While looking i have noticed that a lot of drivers define there private "assert" macro.
It is very similar to warn.

(e.g.)
#define RTL819x_DEBUG
#ifdef RTL819x_DEBUG
#define assert(expr) \
       if (!(expr)) {                                  \
                printk( "Assertion failed! %s,%s,%s,line=%d\n", \
               #expr,__FILE__,__FUNCTION__,__LINE__);          \
       }

WARN is more complicated.  At least with the right debugging options 
turned on, it dumps the stack, via warn_slowpath_common.

julia