrl.complete_count); // The complete counter is not reset
+        assert_eq!(0, ringbuf.start);
+        assert_eq!(16, ringbuf.len());
+
+        /*
+            Read to about the middle of the buffer
+        */
+        dma.setup(vec![
+            TestCircularTransferRequest::PositionRequest(6),
+            TestCircularTransferRequest::PositionRequest(6),
+            TestCircularTransferRequest::GetCompleteCount(0),
+        ]);
+        let mut buf = [0; 6];
+        assert_eq!(6, ringbuf.read(&mut dma, &mut buf).unwrap().0);
+        assert_eq!(6, ringbuf.start);
+
+        /*
+            Now, wrap the DMA controller around
+        */
+        dma.setup(vec![
+            TestCircularTransferRequest::PositionRequest(6),
+            TestCircularTransferRequest::GetCompleteCount(1),
+            TestCircularTransferRequest::PositionRequest(6),
+            TestCircularTransferRequest::GetCompleteCount(1),
+        ]);
+        let mut buf = [0; 6];
+        assert_eq!(6, ringbuf.read(&mut dma, &mut buf).unwrap().0);
+        assert_eq!(12, ringbuf.start);
     }
 
     #[test]
     fn cannot_read_when_dma_writer_overwrites_during_not_wrapping_read() {
+        let mut dma = TestCircularTransfer::new(16);
+
         let mut dma_buf: [u8; 16] = array::from_fn(|idx| idx as u8); // 0, 1, ..., 15
-        let mut ctrl = TestCtrl::new();
         let mut ringbuf = DmaRingBuffer::new(&mut dma_buf);
-        ringbuf.first = 2;
-        ringbuf.ndtr = 6;
-
-        // The dma controller has written 6 + 3 bytes and has reloaded NDTR
-        ctrl.complete_count = 1;
-        ctrl.set_next_ndtr(13);
-
-        let mut buf = [0; 2];
-        assert_eq!(Err(OverrunError), ringbuf.read(&mut ctrl, &mut buf));
 
-        assert_eq!(1, ctrl.complete_count); // The complete counter is not reset
+        assert_eq!(0, ringbuf.start);
+        assert_eq!(16, ringbuf.len());
+
+        /*
+            Read a few bytes
+        */
+        dma.setup(vec![
+            TestCircularTransferRequest::PositionRequest(2),
+            TestCircularTransferRequest::PositionRequest(2),
+            TestCircularTransferRequest::GetCompleteCount(0),
+        ]);
+        let mut buf = [0; 6];
+        assert_eq!(2, ringbuf.read(&mut dma, &mut buf).unwrap().0);
+        assert_eq!(2, ringbuf.start);
+
+        /*
+            Now, overtake the reader
+        */
+        dma.setup(vec![
+            TestCircularTransferRequest::PositionRequest(4),
+            TestCircularTransferRequest::PositionRequest(6),
+            TestCircularTransferRequest::GetCompleteCount(1),
+        ]);
+        let mut buf = [0; 6];
+        assert_eq!(OverrunError, ringbuf.read(&mut dma, &mut buf).unwrap_err());
     }
 
     #[test]
     fn cannot_read_when_dma_writer_overwrites_during_wrapping_read() {
+        let mut dma = TestCircularTransfer::new(16);
+
         let mut dma_buf: [u8; 16] = array::from_fn(|idx| idx as u8); // 0, 1, ..., 15
-        let mut ctrl = TestCtrl::new();
         let mut ringbuf = DmaRingBuffer::new(&mut dma_buf);
-        ringbuf.first = 12;
-        ringbuf.ndtr = 10;
-
-        // The dma controller has written 6 + 13 bytes and has reloaded NDTR
-        ctrl.complete_count = 1;
-        ctrl.set_next_ndtr(3);
-
-        let mut buf = [0; 2];
-        assert_eq!(Err(OverrunError), ringbuf.read(&mut ctrl, &mut buf));
 
-        assert_eq!(1, ctrl.complete_count); // The complete counter is not reset
+        assert_eq!(0, ringbuf.start);
+        assert_eq!(16, ringbuf.len());
+
+        /*
+            Read to close to the end of the buffer
+        */
+        dma.setup(vec![
+            TestCircularTransferRequest::PositionRequest(14),
+            TestCircularTransferRequest::PositionRequest(16),
+            TestCircularTransferRequest::GetCompleteCount(0),
+        ]);
+        let mut buf = [0; 14];
+        assert_eq!(14, ringbuf.read(&mut dma, &mut buf).unwrap().0);
+        assert_eq!(14, ringbuf.start);
+
+        /*
+            Now, overtake the reader
+        */
+        dma.setup(vec![
+            TestCircularTransferRequest::PositionRequest(8),
+            TestCircularTransferRequest::PositionRequest(10),
+            TestCircularTransferRequest::ResetCompleteCount(2),
+        ]);
+        let mut buf = [0; 6];
+        assert_eq!(OverrunError, ringbuf.read(&mut dma, &mut buf).unwrap_err());
     }
 }
diff --git a/embassy-stm32/src/lib.rs b/embassy-stm32/src/lib.rs
index c9df5c1b2..1258a65f1 100644
--- a/embassy-stm32/src/lib.rs
+++ b/embassy-stm32/src/lib.rs
@@ -1,4 +1,4 @@
-#![no_std]
+#![cfg_attr(not(test), no_std)]
 #![cfg_attr(feature = "nightly", feature(async_fn_in_trait, impl_trait_projections))]
 
 // This must go FIRST so that all the other modules see its macros.
diff --git a/embassy-stm32/src/usart/mod.rs b/embassy-stm32/src/usart/mod.rs
index 061c859a8..05ccb8749 100644
--- a/embassy-stm32/src/usart/mod.rs
+++ b/embassy-stm32/src/usart/mod.rs
@@ -13,6 +13,12 @@ use futures::future::{select, Either};
 use crate::dma::{NoDma, Transfer};
 use crate::gpio::sealed::AFType;
 #[cfg(not(any(usart_v1, usart_v2)))]
+#[allow(unused_imports)]
+use crate::pac::usart::regs::Isr as Sr;
+#[cfg(any(usart_v1, usart_v2))]
+#[allow(unused_imports)]
+use crate::pac::usart::regs::Sr;
+#[cfg(not(any(usart_v1, usart_v2)))]
 use crate::pac::usart::Lpuart as Regs;
 #[cfg(any(usart_v1, usart_v2))]
 use crate::pac::usart::Usart as Regs;
@@ -32,7 +38,6 @@ impl<T: BasicInstance> interrupt::Handler<T::Interrupt> for InterruptHandler<T>
 
         let (sr, cr1, cr3) = unsafe { (sr(r).read(), r.cr1().read(), r.cr3().read()) };
 
-        let mut wake = false;
         let has_errors = (sr.pe() && cr1.peie()) || ((sr.fe() || sr.ne() || sr.ore()) && cr3.eie());
         if has_errors {
             // clear all interrupts and DMA Rx Request
@@ -52,35 +57,24 @@ impl<T: BasicInstance> interrupt::Handler<T::Interrupt> for InterruptHandler<T>
                     w.set_dmar(false);
                 });
             }
-
-            wake = true;
-        } else {
-            if cr1.idleie() && sr.idle() {
-                // IDLE detected: no more data will come
-                unsafe {
-                    r.cr1().modify(|w| {
-                        // disable idle line detection
-                        w.set_idleie(false);
-                    });
-                }
-
-                wake = true;
+        } else if cr1.idleie() && sr.idle() {
+            // IDLE detected: no more data will come
+            unsafe {
+                r.cr1().modify(|w| {
+                    // disable idle line detection
+                    w.set_idleie(false);
+                });
             }
+        } else if cr1.rxneie() {
+            // We cannot check the RXNE flag as it is auto-cleared by the DMA controller
 
-            if cr1.rxneie() {
-                // We cannot check the RXNE flag as it is auto-cleared by the DMA controller
-
-                // It is up to the listener to determine if this in fact was a RX event and disable the RXNE detection
-
-                wake = true;
-            }
+            // It is up to the listener to determine if this in fact was a RX event and disable the RXNE detection
+        } else {
+            return;
         }
 
-        if wake {
-            compiler_fence(Ordering::SeqCst);
-
-            s.rx_waker.wake();
-        }
+        compiler_fence(Ordering::SeqCst);
+        s.rx_waker.wake();
     }
 }
 
@@ -1109,9 +1103,9 @@ pub use crate::usart::buffered::InterruptHandler as BufferedInterruptHandler;
 mod buffered;
 
 #[cfg(not(gpdma))]
-mod rx_ringbuffered;
+mod ringbuffered;
 #[cfg(not(gpdma))]
-pub use rx_ringbuffered::RingBufferedUartRx;
+pub use ringbuffered::RingBufferedUartRx;
 
 use self::sealed::Kind;
 
diff --git a/embassy-stm32/src/usart/rx_ringbuffered.rs b/embassy-stm32/src/usart/ringbuffered.rs
index 33b750497..511b71c7f 100644
--- a/embassy-stm32/src/usart/rx_ringbuffered.rs
+++ b/embassy-stm32/src/usart/ringbuffered.rs
@@ -2,13 +2,12 @@ use core::future::poll_fn;
 use core::sync::atomic::{compiler_fence, Ordering};
 use core::task::Poll;
 
-use embassy_hal_common::drop::OnDrop;
 use embassy_hal_common::PeripheralRef;
 use futures::future::{select, Either};
 
 use super::{clear_interrupt_flags, rdr, sr, BasicInstance, Error, UartRx};
-use crate::dma::ringbuffer::OverrunError;
 use crate::dma::RingBuffer;
+use crate::usart::{Regs, Sr};
 
 pub struct RingBufferedUartRx<'d, T: BasicInstance, RxDma: super::RxDma<T>> {
     _peri: PeripheralRef<'d, T>,
@@ -24,7 +23,9 @@ impl<'d, T: BasicInstance, RxDma: super::RxDma<T>> UartRx<'d, T, RxDma> {
 
         let request = self.rx_dma.request();
         let opts = Default::default();
+
         let ring_buf = unsafe { RingBuffer::new_read(self.rx_dma, request, rdr(T::regs()), dma_buf, opts) };
+
         RingBufferedUartRx {
             _peri: self._peri,
             ring_buf,
@@ -42,11 +43,18 @@ impl<'d, T: BasicInstance, RxDma: super::RxDma<T>> RingBufferedUartRx<'d, T, RxD
         Ok(())
     }
 
+    fn stop(&mut self, err: Error) -> Result<usize, Error> {
+        self.teardown_uart();
+
+        Err(err)
+    }
+
     /// Start uart background receive
     fn setup_uart(&mut self) {
         // fence before starting DMA.
         compiler_fence(Ordering::SeqCst);
 
+        // start the dma controller
         self.ring_buf.start();
 
         let r = T::regs();
@@ -58,8 +66,8 @@ impl<'d, T: BasicInstance, RxDma: super::RxDma<T>> RingBufferedUartRx<'d, T, RxD
                 w.set_rxneie(false);
                 // enable parity interrupt if not ParityNone
                 w.set_peie(w.pce());
-                // disable idle line interrupt
-                w.set_idleie(false);
+                // enable idle line interrupt
+                w.set_idleie(true);
             });
             r.cr3().modify(|w| {
                 // enable Error Interrupt: (Frame error, Noise error, Overrun error)
@@ -72,6 +80,8 @@ impl<'d, T: BasicInstance, RxDma: super::RxDma<T>> RingBufferedUartRx<'d, T, RxD
 
     /// Stop uart background receive
     fn teardown_uart(&mut self) {
+        self.ring_buf.request_stop();
+
         let r = T::regs();
         // clear all interrupts and DMA Rx Request
         // SAFETY: only clears Rx related flags
@@ -93,9 +103,6 @@ impl<'d, T: BasicInstance, RxDma: super::RxDma<T>> RingBufferedUartRx<'d, T, RxD
         }
 
         compiler_fence(Ordering::SeqCst);
-
-        self.ring_buf.request_stop();
-        while self.ring_buf.is_running() {}
     }
 
     /// Read bytes that are readily available in the ring buffer.
@@ -111,96 +118,49 @@ impl<'d, T: BasicInstance, RxDma: super::RxDma<T>> RingBufferedUartRx<'d, T, RxD
 
         // Start background receive if it was not already started
         // SAFETY: read only
-        let is_started = unsafe { r.cr3().read().dmar() };
-        if !is_started {
-            self.start()?;
-        }
+        match unsafe { r.cr3().read().dmar() } {
+            false => self.start()?,
+            _ => {}
+        };
 
-        // SAFETY: read only and we only use Rx related flags
-        let s = unsafe { sr(r).read() };
-        let has_errors = s.pe() || s.fe() || s.ne() || s.ore();
-        if has_errors {
-            self.teardown_uart();
-
-            if s.pe() {
-                return Err(Error::Parity);
-            } else if s.fe() {
-                return Err(Error::Framing);
-            } else if s.ne() {
-                return Err(Error::Noise);
-            } else {
-                return Err(Error::Overrun);
-            }
-        }
-
-        self.ring_buf.reload_position();
-        match self.ring_buf.read(buf) {
-            Ok(len) if len == 0 => {}
-            Ok(len) => {
-                assert!(len > 0);
-                return Ok(len);
-            }
-            Err(OverrunError) => {
-                // Stop any transfer from now on
-                // The user must re-start to receive any more data
-                self.teardown_uart();
-                return Err(Error::Overrun);
-            }
-        }
+        check_for_errors(clear_idle_flag(T::regs()))?;
 
         loop {
-            self.wait_for_data_or_idle().await?;
+            match self.ring_buf.read(buf) {
+                Ok((0, _)) => {}
+                Ok((len, _)) => {
+                    return Ok(len);
+                }
+                Err(_) => {
+                    return self.stop(Error::Overrun);
+                }
+            }
 
-            self.ring_buf.reload_position();
-            if !self.ring_buf.is_empty() {
-                break;
+            match self.wait_for_data_or_idle().await {
+                Ok(_) => {}
+                Err(err) => {
+                    return self.stop(err);
+                }
             }
         }
-
-        let len = self.ring_buf.read(buf).map_err(|_err| Error::Overrun)?;
-        assert!(len > 0);
-
-        Ok(len)
     }
 
     /// Wait for uart idle or dma half-full or full
     async fn wait_for_data_or_idle(&mut self) -> Result<(), Error> {
-        let r = T::regs();
-
-        // make sure USART state is restored to neutral state
-        let _on_drop = OnDrop::new(move || {
-            // SAFETY: only clears Rx related flags
-            unsafe {
-                r.cr1().modify(|w| {
-                    // disable idle line interrupt
-                    w.set_idleie(false);
-                });
-            }
-        });
-
-        // SAFETY: only sets Rx related flags
-        unsafe {
-            r.cr1().modify(|w| {
-                // enable idle line interrupt
-                w.set_idleie(true);
-            });
-        }
-
         compiler_fence(Ordering::SeqCst);
 
+        let mut dma_init = false;
         // Future which completes when there is dma is half full or full
         let dma = poll_fn(|cx| {
             self.ring_buf.set_waker(cx.waker());
 
-            compiler_fence(Ordering::SeqCst);
+            let status = match dma_init {
+                false => Poll::Pending,
+                true => Poll::Ready(()),
+            };
 
-            self.ring_buf.reload_position();
-            if !self.ring_buf.is_empty() {
-                // Some data is now available
-                Poll::Ready(())
-            } else {
-                Poll::Pending
-            }
+            dma_init = true;
+            status
         });
 
         // Future which completes when idle line is detected
@@ -210,28 +170,11 @@ impl<'d, T: BasicInstance, RxDma: super::RxDma<T>> RingBufferedUartRx<'d, T, RxD
 
             compiler_fence(Ordering::SeqCst);
 
-            // SAFETY: read only and we only use Rx related flags
-            let sr = unsafe { sr(r).read() };
-
-            // SAFETY: only clears Rx related flags
-            unsafe {
-                // This read also clears the error and idle interrupt flags on v1.
-                rdr(r).read_volatile();
-                clear_interrupt_flags(r, sr);
-            }
+            // Critical section is needed so that IDLE isn't set after
+            // our read but before we clear it.
+            let sr = critical_section::with(|_| clear_idle_flag(T::regs()));
 
-            let has_errors = sr.pe() || sr.fe() || sr.ne() || sr.ore();
-            if has_errors {
-                if sr.pe() {
-                    return Poll::Ready(Err(Error::Parity));
-                } else if sr.fe() {
-                    return Poll::Ready(Err(Error::Framing));
-                } else if sr.ne() {
-                    return Poll::Ready(Err(Error::Noise));
-                } else {
-                    return Poll::Ready(Err(Error::Overrun));
-                }
-            }
+            check_for_errors(sr)?;
 
             if sr.idle() {
                 // Idle line is detected
@@ -243,11 +186,7 @@ impl<'d, T: BasicInstance, RxDma: super::RxDma<T>> RingBufferedUartRx<'d, T, RxD
 
         match select(dma, uart).await {
             Either::Left(((), _)) => Ok(()),
-            Either::Right((Ok(()), _)) => Ok(()),
-            Either::Right((Err(e), _)) => {
-                self.teardown_uart();
-                Err(e)
-            }
+            Either::Right((result, _)) => result,
         }
     }
 }
@@ -257,6 +196,37 @@ impl<T: BasicInstance, RxDma: super::RxDma<T>> Drop for RingBufferedUartRx<'_, T
         self.teardown_uart();
     }
 }
+/// Return an error result if the Sr register has errors
+fn check_for_errors(s: Sr) -> Result<(), Error> {
+    if s.pe() {
+        Err(Error::Parity)
+    } else if s.fe() {
+        Err(Error::Framing)
+    } else if s.ne() {
+        Err(Error::Noise)
+    } else if s.ore() {
+        Err(Error::Overrun)
+    } else {
+        Ok(())
+    }
+}
+
+/// Clear IDLE and return the Sr register
+fn clear_idle_flag(r: Regs) -> Sr {
+    unsafe {
+        // SAFETY: read only and we only use Rx related flags
+
+        let sr = sr(r).read();
+
+        // This read also clears the error and idle interrupt flags on v1.
+        rdr(r).read_volatile();
+        clear_interrupt_flags(r, sr);
+
+        r.cr1().modify(|w| w.set_idleie(true));
+
+        sr
+    }
+}
 
 #[cfg(all(feature = "unstable-traits", feature = "nightly"))]
 mod eio {