nrf: Implement chunked DMA transfers for SPIM peripheral

On some chips (notably nrf52832), the maximum DMA transfer is 255
bytes, which has caused subtle issues while interfacing with various
devices over SPI bus.

2 files changed, 81 insertions, 44 deletions

diff --git a/embassy-nrf/src/spim.rs b/embassy-nrf/src/spim.rs
index 8937159df..2d70732a8 100644
--- a/embassy-nrf/src/spim.rs
+++ b/embassy-nrf/src/spim.rs
@@ -17,7 +17,7 @@ use crate::chip::{EASY_DMA_SIZE, FORCE_COPY_BUFFER_SIZE};
 use crate::gpio::sealed::Pin as _;
 use crate::gpio::{self, AnyPin, Pin as GpioPin, PselBits};
 use crate::interrupt::typelevel::Interrupt;
-use crate::util::{slice_in_ram_or, slice_ptr_parts, slice_ptr_parts_mut};
+use crate::util::{slice_in_ram_or, slice_ptr_len, slice_ptr_parts, slice_ptr_parts_mut};
 use crate::{interrupt, pac, Peripheral};
 
 /// SPIM error
@@ -25,10 +25,6 @@ use crate::{interrupt, pac, Peripheral};
 #[cfg_attr(feature = "defmt", derive(defmt::Format))]
 #[non_exhaustive]
 pub enum Error {
-    /// Supplied TX buffer overflows EasyDMA transmit buffer
-    TxBufferTooLong,
-    /// Supplied RX buffer overflows EasyDMA receive buffer
-    RxBufferTooLong,
     /// EasyDMA can only read from data memory, read only buffers in flash will fail.
     BufferNotInRAM,
 }
@@ -74,9 +70,13 @@ impl<T: Instance> interrupt::typelevel::Handler<T::Interrupt> for InterruptHandl
         let s = T::state();
 
         #[cfg(feature = "_nrf52832_anomaly_109")]
-        if r.events_started.read().bits() != 0 {
-            s.waker.wake();
-            r.intenclr.write(|w| w.started().clear());
+        {
+            // Ideally we should call this only during the first chunk transfer,
+            // but so far calling this every time doesn't seem to be causing any issues.
+            if r.events_started.read().bits() != 0 {
+                s.waker.wake();
+                r.intenclr.write(|w| w.started().clear());
+            }
         }
 
         if r.events_end.read().bits() != 0 {
@@ -209,35 +209,39 @@ impl<'d, T: Instance> Spim<'d, T> {
         spim
     }
 
-    fn prepare(&mut self, rx: *mut [u8], tx: *const [u8]) -> Result<(), Error> {
-        slice_in_ram_or(tx, Error::BufferNotInRAM)?;
-        // NOTE: RAM slice check for rx is not necessary, as a mutable
-        // slice can only be built from data located in RAM.
-
+    fn prepare_dma_transfer(&mut self, rx: *mut [u8], tx: *const [u8], offset: usize, length: usize) {
         compiler_fence(Ordering::SeqCst);
 
         let r = T::regs();
 
-        // Set up the DMA write.
-        let (ptr, tx_len) = slice_ptr_parts(tx);
-        if tx_len > EASY_DMA_SIZE {
-            return Err(Error::TxBufferTooLong);
+        fn xfer_params(ptr: u32, total: usize, offset: usize, length: usize) -> (u32, usize) {
+            if total > offset {
+                (ptr.wrapping_add(offset as _), core::cmp::min(total - offset, length))
+            } else {
+                (ptr, 0)
+            }
         }
 
-        r.txd.ptr.write(|w| unsafe { w.ptr().bits(ptr as _) });
-        r.txd.maxcnt.write(|w| unsafe { w.maxcnt().bits(tx_len as _) });
-
         // Set up the DMA read.
-        let (ptr, rx_len) = slice_ptr_parts_mut(rx);
-        if rx_len > EASY_DMA_SIZE {
-            return Err(Error::RxBufferTooLong);
-        }
-
-        r.rxd.ptr.write(|w| unsafe { w.ptr().bits(ptr as _) });
+        let (ptr, len) = slice_ptr_parts_mut(rx);
+        let (rx_ptr, rx_len) = xfer_params(ptr as _, len as _, offset, length);
+        r.rxd.ptr.write(|w| unsafe { w.ptr().bits(rx_ptr) });
         r.rxd.maxcnt.write(|w| unsafe { w.maxcnt().bits(rx_len as _) });
 
+        // Set up the DMA write.
+        let (ptr, len) = slice_ptr_parts(tx);
+        let (tx_ptr, tx_len) = xfer_params(ptr as _, len as _, offset, length);
+        r.txd.ptr.write(|w| unsafe { w.ptr().bits(tx_ptr) });
+        r.txd.maxcnt.write(|w| unsafe { w.maxcnt().bits(tx_len as _) });
+
+        /*
+        trace!("XFER: offset: {}, length: {}", offset, length);
+        trace!("RX(len: {}, ptr: {=u32:02x})", rx_len, rx_ptr as u32);
+        trace!("TX(len: {}, ptr: {=u32:02x})", tx_len, tx_ptr as u32);
+        */
+
         #[cfg(feature = "_nrf52832_anomaly_109")]
-        {
+        if offset == 0 {
             let s = T::state();
 
             r.events_started.reset();
@@ -260,21 +264,32 @@ impl<'d, T: Instance> Spim<'d, T> {
 
         // Start SPI transaction.
         r.tasks_start.write(|w| unsafe { w.bits(1) });
-
-        Ok(())
     }
 
-    fn blocking_inner_from_ram(&mut self, rx: *mut [u8], tx: *const [u8]) -> Result<(), Error> {
-        self.prepare(rx, tx)?;
+    fn blocking_inner_from_ram_chunk(&mut self, rx: *mut [u8], tx: *const [u8], offset: usize, length: usize) {
+        self.prepare_dma_transfer(rx, tx, offset, length);
 
         #[cfg(feature = "_nrf52832_anomaly_109")]
-        while let Poll::Pending = self.nrf52832_dma_workaround_status() {}
+        if offset == 0 {
+            while self.nrf52832_dma_workaround_status().is_pending() {}
+        }
 
         // Wait for 'end' event.
         while T::regs().events_end.read().bits() == 0 {}
 
         compiler_fence(Ordering::SeqCst);
+    }
 
+    fn blocking_inner_from_ram(&mut self, rx: *mut [u8], tx: *const [u8]) -> Result<(), Error> {
+        slice_in_ram_or(tx, Error::BufferNotInRAM)?;
+        // NOTE: RAM slice check for rx is not necessary, as a mutable
+        // slice can only be built from data located in RAM.
+
+        let xfer_len = core::cmp::max(slice_ptr_len(rx), slice_ptr_len(tx));
+        for offset in (0..xfer_len).step_by(EASY_DMA_SIZE) {
+            let length = core::cmp::min(xfer_len - offset, EASY_DMA_SIZE);
+            self.blocking_inner_from_ram_chunk(rx, tx, offset, length);
+        }
         Ok(())
     }
 
@@ -287,22 +302,23 @@ impl<'d, T: Instance> Spim<'d, T> {
                 tx_ram_buf.copy_from_slice(tx);
                 self.blocking_inner_from_ram(rx, tx_ram_buf)
             }
-            Err(error) => Err(error),
         }
     }
 
-    async fn async_inner_from_ram(&mut self, rx: *mut [u8], tx: *const [u8]) -> Result<(), Error> {
-        self.prepare(rx, tx)?;
+    async fn async_inner_from_ram_chunk(&mut self, rx: *mut [u8], tx: *const [u8], offset: usize, length: usize) {
+        self.prepare_dma_transfer(rx, tx, offset, length);
 
         #[cfg(feature = "_nrf52832_anomaly_109")]
-        poll_fn(|cx| {
-            let s = T::state();
+        if offset == 0 {
+            poll_fn(|cx| {
+                let s = T::state();
 
-            s.waker.register(cx.waker());
+                s.waker.register(cx.waker());
 
-            self.nrf52832_dma_workaround_status()
-        })
-        .await;
+                self.nrf52832_dma_workaround_status()
+            })
+            .await;
+        }
 
         // Wait for 'end' event.
         poll_fn(|cx| {
@@ -316,7 +332,18 @@ impl<'d, T: Instance> Spim<'d, T> {
         .await;
 
         compiler_fence(Ordering::SeqCst);
+    }
+
+    async fn async_inner_from_ram(&mut self, rx: *mut [u8], tx: *const [u8]) -> Result<(), Error> {
+        slice_in_ram_or(tx, Error::BufferNotInRAM)?;
+        // NOTE: RAM slice check for rx is not necessary, as a mutable
+        // slice can only be built from data located in RAM.
 
+        let xfer_len = core::cmp::max(slice_ptr_len(rx), slice_ptr_len(tx));
+        for offset in (0..xfer_len).step_by(EASY_DMA_SIZE) {
+            let length = core::cmp::min(xfer_len - offset, EASY_DMA_SIZE);
+            self.async_inner_from_ram_chunk(rx, tx, offset, length).await;
+        }
         Ok(())
     }
 
@@ -329,7 +356,6 @@ impl<'d, T: Instance> Spim<'d, T> {
                 tx_ram_buf.copy_from_slice(tx);
                 self.async_inner_from_ram(rx, tx_ram_buf).await
             }
-            Err(error) => Err(error),
         }
     }
 
@@ -528,8 +554,6 @@ mod eh02 {
 impl embedded_hal_1::spi::Error for Error {
     fn kind(&self) -> embedded_hal_1::spi::ErrorKind {
         match *self {
-            Self::TxBufferTooLong => embedded_hal_1::spi::ErrorKind::Other,
-            Self::RxBufferTooLong => embedded_hal_1::spi::ErrorKind::Other,
             Self::BufferNotInRAM => embedded_hal_1::spi::ErrorKind::Other,
         }
     }
diff --git a/embassy-nrf/src/util.rs b/embassy-nrf/src/util.rs
index b408c517b..6cdb97f08 100644
--- a/embassy-nrf/src/util.rs
+++ b/embassy-nrf/src/util.rs
@@ -4,6 +4,19 @@ use core::mem;
 const SRAM_LOWER: usize = 0x2000_0000;
 const SRAM_UPPER: usize = 0x3000_0000;
 
+// #![feature(const_slice_ptr_len)]
+// https://github.com/rust-lang/rust/issues/71146
+pub(crate) fn slice_ptr_len<T>(ptr: *const [T]) -> usize {
+    use core::ptr::NonNull;
+    let ptr = ptr.cast_mut();
+    if let Some(ptr) = NonNull::new(ptr) {
+        ptr.len()
+    } else {
+        // We know ptr is null, so we know ptr.wrapping_byte_add(1) is not null.
+        NonNull::new(ptr.wrapping_byte_add(1)).unwrap().len()
+    }
+}
+
 // TODO: replace transmutes with core::ptr::metadata once it's stable
 pub(crate) fn slice_ptr_parts<T>(slice: *const [T]) -> (*const T, usize) {
     unsafe { mem::transmute(slice) }