microblaze: ftrace: add static function tracer

If -pg of gcc is enabled with CONFIG_FUNCTION_TRACER=y. a calling to
_mcount will be inserted into each kernel function. so, there is a
possibility to trace the kernel functions in _mcount.

This patch add the specific _mcount support for static function
tracing. by default, ftrace_trace_function is initialized as
ftrace_stub(an empty function), so, the default _mcount will introduce
very little overhead. after enabling ftrace in user-space, it will jump
to a real tracing function and do static function tracing for us.

Commit message from Wu Zhangjin <wuzhangjin@gmail.com>

Signed-off-by: Michal Simek <monstr@monstr.eu>

arch/microblaze/include/asm/ftrace.h

@@ -1 +1,15 @@
+#ifndef _ASM_MICROBLAZE_FTRACE
+#define _ASM_MICROBLAZE_FTRACE
 
+#ifdef CONFIG_FUNCTION_TRACER
+
+#define MCOUNT_ADDR		((long)(_mcount))
+#define MCOUNT_INSN_SIZE	8 /* sizeof mcount call */
+
+#ifndef __ASSEMBLY__
+extern void _mcount(void);
+extern void ftrace_call_graph(void);
+#endif
+
+#endif /* CONFIG_FUNCTION_TRACER */
+#endif /* _ASM_MICROBLAZE_FTRACE */