TRACE_PRINTK(9) Driver Basics TRACE_PRINTK(9)NAMEtrace_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 “()\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
it's bigger, use do_trace_printk otherwise, optimize it to trace_puts.
Then just let gcc optimize the rest.
COPYRIGHTKernel Hackers Manual 3.11 November 2013 TRACE_PRINTK(9)
