feat: custom DMA channel configuration

See
https://github.com/embassy-rs/embassy/pull/3923#issuecomment-2888810087

The default configuration of the channel which was done in `start()` is
now done in `new()` this allows overriding some settings through the new
`get_dma_channel` function. Only ringbuffers support this;
`LinkedListTransfer` and `Transfer` do not support that yet.

2 files changed, 27 insertions, 6 deletions

diff --git a/embassy-stm32/src/dma/gpdma/mod.rs b/embassy-stm32/src/dma/gpdma/mod.rs
index 7c05715ee..752b39416 100644
--- a/embassy-stm32/src/dma/gpdma/mod.rs
+++ b/embassy-stm32/src/dma/gpdma/mod.rs
@@ -352,6 +352,11 @@ impl AnyChannel {
             .store(table.transfer_count(), Ordering::Relaxed)
     }
 
+    fn get_dma_channel(&self) -> pac::gpdma::Channel {
+        let info = self.info();
+        info.dma.ch(info.num)
+    }
+
     fn start(&self) {
         let info = self.info();
         let ch = info.dma.ch(info.num);
diff --git a/embassy-stm32/src/dma/gpdma/ringbuffered.rs b/embassy-stm32/src/dma/gpdma/ringbuffered.rs
index 4532bda57..66c4b74ec 100644
--- a/embassy-stm32/src/dma/gpdma/ringbuffered.rs
+++ b/embassy-stm32/src/dma/gpdma/ringbuffered.rs
@@ -77,6 +77,9 @@ impl<'a, W: Word> ReadableRingBuffer<'a, W> {
         ];
         let table = Table::new(items);
 
+        // Apply the default configuration to the channel.
+        unsafe { channel.configure_linked_list(&table, Default::default()) };
+
         Self {
             channel,
             ringbuf: ReadableDmaRingBuffer::new(buffer),
@@ -85,14 +88,19 @@ impl<'a, W: Word> ReadableRingBuffer<'a, W> {
     }
 
     /// Start the ring buffer operation.
-    ///
-    /// You must call this after creating it for it to work.
     pub fn start(&mut self) {
-        unsafe { self.channel.configure_linked_list(&self.table, Default::default()) };
         self.table.link(RunMode::Circular);
         self.channel.start();
     }
 
+    /// Get a handle to the GPDMA channel for configuring the DMA.
+    ///
+    /// Usually, **this is not needed** as a default configuration is already
+    /// applied, but it may be useful to setup trigger sources, etc.
+    pub fn get_dma_channel(&mut self) -> stm32_metapac::gpdma::Channel {
+        self.channel.get_dma_channel()
+    }
+
     /// Clear all data in the ring buffer.
     pub fn clear(&mut self) {
         self.ringbuf.reset(&mut DmaCtrlImpl(self.channel.reborrow()));
@@ -226,6 +234,9 @@ impl<'a, W: Word> WritableRingBuffer<'a, W> {
         ];
         let table = Table::new(items);
 
+        // Apply the default configuration to the channel.
+        unsafe { channel.configure_linked_list(&table, Default::default()) };
+
         let this = Self {
             channel,
             ringbuf: WritableDmaRingBuffer::new(buffer),
@@ -236,14 +247,19 @@ impl<'a, W: Word> WritableRingBuffer<'a, W> {
     }
 
     /// Start the ring buffer operation.
-    ///
-    /// You must call this after creating it for it to work.
     pub fn start(&mut self) {
-        unsafe { self.channel.configure_linked_list(&self.table, Default::default()) };
         self.table.link(RunMode::Circular);
         self.channel.start();
     }
 
+    /// Get a handle to the GPDMA channel for configuring the DMA.
+    ///
+    /// Usually, **this is not needed** as a default configuration is already
+    /// applied, but it may be useful to setup trigger sources, etc.
+    pub fn get_dma_channel(&mut self) -> stm32_metapac::gpdma::Channel {
+        self.channel.get_dma_channel()
+    }
+
     /// Clear all data in the ring buffer.
     pub fn clear(&mut self) {
         self.ringbuf.reset(&mut DmaCtrlImpl(self.channel.reborrow()));