Creating a debug target in Linux 2.6 driver module makefile

Question

I'm trying to be able to execute "make debug" at the command line and it will build my driver module with the -DDEBUG_OUTPUT define, which will cause certain sections of code to be compiled in.

In 2.4 kernel makefiles, this is pretty easy. I just create a debug: target and included "-DDEBUG_OUTPUT" in the cc compilation command arguments for that target. Easy.

Unfortunately (for me), 2.6 completely changed how modules are compiled, and I can ONLY seem to find the trivial "all" and "clean" examples, which don't show adding custom defines at compilation time.

I tried this:

  debug:
    make -C $(KERNEL_DIR) SUBDIRS='pwd' -DDEBUG_OUTPUT modules

and got a complaint from make.

I've also tried:

.PHONY: debug

debug:
  make -C $(KERNEL_DIR) SUBDIRS='pwd' EXTRA_CFLAGS="$(EXTRA_CFLAGS) -DDEBUG_OUTPUT" modules

but it is not seeing what EXTRA_CFLAGS contains. I can see from the command line output that it does correctly append the -D onto the existing EXTRA_CFLAGS, which includes the -I for includes dir. However, the driver file won't compile now because it cannot find the includes dir...so somehow it is not seeing what EXTRA_CFLAGS contains.

Answer 1

A "-D" option is not meant to be passed to make: it is a C preprocesseor (cpp) option.

To define DEBUG_OUTPUT for your build you have to add the following line to your Kbuild file:

EXTRA_CFLAGS = -DDEBUG_OUTPUT

Afterwards you can call, as usual:

make -C $(KERNEL_DIR) M=`pwd`

EDIT: If you don't want to edit the Kbuild file, you can have a debug target like this:

INCLUDES="-Imy_include_dir1 -Imy_include_dir2"

.PHONY: debug
debug:
        $(MAKE) -C $(KDIR) M=`pwd` EXTRA_CFLAGS="$(INCLUDES) -DDEBUG_OUTPUT"

EDIT#2:

MY_CFLAGS=-DFOO -DBAR -Imydir1

all:
        $(MAKE) -C $(KDIR) M=`pwd` EXTRA_CFLAGS="$(MY_CFLAGS)"

debug: MY_CFLAGS+=-DDEBUG_OUTPUT
debug:
        $(MAKE) -C $(KDIR) M=`pwd` EXTRA_CFLAGS="$(MY_CFLAGS)"