score: Add _IO_Relax()

This function may be used to burn a couple of processor cycles with
minimum impact on the system bus.  It may be used in busy wait loops.
Since it is a global function, it is possible to wrap it in device
driver test code.

cpukit/include/rtems/score/io.h

@@ -133,6 +133,14 @@ int _IO_Base64url(
   int          wordlen
 );
 
+/**
+ * @brief Issues a couple of no-operation instructions.
+ *
+ * This function may be used to burn a couple of processor cycles with minimum
+ * impact on the system bus.  It may be used in busy wait loops.
+ */
+void _IO_Relax( void );
+
 /** @} */ 
 
 #ifdef __cplusplus

cpukit/score/src/iorelax.c

@@ -0,0 +1,53 @@
+/* SPDX-License-Identifier: BSD-2-Clause */
+
+/**
+ * @file
+ *
+ * @ingroup RTEMSScoreIO
+ *
+ * @brief This source file contains the implementation of _IO_Relax().
+ */
+
+/*
+ * Copyright (C) 2021 embedded brains GmbH (http://www.embedded-brains.de)
+ *
+ * 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/io.h>
+#include <rtems/score/cpuimpl.h>
+
+void _IO_Relax( void )
+{
+  _CPU_Instruction_no_operation();
+  _CPU_Instruction_no_operation();
+  _CPU_Instruction_no_operation();
+  _CPU_Instruction_no_operation();
+  _CPU_Instruction_no_operation();
+  _CPU_Instruction_no_operation();
+  _CPU_Instruction_no_operation();
+  _CPU_Instruction_no_operation();
+}

spec/build/cpukit/librtemscpu.yml

@@ -1418,6 +1418,7 @@ source:
 - cpukit/score/src/interr.c
 - cpukit/score/src/iobase64.c
 - cpukit/score/src/ioprintf.c
+- cpukit/score/src/iorelax.c
 - cpukit/score/src/iovprintf.c
 - cpukit/score/src/isr.c
 - cpukit/score/src/isrisinprogress.c