Enforce scatter gather API statically

2 files changed, 10 insertions, 29 deletions

diff --git a/embassy-mcxa/src/dma.rs b/embassy-mcxa/src/dma.rs
index d076949b8..1f604a340 100644
--- a/embassy-mcxa/src/dma.rs
+++ b/embassy-mcxa/src/dma.rs
@@ -2236,9 +2236,9 @@ pub struct ScatterGatherBuilder<'a, W: Word> {
     _plt: core::marker::PhantomData<&'a mut W>,
 }
 
-impl<W: Word> ScatterGatherBuilder<'_, W> {
+impl<'a, W: Word> ScatterGatherBuilder<'a, W> {
     /// Create a new scatter-gather builder.
-    pub fn new() -> ScatterGatherBuilder<'static, W> {
+    pub fn new() -> Self {
         ScatterGatherBuilder {
             tcds: [Tcd::default(); MAX_SCATTER_GATHER_TCDS],
             count: 0,
@@ -2257,7 +2257,7 @@ impl<W: Word> ScatterGatherBuilder<'_, W> {
     /// # Panics
     ///
     /// Panics if the maximum number of segments (16) is exceeded.
-    pub fn add_transfer<'a>(mut self, src: &'a [W], dst: &'a mut [W]) -> &'a mut Self {
+    pub fn add_transfer<'b: 'a>(&mut self, src: &'b [W], dst: &'b mut [W]) -> &mut Self {
         assert!(self.count < MAX_SCATTER_GATHER_TCDS, "Too many scatter-gather segments");
         assert!(!src.is_empty());
         assert!(dst.len() >= src.len());
@@ -2300,12 +2300,7 @@ impl<W: Word> ScatterGatherBuilder<'_, W> {
     /// # Returns
     ///
     /// A `Transfer` future that completes when the entire chain has executed.
-    ///
-    /// # Safety
-    ///
-    /// All source and destination buffers passed to `add_transfer()` must
-    /// remain valid for the duration of the transfer.
-    pub unsafe fn build<C: Channel>(&mut self, channel: &DmaChannel<C>) -> Result<Transfer<'_>, Error> {
+    pub fn build<C: Channel>(&mut self, channel: &DmaChannel<C>) -> Result<Transfer<'a>, Error> {
         if self.count == 0 {
             return Err(Error::Configuration);
         }
@@ -2360,7 +2355,9 @@ impl<W: Word> ScatterGatherBuilder<'_, W> {
         cortex_m::asm::dsb();
 
         // Load first TCD into hardware
-        channel.load_tcd(&self.tcds[0]);
+        unsafe {
+            channel.load_tcd(&self.tcds[0]);
+        }
 
         // Memory barrier before setting START
         cortex_m::asm::dsb();
@@ -2377,7 +2374,7 @@ impl<W: Word> ScatterGatherBuilder<'_, W> {
     }
 }
 
-impl<W: Word> Default for ScatterGatherBuilder<W> {
+impl<W: Word> Default for ScatterGatherBuilder<'_, W> {
     fn default() -> Self {
         Self::new()
     }
diff --git a/examples/mcxa/src/bin/dma_scatter_gather_builder.rs b/examples/mcxa/src/bin/dma_scatter_gather_builder.rs
index ba8682cee..1691129f6 100644
--- a/examples/mcxa/src/bin/dma_scatter_gather_builder.rs
+++ b/examples/mcxa/src/bin/dma_scatter_gather_builder.rs
@@ -20,8 +20,6 @@
 #![no_std]
 #![no_main]
 
-use core::fmt::Write as _;
-
 use embassy_executor::Spawner;
 use embassy_mcxa::clocks::config::Div8;
 use embassy_mcxa::dma::{DmaChannel, ScatterGatherBuilder};
@@ -97,28 +95,14 @@ async fn main(_spawner: Spawner) {
     defmt::info!("Added 3 transfer segments to chain.");
     defmt::info!("Starting scatter-gather transfer with .await...");
 
-    // TODO START
-    defmt::info!("Destination buffers (after):");
-    defmt::info!("  DST1: {=[?]}", dst1.as_slice());
-    defmt::info!("  DST2: {=[?]}", dst2.as_slice());
-    defmt::info!("  DST3: {=[?]}", dst3.as_slice());
-    // TODO: If we want to make the `builder.build()` below safe, the above prints SHOULD NOT
-    // compile. We need to make sure that the lifetime of the builder reflects that it is
-    // "consuming" the slices until the builder is dropped, since we can access them to print here,
-    // that means that the borrow checker isn't enforcing that yet.
-    todo!("ABOVE CODE SHOULDN'T COMPILE");
-    // TODO END
-
     // Build and execute the scatter-gather chain
     // The build() method:
     // - Links all TCDs together with ESG bit
     // - Sets INTMAJOR on all TCDs
     // - Loads the first TCD into hardware
     // - Returns a Transfer future
-    unsafe {
-        let transfer = builder.build(&dma_ch0).expect("Failed to build scatter-gather");
-        transfer.blocking_wait();
-    }
+    let transfer = builder.build(&dma_ch0).expect("Failed to build scatter-gather");
+    transfer.blocking_wait();
 
     defmt::info!("Scatter-gather transfer complete!");