centos man page for trace_printk

TRACE_PRINTK(9) 						   Driver Basics						   TRACE_PRINTK(9)

NAME
       trace_printk - printf formatting in the ftrace buffer

SYNOPSIS
       trace_printk(fmt, ...);

ARGUMENTS
       fmt
	   the printf format for printing

       ...
	   variable arguments

NOTE
       __trace_printk is an internal function for trace_printk and the ip is passed in via the trace_printk macro.

       This function allows a kernel developer to debug fast path sections that printk is not appropriate for. By scattering in various printk
       like tracing in the code, a developer can quickly see where problems are occurring.

       This is intended as a debugging tool for the developer only. Please refrain from leaving trace_printks scattered around in your code.
       (Extra memory is used for special buffers that are allocated when trace_printk is used)

       A little optization trick is done here. If there's only one argument, there's no need to scan the string for printf formats. The trace_puts
       will suffice. But how can we take advantage of using trace_puts when trace_printk has only one argument? By stringifying the args and
       checking the size we can tell whether or not there are args. __stringify((__VA_ARGS__)) will turn into "()" with a size of 3 when there
       are no args, anything else will be bigger. All we need to do is define a string to this, and then take its size and compare to 3. If it's
       bigger, use do_trace_printk otherwise, optimize it to trace_puts. Then just let gcc optimize the rest.

COPYRIGHT
Kernel Hackers Manual 3.10					     June 2014							   TRACE_PRINTK(9)