Building an out-of-tree Linux kernel module in a separate object directory

Question

I'm confronting the Linux kernel build system (Kbuild, kernel ≥2.6.28) with the directory structure and build system for a larger project. Our project contains an out-of-tree Linux kernel module, and our directory structure looks like this (simplified, obviously):

checkout/src/common/*.c           source files (common to Linux and other platforms)
checkout/src/linux-driver/*.c     source files (for the Linux kernel driver)
checkout/build/linux/Kbuild       Kbuild
tmp/linux-2.6.xx/                 where the Linux kernel is unpacked and configured
output/linux-arm-debug/           where object files must end up

The build process must not modify anything under checkout, and building the module must not modify anything under tmp/linux-2.6.xx. All output files must end up under output/linux-arm-debug (or whatever architecture and debug variant was selected at build time).

I've read kbuild/modules.txt, and started to write my Kbuild file:

MOD_OUTPUT_DIR = ../../../output/linux-$(ARCH)-$(DEBUG)
obj-m += $(MOD_OUTPUT_DIR)/foo_mod.o
$(MOD_OUTPUT_DIR)/our_module-objs := $(MOD_OUTPUT_DIR)/foo_common.o $(MOD_OUTPUT_DIR)/foo_linux.o

This handles storing the object files in a different directory from where Kbuild lives. Now how can I specify that foo_common.o needs to be compiled from …/checkout/src/common/foo_common.c and foo_linux.o from …/checkout/src/linux-driver/foo_linux.c?

Answer 1

Here is a Makefile which does out of source-tree builds for out of kernel-tree modules (adapted from @Mark's comment)...

KDIR ?= /lib/modules/$(shell uname -r)/build
BUILD_DIR ?= $(PWD)/build
BUILD_DIR_MAKEFILE ?= $(PWD)/build/Makefile

default: $(BUILD_DIR_MAKEFILE)
    make -C $(KDIR) M=$(BUILD_DIR) src=$(PWD) modules

$(BUILD_DIR):
    mkdir -p "$@"

$(BUILD_DIR_MAKEFILE): $(BUILD_DIR)
    touch "$@"

clean:
    make -C $(KDIR) M=$(BUILD_DIR) src=$(PWD) clean

Note: You still need a Kbuild file...

obj-m += my_driver.o

Answer 2

I had a similar problem. I modified linux_2_6_34/scripts/Makefile.build as follows.

ifdef SRCDIR
src := $(SRCDIR)
else
src := $(obj)
endif

SRCDIR is the directory source.

To compile the module, run

make -c $(KDIR) M=$(Your_output_dir) SRCDIR=$(your source directory)`

Answer 3

My inelegant but effective solution is to copy the source files into the output tree.

FOO_SOURCES_DIR = $(src)/../../../checkout/src
FOO_MOD_OUTPUT_DIR = ../../../output/linux-$(ARCH)-$(DEBUG)

# Specify the object files
obj-m += $(FOO_MOD_OUTPUT_DIR)/foo_mod.o
FOO_MODULE_OBJS := $(FOO_MOD_OUTPUT_DIR)/foo_common.o $(FOO_MOD_OUTPUT_DIR)/foo_linux.o
$(FOO_MOD_OUTPUT_DIR)/foo_mod-objs := $(FOO_MODULE_OBJS)

# Where to find the sources
$(src)/$(FOO_MOD_OUTPUT_DIR)/foo_common.c: $(FOO_SOURCES_DIR)/common/foo_common.c
$(src)/$(FOO_MOD_OUTPUT_DIR)/foo_linux.c: $(FOO_SOURCES_DIR)/linux-driver/foo_linux.c

# Rules to copy the sources
FOO_COPIED_SOURCES = $(patsubst %.o,$(src)/%.c,$(FOO_MODULE_OBJS))
$(FOO_COPIED_SOURCES):
        $(Q)mkdir -p $(@D)
        cp -f $< $@
clean-files += $(FOO_COPIED_SOURCES)
clean-dirs += $(FOO_MOD_OUTPUT_DIR)

Answer 4

While you haven't mentioned what you've tried so far (or whether you found a solution already), it looks like you just need to continue further down the modules.txt file a bit -- to Section 4.3:

--- 4.3 Several Subdirectories

kbuild can handle files that are spread over several directories.
Consider the following example:

.
|__ src
|   |__ complex_main.c
|   |__ hal
|   |__ hardwareif.c
|   |__ include
|       |__ hardwareif.h
|__ include
    |__ complex.h

To build the module complex.ko, we then need the following
kbuild file:

    --> filename: Kbuild
    obj-m := complex.o
    complex-y := src/complex_main.o
    complex-y += src/hal/hardwareif.o

    ccflags-y := -I$(src)/include
    ccflags-y += -I$(src)/src/hal/include

As you can see, kbuild knows how to handle object files located
in other directories. The trick is to specify the directory
relative to the kbuild file's location. That being said, this
is NOT recommended practice.

For the header files, kbuild must be explicitly told where to
look. When kbuild executes, the current directory is always the
root of the kernel tree (the argument to "-C") and therefore an
absolute path is needed. $(src) provides the absolute path by
pointing to the directory where the currently executing kbuild
file is located.

Answer 5

A bit late, but it looks like O= flag is what you need.

Answer 6

You can set the environment variable KBUILD_OUTPUT. This functions similar to the O= option; however, since it's an environment variable, it can span multiple makefiles where O= can't be passed or an out-of-directory module needs to be built. I was having this same issue with trying to build a set of compat-wireless modules and I needed to use O= for the actual kernel image build.