Describe FAQ/LinuxKernelDebug101 here.

 easiest way is, as always, printk("format", ...)

 almost as easy is dev_info() and friends: 
   dev_info, dev_warn, dev_err, dev_dbg
   they do logging at the KERN_* levels (INFO, WARN, ERROR, DEBUG) respectively
   convenient shorthand, more readable.

 dev_dbg has additional features.
   compiles out, unless #define DEBUG 1 is done before #include <device.h>
     so to activate, recompile, reload
   to see them on console, lower console logging level
     echo 8 > /proc/sys/kernel/printk