fix(SAI): disallow start without initial write

1 files changed, 14 insertions, 13 deletions

diff --git a/embassy-stm32/src/sai/mod.rs b/embassy-stm32/src/sai/mod.rs
index 62b44b77f..057b21980 100644
--- a/embassy-stm32/src/sai/mod.rs
+++ b/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) => {
-                buffer.write_exact(data).await?;
+                if buffer.is_running() {
+                    buffer.write_exact(data).await?;
+                } else {
+                    buffer.write_immediate(data)?;
+                    buffer.start();
+                }
                 Ok(())
             }
             _ => return Err(Error::NotATransmitter),