Taking into account of a change in a kernel source file
 

I would like to know how to modify a kernel source file without having to recompile all the kernel afterward?

(I used the make-kpkg method to compile my kernel.)

I have no clue what "make-kpkg" is :D
But if I remember correctly when I downloaded the "normal" kernel sources and modified the code of a module, just by issuing a "make" (or e.g. "make -j8" if you have 8 CPU cores available on your PC) recompiled only that module and skipped the rest. On the other side, if you change a file that is part of the "core" of the kernel, most of the rest (which probably has a dependency towards what you modified) will be recompiled as well.
:scratch: