Restrict serial API to read only 1 byte at a time to avoid DMA issues.

Author: lulf

nrf-hal-common/src/uarte.rs

@@ -202,7 +202,7 @@ where
     ///
     /// The buffer must have a length of at most 255 bytes.
     pub fn read(&mut self, rx_buffer: &mut [u8]) -> Result<(), Error> {
-        start_read(&*self.0, rx_buffer)?;
+        start_read(&*self.0, rx_buffer, rx_buffer.len())?;
 
         // Wait for transmission to end.
         while self.0.events_endrx.read().bits() == 0 {}
@@ -240,7 +240,7 @@ where
         I: timer::Instance,
     {
         // Start the read.
-        start_read(&self.0, rx_buffer)?;
+        start_read(&self.0, rx_buffer, rx_buffer.len())?;
 
         // Start the timeout timer.
         timer.start(cycles);
@@ -363,7 +363,11 @@ fn stop_write(uarte: &uarte0::RegisterBlock) {
 
 /// Start a UARTE read transaction by setting the control
 /// values and triggering a read task.
-fn start_read(uarte: &uarte0::RegisterBlock, rx_buffer: &mut [u8]) -> Result<(), Error> {
+fn start_read(
+    uarte: &uarte0::RegisterBlock,
+    rx_buffer: &mut [u8],
+    nbytes: usize,
+) -> Result<(), Error> {
     if rx_buffer.len() > EASY_DMA_SIZE {
         return Err(Error::RxBufferTooLong);
     }
@@ -392,7 +396,7 @@ fn start_read(uarte: &uarte0::RegisterBlock, rx_buffer: &mut [u8]) -> Result<(),
         //
         // The MAXCNT field is at least 8 bits wide and accepts the full
         // range of values.
-        unsafe { w.maxcnt().bits(rx_buffer.len() as _) });
+        unsafe { w.maxcnt().bits(nbytes.min(rx_buffer.len()) as _) });
 
     // Start UARTE Receive transaction.
     uarte.tasks_startrx.write(|w|
@@ -700,17 +704,16 @@ where
         }
 
         if in_progress {
-            let b = self.rx_buf[0];
             uarte.events_rxstarted.reset();
 
             finalize_read(uarte);
 
             if uarte.rxd.amount.read().bits() != 1 as u32 {
                 return Err(nb::Error::Other(Error::Receive));
             }
-            Ok(b)
+            Ok(self.rx_buf[0])
         } else {
-            start_read(&uarte, self.rx_buf)?;
+            start_read(&uarte, self.rx_buf, 1)?;
             Err(nb::Error::WouldBlock)
         }
     }