Fix Uart::flush_async (#3186)

* Fix Uart::flush_async

* Apply single-byte workaround

esp-hal/CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - Full-duplex SPI works when mixed with half-duplex SPI (#3176)
+- `Uart::flush_async` should no longer return prematurely (#3186)
 
 ### Removed
 

esp-hal/src/uart.rs

@@ -632,12 +632,18 @@ where
     #[instability::unstable]
     pub fn flush(&mut self) -> Result<(), TxError> {
         while self.tx_fifo_count() > 0 {}
-        // The FSM is in the Idle state for a short while after the last byte is moved
-        // out of the FIFO. It is unclear how long this takes, but 10us seems to be a
-        // good enough duration to wait, for both fast and slow baud rates.
+        self.flush_last_byte();
+        Ok(())
+    }
+
+    fn flush_last_byte(&mut self) {
+        // This function handles an edge case that happens when the TX FIFO count
+        // changes to 0. The FSM is in the Idle state for a short while after
+        // the last byte is moved out of the FIFO. It is unclear how long this
+        // takes, but 10us seems to be a good enough duration to wait, for both
+        // fast and slow baud rates.
         crate::rom::ets_delay_us(10);
         while !self.is_tx_idle() {}
-        Ok(())
     }
 
     /// Checks if the TX line is idle for this UART instance.
@@ -1935,10 +1941,15 @@ impl UartTx<'_, Async> {
     ///
     /// This function is cancellation safe.
     pub async fn flush_async(&mut self) -> Result<(), TxError> {
-        if self.tx_fifo_count() > 0 {
+        // Nothing is guaranteed to clear the Done status, so let's loop here in case Tx
+        // was Done before the last write operation that pushed data into the
+        // FIFO.
+        while self.tx_fifo_count() > 0 {
             UartTxFuture::new(self.uart.reborrow(), TxEvent::Done).await;
         }
 
+        self.flush_last_byte();
+
         Ok(())
     }
 }

hil-test/tests/uart_tx_rx_async.rs

@@ -75,6 +75,43 @@ mod tests {
         }
     }
 
+    #[test]
+    async fn flush_async_does_not_return_prematurely(mut ctx: Context) {
+        // Force TX_DONE to be set
+        ctx.tx.write_async(&[0u8; 10]).await.unwrap();
+
+        let mut read = [0u8; 10];
+        ctx.rx.read_exact_async(&mut read).await.unwrap();
+
+        // The flush should not return until the data is actually sent, regardless of
+        // previous TX_DONE status.
+        for _ in 0..10 {
+            ctx.tx.write_async(&[1u8; 10]).await.unwrap();
+            ctx.tx.flush_async().await.unwrap();
+
+            let read_count = ctx.rx.read_buffered(&mut read).unwrap();
+
+            assert_eq!(read_count, 10);
+            assert_eq!(&read, &[1u8; 10]);
+        }
+    }
+
+    #[test]
+    async fn flush_async_does_not_return_prematurely_even_for_short_writes(mut ctx: Context) {
+        let mut read = [0u8; 2];
+        // The flush should not return until the data is actually sent, regardless of
+        // previous TX_DONE status.
+        for i in 0..10 {
+            ctx.tx.write_async(&[i as u8]).await.unwrap();
+            ctx.tx.flush_async().await.unwrap();
+
+            let read_count = ctx.rx.read_buffered(&mut read).unwrap();
+
+            assert_eq!(read_count, 1);
+            assert_eq!(read[0], i as u8);
+        }
+    }
+
     #[test]
     async fn rx_overflow_is_detected(mut ctx: Context) {
         let mut to_send: &[u8] = &[0; 250];