CODEWITHSUNDEEP
gccmakefilelinux-kernellinux-device-drivercompiler-flags
superK
superK

Reputation: 3972

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

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 wonder if I can set gcc's CFLAGS for this specific Makefile. When I change default section to (note the addition of the -O2):

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

The change does not take effect (i.e., - the -O2 is NOT added to the CFLAGS).

Any help? Thanks a lot.

Upvotes: 28

Views: 35188

Answers (3)

Zhaoyang
Zhaoyang

Reputation: 71

default:
    $(MAKE) CFLAGS_MODULE="your c flags" -C /lib/modules/$(shell uname -r)/build M=$(shell pwd) modules

Upvotes: 1

Benjamin Leinweber
Benjamin Leinweber

Reputation: 2944

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

Upvotes: 19

eepp
eepp

Reputation: 7575

-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.

Upvotes: 34

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