I'm trying to write a kernel module under drivers/mtd/devices. The following ups are the files I try to compile:
mtdram2.c
AAtree.c
minmaxHeap.c
logNRPQ.c
RPQs.h

mtdram2.c is the main program. It will call some functions from logNRPQ.c. logNRPQ.c will call functions from AAtree.c and minmaxHeap.c. All the function prototypes are declared in RPQs.h.

I modified drivers/mtd/devices/Makefile to compile my program.
This is what I added in the Makefile:

obj-m := mtdram2.o
mtdram2-m += RPQs.o AAtree.o minmaxHeap.o logNRPQ.o

The problem is that there are warnings like
*** Warning: "function_name" [drivers/mtd/devices/mtdram2.ko] undefined!

Does anyone know what the problem is? Is there something wrong with my Makefile?

Thanks for help.