Add support for IDLE Thread stack allocator

Add a stack allocator hook specifically for allocation of IDLE thread stacks.
This allows the user to decide if IDLE thread stacks are statically allocated
or handled by the same custom allocator mechanism as other thread stacks.

Closes #4524.

2 files changed, 68 insertions, 3 deletions

diff --git a/cpukit/score/src/stackallocatorforidle.c b/cpukit/score/src/stackallocatorforidle.c
new file mode 100644
index 0000000000..7c4fd10c7d
--- /dev/null
+++ b/cpukit/score/src/stackallocatorforidle.c
@@ -0,0 +1,59 @@
+/*
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (C) 2021 OAR Corporation
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include <rtems/score/stack.h>
+#include <rtems/score/thread.h>
+
+/**
+ * @brief Default stack allocator allocate for idle handler.
+ *
+ * The allocate for idle handler is optional even when the user thread stack
+ * allocator and deallocator are configured.
+ *
+ * The default allocator for IDLE thread stacks gets the memory from a
+ * statically allocated area provided via confdefs.h.
+ *
+ * @param cpu Index of the CPU for the IDLE thread using this stack
+ * @param stack_size The size of the stack area to allocate in bytes.
+ *
+ * @retval NULL Not enough memory (never returned).
+ * @retval other Pointer to begin of stack area.
+ */
+static void *_Stack_Allocator_allocate_for_idle_default(
+  uint32_t  cpu,
+  size_t    stack_size
+)
+{
+  return &_Thread_Idle_stacks[ cpu * stack_size ];
+}
+
+const Stack_Allocator_allocate_for_idle _Stack_Allocator_allocate_for_idle =
+        _Stack_Allocator_allocate_for_idle_default;
diff --git a/cpukit/score/src/threadcreateidle.c b/cpukit/score/src/threadcreateidle.c
index f5ffc9bad8..86e117e70f 100644
--- a/cpukit/score/src/threadcreateidle.c
+++ b/cpukit/score/src/threadcreateidle.c
@@ -49,9 +49,15 @@ static void _Thread_Create_idle_for_CPU( Per_CPU_Control *cpu )
   config.is_preemptible = true;
   config.stack_size = _Thread_Idle_stack_size
     + CPU_IDLE_TASK_IS_FP * CONTEXT_FP_SIZE;
-  config.stack_area = &_Thread_Idle_stacks[
-    _Per_CPU_Get_index( cpu ) * config.stack_size
-  ];
+
+  /*
+   * The IDLE thread stacks may be statically allocated or there may be a
+   * custom allocator provided just as with user threads.
+   */
+  config.stack_area = (*_Stack_Allocator_allocate_for_idle)(
+    _Per_CPU_Get_index( cpu ),
+    config.stack_size
+  );
 
   /*
    *  The entire workspace is zeroed during its initialization.  Thus, all