View source for Howto build a kernel module out of the kernel tree

== Yocto recipe to build a kernel module out of the kernel tree ==

While it is always preferable to work with sources integrated into the Linux kernel sources, if you need an external kernel module, you could start from this simple recipe based on [http://www.yoctoproject.org/docs/current/kernel-dev/kernel-dev.html#incorporating-out-of-tree-modules Yocto Project kernel Development Manual]. This GPL2 recipe is available as a template from which you can create your own out-of-tree Linux kernel module recipe.


 $ tree
 .
 ├── files
 │   ├── COPYING
 │   ├── hellokernel.c
 │   └── Makefile
 └── kernmodule.bb



''' COPYING '''

Copy COPYING file into your files

 cp meta-skeleton/recipes-kernel/hello-mod/files/COPYING  YOURPATH/kernmodule/files/



'''kernmodule.bb'''

 #
 # Yocto recipe to build a kernel module out of the kernel tree
 # kernmodule.bb  
 # Marco Cavallini - KOAN sas - www.koansoftware.com
 #
 
 DESCRIPTION = "Hello kernel module out of the kernel tree"
 SECTION = "examples"
 LICENSE = "GPLv2"
 LIC_FILES_CHKSUM = "file://COPYING;md5=12f884d2ae1ff87c09e5b7ccc2c4ca7e"
 PR = "r0"
 
 inherit module
 
 SRC_URI =   "file://hellokernel.c \
 			file://Makefile \
 			file://COPYING \
 			"
 
 S = "${WORKDIR}"

'''Makefile'''

 obj-m += hellokernel.o
 SRC := $(shell pwd)
 
 all:
 	$(MAKE) -C $(KERNEL_SRC) M=$(SRC) modules
 
 modules_install:
 	$(MAKE) -C $(KERNEL_SRC) M=$(SRC) modules_install
 

''' hellokernel.c '''

 /*
  * hello.c - The simplest and dirty kernel module
  */
 
 #include <linux/module.h>
 #include <linux/kernel.h>
 
 int init_module(void)
 {
   printk(KERN_INFO "Hello world\n");
   
   return 0;
 }
 
 void cleanup_module(void)
 {
   printk(KERN_INFO "Goodbye world\n");
 }


----


== ISSUE : Custom kernel module QA Issue warnings during do_package task execution ==

In case the module compiles with no errors, but a QA warning is thrown as shown below. 

The module does not successfully get installed into build images.

 WARNING: mymodule-1.0-r0 do_package: QA Issue: hellokernel: Files/directories were installed but not shipped in any package:
  /lib
  /lib/modules
  /lib/modules/5.4.209-yocto-standard
  /lib/modules/5.4.209-yocto-standard/extra


To fix this issue, add below content to your custom kernel module recipe.

 FILES_${PN} += "${base_libdir}/modules/"