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

Question

Eg: a common device module's Makefile

obj-m:=jc.o

default:
    $(MAKE) -C /lib/modules/$(shell uname -r)/build M=$(shell pwd) modules
clean:
    $(MAKE) -C /lib/modules/$(shell uname -r)/build M=$(shell pwd) modules clean

I consider if I can set CFLAGS to the file. When I change default section to

$(MAKE) -O2 -C /lib/modules/$(shell uname -r)/build M=$(shell pwd) modules

But it didn't work.

Any help? Thanks a lot.

Answer 1

-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.

Answer 2

You can also use

ccflags-y := -O2

This will be applied to all of the source files compiled for your module with the Makefile. This is indirectly documented in the link provided by eepp in Section 4.2