Yocto: Add hello world kernel module

In this section, you are going to learn

How to create a simple kernel module in yocto ?

How to generate full yocto image for RPI-4B ?

How to flash full yocto image for RPI-4B ?

How to run minicom and work remotely with RPI-4B ?

Topics in this section,

How to clone and build base Yocto image ?

Step 1 : Add custom kernel module

Step 1.1: Create a new meta layer inside yocto build directory

Step 1.2: Create layer.conf in conf directory

Step 1.3: Create source file for sample kernel module

Step 1.4: Create recipe file for sample kernel module

Step 1.5: Explanation of recipe file part by part

Step 2 : Add configurations in .conf files

Step 2.1: Add the meta custom layer in bblayers.conf

Step 2.2: Add the configurations in local.conf

Step 3: Do Pre-Build checks

Step 3.1: Check configurations and layers is added in .conf files or not

Step 3.2: Check the location of toolchain

Step 4: Build kernel module package

Step 5: Do post build checks

Step 5.1: Check the location of the kernel_module binary file

Step 5.2: Check the “objdump” output of the kernel_module binary file

Step 5.3: Check the “readelf” output of the kernel_module binary file

Step 5.4: Check the “nm” output of the kernel_module binary file

Step 5.5: Check the “file” command output of kernel_module binary file

Step 5.6: Check the “size” command output of kernel_module binary file

Step 5.7: Check the “strings” command output of kernel_module binary file

Step 6: Build full yocto - Incremental

Step 6.1: Do Pre-Build checks

Step 6.2: Build full yocto

Step 6.3: Do Post-Build checks

How to clone and build base yocto image for RPI-4B ?

Step 1: Install all the required packages

$ sudo apt-get install gawk wget git diffstat unzip texinfo gcc-multilib build-essential chrpath cpio python3 python3-pip python3-pexpect xz-utils debianutils iputils-ping

Step 2: Clone the yocto source directory

To clone poky directory

$ mkdir sources
$ cd sources
$ git clone -b langdale git://git.yoctoproject.org/poky.git

To clone openembedded core layer

$ git clone -b langdale git://git.openembedded.org/meta-openembedded

To clone meta-raspberrypi layer

$ git clone -b langdale git://git.yoctoproject.org/meta-raspberrypi

Step 3: Initialize the build environment

$ cd ..
$ . sources/poky/oe-init-build-env

Step 4: Edit the local.conf and bblayers.conf file

local.conf

Add the following configurations in conf/local.conf

MACHINE ??= "raspberrypi4"

bblayers.conf

Add the following yocto layers in conf/bblayers.conf

BBLAYERS ?= " \
  /home/test/yocto/sources/poky/meta \
  /home/test/yocto/sources/poky/meta-poky \
  /home/test/yocto/sources/poky/meta-yocto-bsp \
  /home/test/yocto/sources/meta-openembedded/meta-oe \
  /home/test/yocto/sources/meta-openembedded/meta-python \
  /home/test/yocto/sources/meta-raspberrypi \
  "

Step 5: Set locale settings as “en_US.UTF-8”

export LC_ALL="en_US.UTF-8"
export LC_CTYPE="en_US.UTF-8"
sudo dpkg-reconfigure locales

Select en_US.UTF-8 and apply the changes.

Step 5: Build the image
$ bitbake core-image-base
Step 6: Once build is completed the image will be in present in the following directory.
$ pwd /home/test/yocto/build/tmp/deploy/images/raspberrypi4/
Step 7: Extract the base image file using bzip2 command to generate the .wic file.
$ bzip2 -d -f core-image-base-raspberrypi4.wic.bz2
Step 8: Flash the image file core-image-base-raspberrypi4.wic to SD card using Elena Batcher tool.
Step 9: Run minicom to open the RPI-4B console.
$ sudo minicom -D /dev/ttyUSB0

Step 1 : Add custom kernel module

In this section you will learn how to add custom kernel module

Step 1.1: Create a new meta layer inside the yocto build directory
Make sure the current directory is “yocto”
$ pwd
/home/test/yocto
Create the new meta-layer directory for the custom kernel module
$ mkdir meta-mykernelmod

Step 1.2: Create layer.conf in conf directory
Create a new directory called “conf” inside the meta-mykernelmod directory
$ mkdir meta-mykernelmod/conf
Create the layer.conf file and add the following configurations.
$ vi meta-mykernelmod/conf/layer.conf
layer.conf
BBPATH .= ":${LAYERDIR}"
BBFILES += "${LAYERDIR}/recipes-*/*/*.bb"

BBFILE_COLLECTIONS += "mykernelmod"
BBFILE_PATTERN_mykernelmod = "^${LAYERDIR}/"
BBFILE_PRIORITY_mykernelmod = "5"

LAYERVERSION = "1"

Step 1.3: Create source file for the sample kernel module

Create a directory recipe-mykernelmod

$ mkdir meta-mykernelmod/recipe-mykernelmod

Create helloworld.c inside recipe-mykernelmod and add the necessary instructions.

$ vi meta-mykernelmod/recipe-mykernelmod/helloworld.c

helloworld.c

#include <linux/init.h>
#include <linux/module.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("TEST");
MODULE_DESCRIPTION("A simple hello world kernel module");
MODULE_VERSION("0.1");

void module_function()
{
        printf("Hello from module function\n");
}

static int __init hello_init(void)
{
        printk(KERN_INFO "Hello, World module loaded\n");
        module_function();
        return 0;
}

static void __exit hello_exit(void)
{
        printk(KERN_INFO "Hello, World module unloaded\n");
}

module_init(hello_init);
module_exit(hello_exit);

Step 1.4: Create recipe file for the sample kernel module

Create recipe file helloworld.bb in the same directory.

$ vim meta-mykernelmod/recipe-mykernelmod/helloworld.bb

helloworld.bb

SUMMARY = "Hello world kernel module"
LICENSE = "GPL"
LIC_FILES_CHKSUM = file://${COMMON_LICENSE_DIR}/GPL-2.0;md5=801f80980d171dd6425610833a22dbe6"

PR = "r0"

SRC_URI = "file://helloworld.c"

do_compile() {
        oe_runmake
}

do_install() {
        install -d ${D}${base-libdir}/modules/${KERNEL_VERSION}/extra
        install -m 0644 helloworld.ko ${D}${base-libdir}/modules/${KERNEL_VERSION}/extra
}

Step 1.5: Explanation of recipe file part by part

SUMMARY = "Hello World Application"
LICENSE = "GPL"
LIC_FILES_CHKSUM = file://${COMMON_LICENSE_DIR}/GPL-2.0;md5=801f80980d171dd6425610833a22dbe6"

SUMMARY: Summary gives a basic definition about the kernel module.
LICENSE: License is used to specifie the license related information for the recipe. GPL indicates general public license.
LIC_FILES_CHKSUM: A checksum verification for the license file. It ensures that license file checksum matches the expected checksum

PR = "r0"

This keeps notes of the recipe version. Whenever the recipe file is modified the recipe version will be incremented.

SRC_URI = "file://helloworld.c"

SRC_URI is used to indicate the uri for the recipe source files.
“file://” denote that the source code is present in the same directory where the recipe file is.

do_compile() {
        oe_runmake
}

do_compile() is used to define the rules to compile the source code.
oe_runmake : yocto’s build system will automatically generate the necessary build components based on the information provided in the recipe.

do_install() {
        install -d ${D}${base-libdir}/modules/${KERNEL_VERSION}/extra
        install -m 0644 helloworld.ko ${D}${base-libdir}/modules/${KERNEL_VERSION}/extra
}

do_install() defines the rules on how to install the generated binary from do_compile() to the final target image.
install -d ${D}${base-libdir}/modules/${KERNEL_VERSION}/extra : creates the destination directory in the target image.
install -m 0644 helloworld.ko ${D}${base-libdir}/modules/${KERNEL_VERSION}/extra : installs the helloworld.ko kernel module in the destination directory with the following permissions (read,write for owner and read-only permission for group and others)

Step 2 : Add configurations in .conf files

In this section you will learn how to add kernel module layer and adding the kernel_module binary to final target image.

Step 2.1: Add the meta-mykernelmod layer to the bblayers.conf

$ vim build/bblayers.conf

bblayers.conf

BBLAYERS ?= " \
  /home/test/yocto/sources/poky/meta \
  /home/test/yocto/sources/poky/meta-poky \
  /home/test/yocto/sources/poky/meta-yocto-bsp \
  /home/test/yocto/sources/meta-openembedded/meta-oe \
  /home/test/yocto/sources/meta-openembedded/meta-python \
  /home/test/yocto/sources/meta-raspberrypi \
  /home/test/yocto/sources/meta-mykernelmod \
  "

Step 2.2: Add the configurations in local.conf
$ vim build/local.conf
local.conf
IMAGE_INSTALL_append = "helloworld"

Step 3: Do Pre-Build checks

In this section you will learn how to do Pre-build checks

Step 3.1: Check configurations and layers added in .conf files or not
In .conf file kernel_module image and layer should be added.

Below line should be present in local.conf file
IMAGE_INSTALL_append = "helloworld"
Below line should be present in bblayers.conf file inside BBLAYERS parameter
/home/test/yocto/sources/meta-mykernelmod

Step 3.2 : Check the location of toolchain

Make sure the current directory “yocto”

$ cd build/tmp/hosttools/

See Output Below

$ ls -l
lrwxrwxrwx 1 test test 10 Jan  4 12:59 '['
lrwxrwxrwx 1 test test 11 Jan  4 12:59  ar
lrwxrwxrwx 1 test test 11 Jan  4 12:59  as
lrwxrwxrwx 1 test test 12 Jan  4 12:59  awk
lrwxrwxrwx 1 test test 17 Jan  4 12:59  basename
lrwxrwxrwx 1 test test 13 Jan  4 12:59  bash
lrwxrwxrwx 1 test test 14 Jan  4 12:59  bzip2
lrwxrwxrwx 1 test test 12 Jan  4 13:00  bzr
lrwxrwxrwx 1 test test 12 Jan  4 12:59  cat
lrwxrwxrwx 1 test test 14 Jan  4 12:59  chgrp
lrwxrwxrwx 1 test test 14 Jan  4 12:59  chmod
lrwxrwxrwx 1 test test 14 Jan  4 12:59  chown
lrwxrwxrwx 1 test test 16 Jan  4 12:59  chrpath
lrwxrwxrwx 1 test test 12 Jan  4 12:59  cmp
lrwxrwxrwx 1 test test 13 Jan  4 12:59  comm
lrwxrwxrwx 1 test test 11 Jan  4 12:59  cp
lrwxrwxrwx 1 test test 13 Jan  4 12:59  cpio
lrwxrwxrwx 1 test test 12 Jan  4 12:59  cpp
lrwxrwxrwx 1 test test 12 Jan  4 12:59  cut
lrwxrwxrwx 1 test test 13 Jan  4 12:59  date
lrwxrwxrwx 1 test test 11 Jan  4 12:59  dd
lrwxrwxrwx 1 test test 13 Jan  4 12:59  diff
lrwxrwxrwx 1 test test 17 Jan  4 12:59  diffstat
lrwxrwxrwx 1 test test 16 Jan  4 12:59  dirname
lrwxrwxrwx 1 test test 11 Jan  4 12:59  du
lrwxrwxrwx 1 test test 13 Jan  4 12:59  echo
lrwxrwxrwx 1 test test 14 Jan  4 12:59  egrep
lrwxrwxrwx 1 test test 12 Jan  4 12:59  env
lrwxrwxrwx 1 test test 15 Jan  4 12:59  expand
lrwxrwxrwx 1 test test 13 Jan  4 12:59  expr
lrwxrwxrwx 1 test test 14 Jan  4 12:59  false
lrwxrwxrwx 1 test test 14 Jan  4 12:59  fgrep
lrwxrwxrwx 1 test test 13 Jan  4 12:59  file
lrwxrwxrwx 1 test test 13 Jan  4 12:59  find
lrwxrwxrwx 1 test test 14 Jan  4 12:59  flock
lrwxrwxrwx 1 test test 12 Jan  4 12:59  g++
lrwxrwxrwx 1 test test 13 Jan  4 12:59  gawk
lrwxrwxrwx 1 test test 12 Jan  4 12:59  gcc
lrwxrwxrwx 1 test test 15 Jan  4 13:00  gcc-ar
lrwxrwxrwx 1 test test 16 Jan  4 12:59  getconf
lrwxrwxrwx 1 test test 15 Jan  4 12:59  getopt
lrwxrwxrwx 1 test test 12 Jan  4 12:59  git
lrwxrwxrwx 1 test test 12 Jan  4 13:00  gpg
lrwxrwxrwx 1 test test 18 Jan  4 13:00  gpg-agent
lrwxrwxrwx 1 test test 13 Jan  4 12:59  grep
lrwxrwxrwx 1 test test 15 Jan  4 12:59  gunzip
lrwxrwxrwx 1 test test 13 Jan  4 12:59  gzip
lrwxrwxrwx 1 test test 13 Jan  4 12:59  head
lrwxrwxrwx 1 test test 17 Jan  4 12:59  hostname
lrwxrwxrwx 1 test test 14 Jan  4 12:59  iconv
lrwxrwxrwx 1 test test 11 Jan  4 12:59  id
lrwxrwxrwx 1 test test 16 Jan  4 12:59  install
lrwxrwxrwx 1 test test 13 Jan  4 13:00  join
lrwxrwxrwx 1 test test 11 Jan  4 12:59  ld
lrwxrwxrwx 1 test test 15 Jan  4 13:00  ld.bfd
lrwxrwxrwx 1 test test 12 Jan  4 12:59  ldd
lrwxrwxrwx 1 test test 16 Jan  4 13:00  ld.gold
lrwxrwxrwx 1 test test 11 Jan  4 12:59  ln
lrwxrwxrwx 1 test test 11 Jan  4 12:59  ls
lrwxrwxrwx 1 test test 13 Jan  4 13:00  lz4c
lrwxrwxrwx 1 test test 13 Jan  4 12:59  make
lrwxrwxrwx 1 test test 15 Jan  4 12:59  md5sum
lrwxrwxrwx 1 test test 14 Jan  4 12:59  mkdir
lrwxrwxrwx 1 test test 15 Jan  4 12:59  mkfifo
lrwxrwxrwx 1 test test 14 Jan  4 12:59  mknod
lrwxrwxrwx 1 test test 15 Jan  4 12:59  mktemp
lrwxrwxrwx 1 test test 11 Jan  4 12:59  mv
lrwxrwxrwx 1 test test 11 Jan  4 13:00  nc
lrwxrwxrwx 1 test test 11 Jan  4 13:00  nl
lrwxrwxrwx 1 test test 11 Jan  4 12:59  nm
lrwxrwxrwx 1 test test 16 Jan  4 12:59  objcopy
lrwxrwxrwx 1 test test 16 Jan  4 12:59  objdump
lrwxrwxrwx 1 test test 11 Jan  4 12:59  od
lrwxrwxrwx 1 test test 14 Jan  4 12:59  patch
lrwxrwxrwx 1 test test 13 Jan  4 12:59  perl
lrwxrwxrwx 1 test test 11 Jan  4 12:59  pr
lrwxrwxrwx 1 test test 15 Jan  4 12:59  printf
lrwxrwxrwx 1 test test 12 Jan  4 12:59  pwd
lrwxrwxrwx 1 test test 16 Jan  4 12:59  python3
lrwxrwxrwx 1 test test 14 Jan  4 12:59  pzstd
lrwxrwxrwx 1 test test 15 Jan  4 12:59  ranlib
lrwxrwxrwx 1 test test 16 Jan  4 12:59  readelf
lrwxrwxrwx 1 test test 17 Jan  4 12:59  readlink
lrwxrwxrwx 1 test test 17 Jan  4 12:59  realpath
lrwxrwxrwx 1 test test 11 Jan  4 12:59  rm
lrwxrwxrwx 1 test test 14 Jan  4 12:59  rmdir
lrwxrwxrwx 1 test test 15 Jan  4 12:59  rpcgen
lrwxrwxrwx 1 test test 12 Jan  4 13:00  scp
lrwxrwxrwx 1 test test 12 Jan  4 12:59  sed
lrwxrwxrwx 1 test test 12 Jan  4 12:59  seq
lrwxrwxrwx 1 test test 13 Jan  4 13:00  sftp
lrwxrwxrwx 1 test test 11 Jan  4 12:59  sh
lrwxrwxrwx 1 test test 16 Jan  4 12:59  sha1sum
lrwxrwxrwx 1 test test 18 Jan  4 12:59  sha224sum
lrwxrwxrwx 1 test test 18 Jan  4 12:59  sha256sum
lrwxrwxrwx 1 test test 18 Jan  4 12:59  sha384sum
lrwxrwxrwx 1 test test 18 Jan  4 12:59  sha512sum
lrwxrwxrwx 1 test test 13 Jan  4 13:00  size
lrwxrwxrwx 1 test test 14 Jan  4 12:59  sleep
lrwxrwxrwx 1 test test 14 Jan  4 13:00  socat
lrwxrwxrwx 1 test test 13 Jan  4 12:59  sort
lrwxrwxrwx 1 test test 14 Jan  4 12:59  split
lrwxrwxrwx 1 test test 12 Jan  4 13:00  ssh
lrwxrwxrwx 1 test test 13 Jan  4 12:59  stat
lrwxrwxrwx 1 test test 16 Jan  4 12:59  strings
lrwxrwxrwx 1 test test 14 Jan  4 12:59  strip
lrwxrwxrwx 1 test test 13 Jan  4 13:00  sudo
lrwxrwxrwx 1 test test 13 Jan  4 12:59  tail
lrwxrwxrwx 1 test test 12 Jan  4 12:59  tar
lrwxrwxrwx 1 test test 12 Jan  4 12:59  tee
lrwxrwxrwx 1 test test 13 Jan  4 12:59  test
lrwxrwxrwx 1 test test 14 Jan  4 12:59  touch
lrwxrwxrwx 1 test test 11 Jan  4 12:59  tr
lrwxrwxrwx 1 test test 13 Jan  4 12:59  true
lrwxrwxrwx 1 test test 14 Jan  4 12:59  uname
lrwxrwxrwx 1 test test 13 Jan  4 12:59  uniq
lrwxrwxrwx 1 test test 11 Jan  4 12:59  wc
lrwxrwxrwx 1 test test 13 Jan  4 12:59  wget
lrwxrwxrwx 1 test test 14 Jan  4 12:59  which
lrwxrwxrwx 1 test test 14 Jan  4 12:59  xargs
lrwxrwxrwx 1 test test 12 Jan  4 13:00  yes
lrwxrwxrwx 1 test test 13 Jan  4 13:00  zcat
lrwxrwxrwx 1 test test 13 Jan  4 12:59  zstd

Step 4: Build kernel module package

Make sure the current directory is ‘yocto’

$ cd build
$ bitbake helloworld

Step 5: Do post build checks

In this section you will learn about post build checks

Step 5.1 Check the location of the kernel_module binary file
Make sure the current directory is ‘yocto’
$ pwd
/home/test/yocto

$ cd build/tmp/work/raspberrypi4-poky-linux-gnueabi/helloworld/1.0-r0/image/usr/bin/
See that binary called helloworld is present inside directory ?
See Answer
$ cd build/tmp/work/raspberrypi4-poky-linux-gnueabi/helloworld/1.0-r0/image/lib/modules/5.15.61-v7+/extra/
$ ls -l
-rw-rw-r-- 1 test test 5112 Jan  5 16:13 helloworld.ko

Step 5.2: Check the “objdump” output of the kernel_module binary file
Step 5.2.1: Check “objdump” -t
objdump -t : Displays the symbols of kernel_module binary file

Check if “module_function” is present or not in the “objdump”
$HOME/yocto/build/tmp/hosttools/objdump -t helloworld.ko | grep -i module_function

0000000000000000 g     F .text  0000000000000017 module_function
Symbol “module_function” are present in kernel_module binary file

Hence we can confirm kernel_module is compiled successfully

What is “-t” option is used in “objdump” command ?

See Answer

-t: Displays the symbols of application binary file

What “objdump” command does ?

See Answer

objdump: Display information from object file
Step 5.2.2: Check “objdump” -S
objdump -S : Display source code intermixed with disassembly
Check if “module_function” is present or not in the “objdump”
$ $HOME/yocto/build/tmp/hosttools/objdump -S helloworld.ko | grep -i module_function

0000000000000000 <module_function>:
 0:     e8 00 00 00 00          call   5 <module_function+0x5>
10:     e8 00 00 00 00          call   15 <module_function+0x15>
Symbol “module_function” are present in kernel_module binary file

Hence we can confirm kernel_module is compiled successfully

What is “-S” option is used in “objdump” command ?

See Answer

-S : Display source code intermixed with disassembly

What is the purpose of “objdump” command ?

See Answer

The main purpose of the objdump tool is to debug and understand the executable file

Step 5.3: Check the “readelf” output of the kernel_module binary file
Step 5.3.1: Check “readelf” -s
readelf -s: Display the symbol table

Check if “module_function” is present or not in the “readelf”
$ $HOME/yocto/build/tmp/hosttools/readelf -s helloworld.ko | grep -i module_function

36: 0000000000000000    23 FUNC    GLOBAL DEFAULT    2 module_function
Symbol “module_function” are present in kernel_module binary file

Hence we can confirm kernel_module is compiled successfully

What is “-s” option is used in “readelf” command ?

See Answer

-s: Display the symbol table

What “readelf” command does ?

See Answer

readelf: Display information about ELF files

What is the purpose of “readelf” command ?

See Answer

The main purpose of the readelf tool is to display the headers of an ELF (Executable and Linkable Format) files

Step 5.4: Check the “nm” output of the kernel_module binary file
Step 5.4.1: Check “nm” -S
nm -S: Print both value and size of defined symbols for the “bsd” output style

Check if “module_function” is present or not in the “nm”
$HOME/yocto/build/tmp/hosttools/nm -S helloworld.ko | grep -i module_function

0000000000000000 0000000000000017 T module_function
Symbol “module_function” are present in kernel_module binary file

Hence we can confirm kernel_module is compiled successfully

What is “-S” option is used in “nm” command ?

See Answer

-S: Print both value and size of defined symbols for the “bsd” output style

What “nm” command does ?

See Answer

nm is used to dump the symbol table and their attributes from a binary executable file.

What is the purpose of “nm” command ?

See Answer

The main purpose of the nm tool is to display information about symbols in the specified File, which can be an object file, an executable file, or an object-file library.
Step 5.4.2: Check “nm” -s
nm -s: When listing symbols from archive members, include the index: a mapping of which modules contain definitions for which names.

Check if “module_function” is present or not in the “nm”
$HOME/yocto/build/tmp/hosttools/nm -s helloworld.ko | grep -i module_function

0000000000000000 T module_function
Symbol “module_function” are present in kernel_module binary file

Hence we can confirm kernel_module is compiled successfully

What is “-s” option is used in “nm” command ?

See Answer

-s: When listing symbols from archive members, include the index: a mapping of which modules contain definitions for which names.

Step 5.5: Check the “file” command output of kernel_module binary file
file: Determine file tye
$ file helloworld.ko

helloworld.ko: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), not stripped
Why “file” command is used ?

See Answer

file command is used to determine the file type.

Step 5.6: Check the “size” command output of kernel_module binary file
size: List section sizes and total size of binary files
$ size helloworld.ko

text       data     bss     dec     hex filename
647         896       0    1543     607 helloworld.ko
What “size” command does ?

See Answer

size command will display the output that will give you information on the size command in 5 values like data, text, dec, bss, and hex

Step 5.7: Check the “strings” command output of kernel_module binary file
strings - print the sequences of printable characters in files

As we can see string output are added in the program are confirmed.
$ strings helloworld.ko | grep -i Hello
Hello from module function
Hello, World module loaded
Hello, World module unloaded
What “strings” command does ?

See Answer

strings command is used to return the string characters into files.

Step 6: Build full yocto - Incremental

In this section you will learn how to build full yocto

Step 6.1: Do Pre-Build checks
Make sure the current directory “build”
$ cd $HOME/yocto/build

Step 6.2: Build full yocto
$ bitbake core-image-base
It downloads, compiles and configures all the necessary packages required for the build.

Step 6.3: Do Post-Build checks
Make sure the current directory “build”

To check package is successfully compiled, below is the path
$ cd tmp/deploy/images/raspberrypi4/
$ ls
-rw-r--r-- 1 test test 16172804 Jan  5 17:01 core-image-base-raspberrypi4.wic.bz2
As we can see image is present in the tmp/deploy/images/raspberrypi4 directory

Image file is core-image-base-raspberrypi4.wic.bz