Issue #2059 use write_immediate. Add docs. Remove half-write workaround

embassy-stm32/src/dma/dma.rs

@@ -876,6 +876,18 @@ impl<'a, C: Channel, W: Word> WritableRingBuffer<'a, C, W> {
         self.ringbuf.clear(&mut DmaCtrlImpl(self.channel.reborrow()));
     }
 
+    /// Write elements directly to the raw buffer.
+    /// This can be used to fill the buffer before starting the DMA transfer.
+    pub fn write_immediate(&mut self, buf: &[W]) -> Result<(usize, usize), OverrunError> {
+        self.ringbuf.write_immediate(
+            &mut DmaCtrlImpl {
+                channel: self.channel.reborrow(),
+                word_size: W::size(),
+            },
+            buf,
+        )
+    }
+
     /// Write elements from the ring buffer
     /// Return a tuple of the length written and the length remaining in the buffer
     pub fn write(&mut self, buf: &[W]) -> Result<(usize, usize), OverrunError> {

embassy-stm32/src/dma/gpdma.rs

@@ -731,8 +731,10 @@ impl<'a, C: Channel, W: Word> WritableRingBuffer<'a, C, W> {
         )
     }
 
-    pub fn prime(&mut self, buf: &[W]) -> Result<(usize, usize), OverrunError> {
-        self.ringbuf.prime(
+    /// Write elements directly to the raw buffer.
+    /// This can be used to fill the buffer before starting the DMA transfer.
+    pub fn write_immediate(&mut self, buf: &[W]) -> Result<(usize, usize), OverrunError> {
+        self.ringbuf.write_immediate(
             &mut DmaCtrlImpl {
                 channel: self.channel.reborrow(),
                 word_size: W::size(),

embassy-stm32/src/dma/ringbuffer.rs

@@ -260,15 +260,11 @@ impl<'a, W: Word> WritableDmaRingBuffer<'a, W> {
 
     /// The current position of the ringbuffer
     fn pos(&self, dma: &mut impl DmaCtrl) -> usize {
-        let result = self.cap() - dma.get_remaining_transfers();
-        if result >= self.cap() / 2 {
-            self.cap() / 2
-        } else {
-            0
-        }
+        self.cap() - dma.get_remaining_transfers()
     }
 
-    pub fn prime(&mut self, dma: &mut impl DmaCtrl, buffer: &[W]) -> Result<(usize, usize), OverrunError> {
+    /// Write elements directl to the buffer. This should be done before the DMA is started.
+    pub fn write_immediate(&mut self, dma: &mut impl DmaCtrl, buffer: &[W]) -> Result<(usize, usize), OverrunError> {
         if self.end != 0 {
             return Err(OverrunError);
         }

embassy-stm32/src/sai/mod.rs

@@ -435,6 +435,8 @@ impl MasterClockDivider {
     }
 }
 
+pub const MAX_FRAME_LENGTH: u8 = 0;
+
 /// [`SAI`] configuration.
 #[non_exhaustive]
 #[derive(Copy, Clone)]
@@ -853,13 +855,6 @@ impl<'d, T: Instance, C: Channel, W: word::Word> SubBlock<'d, T, C, W> {
         }
     }
 
-    pub fn prime(self: &mut Self, prime_buffer: &[W]) -> Result<(usize, usize), Error> {
-        match self.ring_buffer {
-            RingBuffer::Writable(ref mut rb) => Ok(rb.prime(prime_buffer)?),
-            RingBuffer::Readable(_) => Err(Error::NotATransmitter),
-        }
-    }
-
     fn is_transmitter(ring_buffer: &RingBuffer<C, W>) -> bool {
         match ring_buffer {
             RingBuffer::Writable(_) => true,
@@ -943,7 +938,11 @@ impl<'d, T: Instance, C: Channel, W: word::Word> SubBlock<'d, T, C, W> {
                 w.set_fspol(config.frame_sync_polarity.fspol());
                 w.set_fsdef(config.frame_sync_definition.fsdef());
                 w.set_fsall(config.frame_sync_active_level_length.0 as u8 - 1);
-                w.set_frl(config.frame_length - 1);
+                if config.frame_length == MAX_FRAME_LENGTH {
+                    w.set_frl(255);
+                } else {
+                    w.set_frl(config.frame_length - 1);
+                }
             });
 
             ch.slotr().modify(|w| {
@@ -1030,6 +1029,16 @@ impl<'d, T: Instance, C: Channel, W: word::Word> SubBlock<'d, T, C, W> {
         }
     }
 
+    /// Write elements directly to the raw buffer.
+    /// This should be used before starting the audio stream. This will give the CPU time to
+    /// prepare the next audio frame while the initial audio frame is playing.
+    pub fn write_immediate(self: &mut Self, source_buffer: &[W]) -> Result<(usize, usize), Error> {
+        match self.ring_buffer {
+            RingBuffer::Writable(ref mut rb) => Ok(rb.write_immediate(source_buffer)?),
+            RingBuffer::Readable(_) => Err(Error::NotATransmitter),
+        }
+    }
+
     pub async fn write(&mut self, data: &[W]) -> Result<(), Error> {
         match &mut self.ring_buffer {
             RingBuffer::Writable(buffer) => {