HelloWorld kernel module

Building an out-of-tree kernel module from scratch

Steps

1. cd ~/poky : Replace with your poky path
2. create a new layer. let say meta-hello-yocto: bitbake-layers create-layer ./meta-hello-yocto
3. initialize the build environment variables : source oe-init-build-env
4. cd build/ if not already there.
5. add the newly created layer to the build-system: bitbake-layers add-layer "../meta-hello-yocto". This should automatically update the bblayers.conf file under build/conf
6. cd ~/poky/meta-hello-yocto
7. remove the receipes-exaple directory :rm -rf recipes-example/
8. create a dir . let say recepies-kernel: mkdir recipes-kernel
9. cd recipes-kernel/
10. make a directory. let say hello-mod: mkdir hello-mod
11. cd hello-mod
12. create a file name, say kernel-module-hello_0.1.bb with the following content:

SUMMARY = "Example of how to build an external Linux kernel module"
LICENSE = "GPLv2"
LIC_FILES_CHKSUM = "file://COPYING;md5=12f884d2ae1ff87c09e5b7ccc2c4ca7e"

inherit module

SRC_URI = "file://Makefile \
           file://hello.c \
           file://COPYING \
          "

S = "${WORKDIR}"

# The inherit of module.bbclass will automatically name module packages with
# "kernel-module-" prefix as required by the oe-core build environment.

RPROVIDES_${PN} += "kernel-module-hello"

13. mkdir files
14. cd files
15. create a file named hello.c with the following content:

/******************************************************************************
 *
 *   Copyright (C) 2011  Intel Corporation. All rights reserved.
 *
 *   This program is free software;  you can redistribute it and/or modify
 *   it under the terms of the GNU General Public License as published by
 *   the Free Software Foundation; version 2 of the License.
 *
 *   This program is distributed in the hope that it will be useful,
 *   but WITHOUT ANY WARRANTY;  without even the implied warranty of
 *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See
 *   the GNU General Public License for more details.
 *
 *   You should have received a copy of the GNU General Public License
 *   along with this program;  if not, write to the Free Software
 *   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 *
 *****************************************************************************/

#include <linux/module.h>

int init_module(void)
{
	printk("Hello World!\n");
	return 0;
}

void cleanup_module(void)
{
	printk("Goodbye Cruel World!\n");
}

MODULE_LICENSE("GPL");

16. create a file named COPYING (optional)

17. create a Makefile

obj-m := hello.o

SRC := $(shell pwd)

all:
	$(MAKE) -C $(KERNEL_SRC) M=$(SRC)

modules_install:
	$(MAKE) -C $(KERNEL_SRC) M=$(SRC) modules_install

clean:
	rm -f *.o *~ core .depend .*.cmd *.ko *.mod.c
	rm -f Module.markers Module.symvers modules.order
	rm -rf .tmp_versions Modules.symvers

18. tree should be like this

.
├── conf
│   └── layer.conf
├── COPYING.MIT
├── README
└── recipes-kernel
    └── hello-mod
        ├── files
        │   ├── COPYING
        │   ├── hello.c
        │   └── Makefile
        └── kernel-module-hello_0.1.bb

4 directories, 7 files

19. recheck the layers bitbake-layers show-layers

20. update the poky/build/conf/local.conf file by appending MACHINE_ESSENTIAL_EXTRA_DEPENDS +="kernel-module-hello"

21. compile the the code: cd ~/poky/build && bitbake kernel-module-hello

22. bitbake package-index

23. re-build the image ( in this example core-image-sato) : bitbake core-image-sato

24. run the image in QEMU: runqemu qemux86-64 : (in this example I'm using qemux86-64 , add nographic to the command if you are remotely logged into a machine )

25. modprobe hello

26. ⚡