arch: rp2040: Add stack coloration for the idle task

Summary:
- This commit adds stack coloration for the idle task

Impact:
- rp2040 with CONFIG_STACK_COLORATION=y

Testing:
- Tested with nsh, nshsram and smp configurations
- NOTE: CONFIG_STACK_COLORATION=y needs to be added

arch/arm/src/rp2040/rp2040_start.c

@@ -51,6 +51,14 @@
 
 const uintptr_t g_idle_topstack = IDLE_STACK;
 
+/****************************************************************************
+ * Private Function prototypes
+ ****************************************************************************/
+
+#ifdef CONFIG_STACK_COLORATION
+static inline void go_nx_start(void *pv, unsigned int nbytes);
+#endif
+
 /****************************************************************************
  * Private Functions
  ****************************************************************************/
@@ -69,6 +77,51 @@ const uintptr_t g_idle_topstack = IDLE_STACK;
 #  define showprogress(c)
 #endif
 
+/****************************************************************************
+ * Name: go_nx_start
+ ****************************************************************************/
+
+#ifdef CONFIG_STACK_COLORATION
+static inline void go_nx_start(void *pv, unsigned int nbytes)
+{
+  /* Set the IDLE stack to the stack coloration value then jump to
+   * nx_start().  We take extreme care here because were currently
+   * executing on this stack.
+   *
+   * We want to avoid sneak stack access generated by the compiler.
+   * NOTE: this function must be inlined so that SRAM boot can work.
+   */
+
+  __asm__ __volatile__
+  (
+    "\t.global nx_start\n"
+    "\tlsr  %1, %1, #2\n"       /* %1 = nwords = nbytes >> 2 */
+    "\tcmp  %1, #0\n"           /* Check (nwords == 0) */
+    "\tbeq  2f\n"               /* (should not happen) */
+
+    "\tmov  r2, #3\n"
+    "\tadd  %0, %0, #3\n"
+    "\tbic  %0, r2\n"           /* %0 = Aligned stackptr */
+    "\tldr  r2, =0xdeadbeef\n"  /* R2 = STACK_COLOR = 0xdeadbeef */
+
+    "1:\n"                      /* Top of the loop */
+    "\tstr  r2, [%0, #0]\n"     /* Save stack color word */
+    "\tadd  %0, %0, #4\n"       /* Increment stackptr */
+    "\tsub  %1, %1, #1\n"       /* %1 nwords-- */
+    "\tcmp  %1, #0\n"           /* Check (nwords == 0) */
+    "\tbne  1b\n"               /* Bottom of the loop */
+
+    "2:\n"
+    "\tmov  r2, #0\n"
+    "\tmov  r14, r2\n"          /* LR = return address (none) */
+    "\tbl   nx_start\n"         /* Branch to nx_start */
+    :
+    : "r" (pv), "r" (nbytes)
+    : "r2"
+  );
+}
+#endif
+
 /****************************************************************************
  * Public Functions
  ****************************************************************************/
@@ -169,7 +222,14 @@ void __start(void)
 
   showprogress('\r');
   showprogress('\n');
+
+#ifdef CONFIG_STACK_COLORATION
+  /* Set the IDLE stack to the coloration value and jump into nx_start() */
+
+  go_nx_start((FAR void *)&_ebss, CONFIG_IDLETHREAD_STACKSIZE);
+#else
   nx_start();
+#endif
 
   /* Shouldn't get here */