Man page 9 : trace_printk

trace_printk - printf formatting in the ftrace buffer

trace_printk( fmt , ... );

fmt
    the printf format for printing

...
    variable arguments

__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 thereAqs only one argument, thereAqs 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 « ()\0 » 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 itAqs bigger, use do_trace_printk otherwise, optimize it to trace_puts. Then just let gcc optimize the rest.