Is it possible to set CFLAGS to a linux kernel module Makefile?

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-O2 would be an option to make (or $(MAKE), as you’re using it) in what you tried. Obviously, the compiler (probably gcc) needs this flag, not make.

Kbuild understands a make variable named CFLAGS_modulename.o to add specific C flags when compiling this unit. In your case, your module object will be jc.o, so you can specify:

CFLAGS_jc.o := -O2

and it should work. Add V=1 to your $(MAKE) lines to get a verbose output and you should see -O2 when jc.c is being compiled.

You can find more about compiling modules in the official documentation.

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