Homepr_debug() http://www.kernel.org/doc/local/pr_debug.txt
Some files call pr_debug(), which is ordinarily an empty macro that discards
its arguments at compile time. To enable debugging output, build the
appropriate file with -DDEBUG by adding
CFLAGS_[filename].o := -DDEBUG
to the makefile.
For example, to see all attempts to spawn a usermode helper (such as
/sbin/hotplug), add to lib/Makefile the line:
CFLAGS_kobject_uevent.o := -DDEBUG
Then boot the new kernel, do something that spawns a usermode helper, and
use the “dmesg” command to view the pr_debug() output.
