rp2040: fix RP2350 hard fault in unaligned_memcpy to USB DPRAM

Use volatile byte accesses to prevent the compiler from widening
the byte-by-byte copy loop into 16/32-bit accesses, which cause
a hard fault on RP2350 when targeting USB DPRAM (device memory).

Closes #3554

1 files changed, 6 insertions, 2 deletions

diff --git a/src/portable/raspberrypi/rp2040/rp2040_usb.c b/src/portable/raspberrypi/rp2040/rp2040_usb.c
index 1b13934d3..f5ba81dd7 100644
--- a/src/portable/raspberrypi/rp2040/rp2040_usb.c
+++ b/src/portable/raspberrypi/rp2040/rp2040_usb.c
@@ -51,10 +51,14 @@ critical_section_t rp2usb_lock;
 //--------------------------------------------------------------------+
 // Implementation
 //--------------------------------------------------------------------+
-// Provide own byte by byte memcpy as not all copies are aligned
+// Provide own byte by byte memcpy as not all copies are aligned.
+// Use volatile to prevent compiler from widening to 16/32-bit accesses
+// which cause hard fault on RP2350 when dst/src points to USB DPRAM.
 static void unaligned_memcpy(uint8_t *dst, const uint8_t *src, size_t n) {
+  volatile uint8_t *vdst = dst;
+  const volatile uint8_t *vsrc = src;
   while (n--) {
-    *dst++ = *src++;
+    *vdst++ = *vsrc++;
   }
 }