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 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.

$$PWD can replace $(shell pwd)

eepp
– eepp

2013-08-28 03:57:53 +00:00
Commented Aug 28, 2013 at 3:57 — eepp
– eepp, Commented Aug 28, 2013 at 3:57

eepp · Accepted Answer · 2013-08-28 03:51:54Z

35

-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.

answered Aug 28, 2013 at 3:51

eepp

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1 Comment

Guy Avraham Over a year ago

what about the CFLAGS that are (potentially) set by inner "Makefile(s)" and/or the defualt kernel flags configuration/settings? Will it "collide" with the flags set for the speicifc external module?

Benjamin Leinweber · Accepted Answer · 2017-09-14 16:47:07Z

19

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

edited Sep 14, 2017 at 16:47

answered Aug 28, 2013 at 14:10

Benjamin Leinweber

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4 Comments

MarSoft Over a year ago

This will override ccflags-y setting in some inner Makefiles, and as a result will break .h file lookup.

MarSoft Over a year ago

For me, it helped to use EXTRA_CFLAGS instead. It is referenced in scripts/Makefile.lib as a thing for "backwards compatibility" and it is added to ccflags-y, thus not breaking anything.

fdk1342 Over a year ago

ccflags-y += -O2 should work. It was shown in Documentation/kbuild/modules.txt.

Charles Lohr Over a year ago

Is there a way to do this by buildroot configuration instead of needing to modify Makefiles?

Zhaoyang · Accepted Answer · 2024-09-24 03:00:22Z

1

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

answered Sep 24, 2024 at 3:00

Zhaoyang

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