Merge #1370

1370: stm32/i2c: fix races when using dma. r=Dirbaio a=xoviat

This change addresses two races:

1. It removes the `chunks_transferred` state variable that is modified inside the interrupt. Analysis of the code reveals that the only time the waker can be woken is when `chunks_transferred` is incremented. Therefore, waking is enough to signal the `poll_fn` that the `chunks_transferred` has incremented. Moving to `remaining_len` clarifies the code, since there is no need to track how many chunks are remaining.
2. It moves the start of the transfer until after the waker is registered, which could theoretically occur if the clock speed is very low, but probably never would even if this wasn't fixed.

There is another race that I noticed: between writes the waker may not yet be registered. In that case, the code would simply be stuck and the `poll_fn` would never be woken. There is no way to resolve this without broadening the scope of the analysis, and this will likely never occur. 

Co-authored-by: xoviat <xoviat@users.noreply.github.com>

embassy-stm32/src/i2c/v2.rs

@@ -1,6 +1,5 @@
 use core::cmp;
 use core::future::poll_fn;
-use core::sync::atomic::{AtomicUsize, Ordering};
 use core::task::Poll;
 
 use embassy_embedded_hal::SetConfig;
@@ -35,14 +34,12 @@ impl Default for Config {
 
 pub struct State {
     waker: AtomicWaker,
-    chunks_transferred: AtomicUsize,
 }
 
 impl State {
     pub(crate) const fn new() -> Self {
         Self {
             waker: AtomicWaker::new(),
-            chunks_transferred: AtomicUsize::new(0),
         }
     }
 }
@@ -130,10 +127,7 @@ impl<'d, T: Instance, TXDMA, RXDMA> I2c<'d, T, TXDMA, RXDMA> {
         let isr = regs.isr().read();
 
         if isr.tcr() || isr.tc() {
-            let state = T::state();
+            T::state().waker.wake();
-            let transferred = state.chunks_transferred.load(Ordering::Relaxed);
-            state.chunks_transferred.store(transferred + 1, Ordering::Relaxed);
-            state.waker.wake();
         }
         // The flag can only be cleared by writting to nbytes, we won't do that here, so disable
         // the interrupt
@@ -457,12 +451,6 @@ impl<'d, T: Instance, TXDMA, RXDMA> I2c<'d, T, TXDMA, RXDMA> {
         TXDMA: crate::i2c::TxDma<T>,
     {
         let total_len = write.len();
-        let completed_chunks = total_len / 255;
-        let total_chunks = if completed_chunks * 255 == total_len {
-            completed_chunks
-        } else {
-            completed_chunks + 1
-        };
 
         let dma_transfer = unsafe {
             let regs = T::regs();
@@ -480,7 +468,6 @@ impl<'d, T: Instance, TXDMA, RXDMA> I2c<'d, T, TXDMA, RXDMA> {
         };
 
         let state = T::state();
-        state.chunks_transferred.store(0, Ordering::Relaxed);
         let mut remaining_len = total_len;
 
         let on_drop = OnDrop::new(|| {
@@ -495,33 +482,35 @@ impl<'d, T: Instance, TXDMA, RXDMA> I2c<'d, T, TXDMA, RXDMA> {
             }
         });
 
-        // NOTE(unsafe) self.tx_dma does not fiddle with the i2c registers
-        if first_slice {
-            unsafe {
-                Self::master_write(
-                    address,
-                    total_len.min(255),
-                    Stop::Software,
-                    (total_chunks != 1) || !last_slice,
-                    &check_timeout,
-                )?;
-            }
-        } else {
-            unsafe {
-                Self::master_continue(total_len.min(255), (total_chunks != 1) || !last_slice, &check_timeout)?;
-                T::regs().cr1().modify(|w| w.set_tcie(true));
-            }
-        }
-
         poll_fn(|cx| {
             state.waker.register(cx.waker());
-            let chunks_transferred = state.chunks_transferred.load(Ordering::Relaxed);
 
-            if chunks_transferred == total_chunks {
+            let isr = unsafe { T::regs().isr().read() };
+            if remaining_len == total_len {
+                // NOTE(unsafe) self.tx_dma does not fiddle with the i2c registers
+                if first_slice {
+                    unsafe {
+                        Self::master_write(
+                            address,
+                            total_len.min(255),
+                            Stop::Software,
+                            (total_len > 255) || !last_slice,
+                            &check_timeout,
+                        )?;
+                    }
+                } else {
+                    unsafe {
+                        Self::master_continue(total_len.min(255), (total_len > 255) || !last_slice, &check_timeout)?;
+                        T::regs().cr1().modify(|w| w.set_tcie(true));
+                    }
+                }
+            } else if !(isr.tcr() || isr.tc()) {
+                // poll_fn was woken without an interrupt present
+                return Poll::Pending;
+            } else if remaining_len == 0 {
                 return Poll::Ready(Ok(()));
-            } else if chunks_transferred != 0 {
+            } else {
-                remaining_len = remaining_len.saturating_sub(255);
+                let last_piece = (remaining_len <= 255) && last_slice;
-                let last_piece = (chunks_transferred + 1 == total_chunks) && last_slice;
 
                 // NOTE(unsafe) self.tx_dma does not fiddle with the i2c registers
                 unsafe {
@@ -531,6 +520,8 @@ impl<'d, T: Instance, TXDMA, RXDMA> I2c<'d, T, TXDMA, RXDMA> {
                     T::regs().cr1().modify(|w| w.set_tcie(true));
                 }
             }
+
+            remaining_len = remaining_len.saturating_sub(255);
             Poll::Pending
         })
         .await?;
@@ -559,12 +550,6 @@ impl<'d, T: Instance, TXDMA, RXDMA> I2c<'d, T, TXDMA, RXDMA> {
         RXDMA: crate::i2c::RxDma<T>,
     {
         let total_len = buffer.len();
-        let completed_chunks = total_len / 255;
-        let total_chunks = if completed_chunks * 255 == total_len {
-            completed_chunks
-        } else {
-            completed_chunks + 1
-        };
 
         let dma_transfer = unsafe {
             let regs = T::regs();
@@ -580,7 +565,6 @@ impl<'d, T: Instance, TXDMA, RXDMA> I2c<'d, T, TXDMA, RXDMA> {
         };
 
         let state = T::state();
-        state.chunks_transferred.store(0, Ordering::Relaxed);
         let mut remaining_len = total_len;
 
         let on_drop = OnDrop::new(|| {
@@ -593,27 +577,29 @@ impl<'d, T: Instance, TXDMA, RXDMA> I2c<'d, T, TXDMA, RXDMA> {
             }
         });
 
-        // NOTE(unsafe) self.rx_dma does not fiddle with the i2c registers
-        unsafe {
-            Self::master_read(
-                address,
-                total_len.min(255),
-                Stop::Software,
-                total_chunks != 1,
-                restart,
-                &check_timeout,
-            )?;
-        }
-
         poll_fn(|cx| {
             state.waker.register(cx.waker());
-            let chunks_transferred = state.chunks_transferred.load(Ordering::Relaxed);
 
-            if chunks_transferred == total_chunks {
+            let isr = unsafe { T::regs().isr().read() };
+            if remaining_len == total_len {
+                // NOTE(unsafe) self.rx_dma does not fiddle with the i2c registers
+                unsafe {
+                    Self::master_read(
+                        address,
+                        total_len.min(255),
+                        Stop::Software,
+                        total_len > 255,
+                        restart,
+                        &check_timeout,
+                    )?;
+                }
+            } else if !(isr.tcr() || isr.tc()) {
+                // poll_fn was woken without an interrupt present
+                return Poll::Pending;
+            } else if remaining_len == 0 {
                 return Poll::Ready(Ok(()));
-            } else if chunks_transferred != 0 {
+            } else {
-                remaining_len = remaining_len.saturating_sub(255);
+                let last_piece = remaining_len <= 255;
-                let last_piece = chunks_transferred + 1 == total_chunks;
 
                 // NOTE(unsafe) self.rx_dma does not fiddle with the i2c registers
                 unsafe {
@@ -623,6 +609,8 @@ impl<'d, T: Instance, TXDMA, RXDMA> I2c<'d, T, TXDMA, RXDMA> {
                     T::regs().cr1().modify(|w| w.set_tcie(true));
                 }
             }
+
+            remaining_len = remaining_len.saturating_sub(255);
             Poll::Pending
         })
         .await?;