ftrace: printk formatting infrastructure

This patch adds a feature that can help kernel developers debug their
code using ftrace.

  int ftrace_printk(const char *fmt, ...);

This records into the ftrace buffer using printf formatting. The entry
size in the buffers are still a fixed length. A new type has been added
that allows for more entries to be used for a single recording.

The start of the print is still the same as the other entries.

It returns the number of characters written to the ftrace buffer.

For example:

Having a module with the following code:

static int __init ftrace_print_test(void)
{
        ftrace_printk("jiffies are %ld\n", jiffies);
        return 0;
}

Gives me:

  insmod-5441  3...1 7569us : ftrace_print_test: jiffies are 4296626666

for the latency_trace file and:

          insmod-5441  [03]  1959.370498: ftrace_print_test jiffies are 4296626666

for the trace file.

Note: Only the infrastructure should go into the kernel. It is to help
facilitate debugging for other kernel developers. Calls to ftrace_printk
is not intended to be left in the kernel, and should be frowned upon just
like scattering printks around in the code.

But having this easily at your fingertips helps the debugging go faster
and bugs be solved quicker.

Maybe later on, we can hook this with markers and have their printf format
be sucked into ftrace output.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/ftrace.h

@@ -157,9 +157,18 @@ static inline void __ftrace_enabled_restore(int enabled)
 #ifdef CONFIG_TRACING
 extern void
 ftrace_special(unsigned long arg1, unsigned long arg2, unsigned long arg3);
+# define ftrace_printk(x...) __ftrace_printk(_THIS_IP_, x)
+extern int
+__ftrace_printk(unsigned long ip, const char *fmt, ...)
+	__attribute__ ((format (printf, 2, 3)));
 #else
 static inline void
 ftrace_special(unsigned long arg1, unsigned long arg2, unsigned long arg3) { }
+static inline int
+ftrace_printk(const char *fmt, ...) __attribute__ ((format (printf, 1, 0)))
+{
+	return 0;
+}
 #endif
 
 #ifdef CONFIG_FTRACE_MCOUNT_RECORD
@@ -173,4 +182,5 @@ ftrace_init_module(unsigned long *start, unsigned long *end) { }
 static inline void ftrace_release(void *start, unsigned long size) { }
 #endif
 
+
 #endif /* _LINUX_FTRACE_H */