rp2040: Load vectortable into ram

Load the interrupt vector table into ram at startup.  This reduces the
chance of a flash cache access causing timing instability.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

2 files changed, 27 insertions, 0 deletions

diff --git a/src/rp2040/main.c b/src/rp2040/main.c
index 0b144d0b..e7b64e5f 100644
--- a/src/rp2040/main.c
+++ b/src/rp2040/main.c
@@ -17,6 +17,26 @@
 
 
 /****************************************************************
+ * Ram IRQ vector table
+ ****************************************************************/
+
+// Copy vector table to ram and activate it
+static void
+enable_ram_vectortable(void)
+{
+    // Symbols created by rp2040_link.lds.S linker script
+    extern uint32_t _ram_vectortable_start, _ram_vectortable_end;
+    extern uint32_t _text_vectortable_start;
+
+    uint32_t count = (&_ram_vectortable_end - &_ram_vectortable_start) * 4;
+    __builtin_memcpy(&_ram_vectortable_start, &_text_vectortable_start, count);
+    barrier();
+
+    SCB->VTOR = (uint32_t)&_ram_vectortable_start;
+}
+
+
+/****************************************************************
  * Bootloader
  ****************************************************************/
 
@@ -145,6 +165,7 @@ clock_setup(void)
 void
 armcm_main(void)
 {
+    enable_ram_vectortable();
     clock_setup();
     sched_main();
 }
diff --git a/src/rp2040/rp2040_link.lds.S b/src/rp2040/rp2040_link.lds.S
index fd178847..9b0264a2 100644
--- a/src/rp2040/rp2040_link.lds.S
+++ b/src/rp2040/rp2040_link.lds.S
@@ -37,6 +37,12 @@ SECTIONS
     . = ALIGN(4);
     _data_flash = .;
 
+    .ram_vectortable (NOLOAD) : {
+        _ram_vectortable_start = .;
+        . = . + ( _text_vectortable_end - _text_vectortable_start ) ;
+        _ram_vectortable_end = .;
+    } > ram
+
     .data : AT (_data_flash)
     {
         . = ALIGN(4);