fix(SAI): disallow start without initial write

embassy-stm32/src/dma/dma_bdma.rs

@@ -828,7 +828,6 @@ impl<'a, W: Word> ReadableRingBuffer<'a, W> {
     /// You must call this after creating it for it to work.
     pub fn start(&mut self) {
         self.channel.start();
-        self.clear();
     }
 
     /// Clear all data in the ring buffer.
@@ -981,7 +980,6 @@ impl<'a, W: Word> WritableRingBuffer<'a, W> {
     /// You must call this after creating it for it to work.
     pub fn start(&mut self) {
         self.channel.start();
-        self.clear();
     }
 
     /// Clear all data in the ring buffer.
@@ -991,7 +989,6 @@ impl<'a, W: Word> WritableRingBuffer<'a, W> {
 
     /// Write elements directly to the raw buffer.
     /// This can be used to fill the buffer before starting the DMA transfer.
-    #[allow(dead_code)]
     pub fn write_immediate(&mut self, buf: &[W]) -> Result<(usize, usize), Error> {
         self.ringbuf.write_immediate(buf)
     }

embassy-stm32/src/sai/mod.rs

@@ -958,13 +958,14 @@ impl<'d, T: Instance, W: word::Word> Sai<'d, T, W> {
     }
 
     /// Start the SAI driver.
-    pub fn start(&mut self) {
+    ///
+    /// Only receivers can be started. Transmitters are started on the first writing operation.
+    pub fn start(&mut self) -> Result<(), Error> {
         match self.ring_buffer {
-            RingBuffer::Writable(ref mut rb) => {
-                rb.start();
-            }
+            RingBuffer::Writable(_) => Err(Error::NotAReceiver),
             RingBuffer::Readable(ref mut rb) => {
                 rb.start();
+                Ok(())
             }
         }
     }
@@ -981,14 +982,6 @@ impl<'d, T: Instance, W: word::Word> Sai<'d, T, W> {
         rcc::enable_and_reset::<T>();
     }
 
-    /// Flush.
-    pub fn flush(&mut self) {
-        let ch = T::REGS.ch(self.sub_block as usize);
-        ch.cr1().modify(|w| w.set_saien(false));
-        ch.cr2().modify(|w| w.set_fflush(true));
-        ch.cr1().modify(|w| w.set_saien(true));
-    }
-
     /// Enable or disable mute.
     pub fn set_mute(&mut self, value: bool) {
         let ch = T::REGS.ch(self.sub_block as usize);
@@ -1012,6 +1005,9 @@ impl<'d, T: Instance, W: word::Word> Sai<'d, T, W> {
 
     /// Write data to the SAI ringbuffer.
     ///
+    /// The first write starts the DMA after filling the ring buffer with the provided data.
+    /// This ensures that the DMA does not run before data is available in the ring buffer.
+    ///
     /// This appends the data to the buffer and returns immediately. The
     /// data will be transmitted in the background.
     ///
@@ -1019,7 +1015,12 @@ impl<'d, T: Instance, W: word::Word> Sai<'d, T, W> {
     pub async fn write(&mut self, data: &[W]) -> Result<(), Error> {
         match &mut self.ring_buffer {
             RingBuffer::Writable(buffer) => {
+                if buffer.is_running() {
                     buffer.write_exact(data).await?;
+                } else {
+                    buffer.write_immediate(data)?;
+                    buffer.start();
+                }
                 Ok(())
             }
             _ => return Err(Error::NotATransmitter),

examples/stm32h7/src/bin/sai.rs

@@ -108,7 +108,6 @@ async fn main(_spawner: Spawner) {
     let mut sai_receiver = Sai::new_synchronous(sub_block_rx, p.PE3, p.DMA1_CH1, rx_buffer, rx_config);
 
     sai_receiver.start();
-    sai_transmitter.start();
 
     let mut buf = [0u32; HALF_DMA_BUFFER_LENGTH];