ftrace: Add sample module that uses register_ftrace_direct()

Add a sample module that shows a simple use case for
regsiter_ftrace_direct(), and how to use it.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

samples/ftrace/ftrace-direct.c

@@ -0,0 +1,45 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <linux/module.h>
+
+#include <linux/sched.h> /* for wake_up_process() */
+#include <linux/ftrace.h>
+
+void my_direct_func(struct task_struct *p)
+{
+	trace_printk("wakeing up %s-%d\n", p->comm, p->pid);
+}
+
+extern void my_tramp(void *);
+
+asm (
+"	.pushsection    .text, \"ax\", @progbits\n"
+"   my_tramp:"
+"	pushq %rbp\n"
+"	movq %rsp, %rbp\n"
+"	pushq %rdi\n"
+"	call my_direct_func\n"
+"	popq %rdi\n"
+"	leave\n"
+"	ret\n"
+"	.popsection\n"
+);
+
+
+static int __init ftrace_direct_init(void)
+{
+	return register_ftrace_direct((unsigned long)wake_up_process,
+				     (unsigned long)my_tramp);
+}
+
+static void __exit ftrace_direct_exit(void)
+{
+	unregister_ftrace_direct((unsigned long)wake_up_process,
+				 (unsigned long)my_tramp);
+}
+
+module_init(ftrace_direct_init);
+module_exit(ftrace_direct_exit);
+
+MODULE_AUTHOR("Steven Rostedt");
+MODULE_DESCRIPTION("Example use case of using register_ftrace_direct()");
+MODULE_LICENSE("GPL");