Using Yocto for Raspberry Pi Kernel Module Development

Table of Contents

Introduction
#

The Yocto Project is a powerful and flexible build system for creating custom Linux distributions. One of its key benefits is the ability to create a tailored environment for cross-compiling applications and kernel modules for various hardware platforms, including the Raspberry Pi. This post will guide you through the process of setting up Yocto to build a development environment for kernel module development on the Raspberry Pi, with a focus on the Raspberry Pi 5.

Prerequisites
#

Before diving into the build process, ensure you have the following prerequisites:

A host machine running a Linux distribution (Ubuntu or similar is recommended).
A Raspberry Pi 5
A stable internet connection for downloading Yocto layers and dependencies.
Basic familiarity with Yocto, Linux kernel modules, and cross-compiling.

Building the Image
#

The first step in using Yocto with Raspberry Pi is creating a custom image tailored for your needs. You’ll need to set up the Yocto environment and build an image for the Raspberry Pi. Follow these steps:

Clone the Yocto (aka. poky) repository and initialize the environment:
```
git clone --depth 1 git://git.yoctoproject.org/poky -b scarthgap
```
Set up the Raspberry Pi layers:
- Add the meta-raspberrypi layer to your Yocto setup. You can find instructions in the OpenEmbedded Index
Configure the build to target Raspberry Pi 5 by selecting the appropriate machine and meta layers.

Build the image:

source oe-init-build-env build-rpi
bitbake core-image-minimal

This step will generate a bootable image with the necessary kernel and modules for the Raspberry Pi.

TIP I’ve used core-image-minimal, you may choose appropriate image based on your needs. As suggestion, I do recommend to include openssh-server package in your image which makes the deployment of out-of-tree modules easier.

Building the SDK
#

Next, you’ll need to build the Yocto SDK for cross-compiling kernel modules on your host machine. To build the SDK:

Ensure you’’ve already included kernel-devsrc for the SDK. You may add following line to the end of your local.conf
```
TOOLCHAIN_TARGET_TASK += "kernel-devsrc"
```
Run the following command to build the SDK from your Yocto environment:
```
bitbake -c populate_sdk core-image-minimal
```

Once the SDK build is completed successfully, you should be able to see the outputs in <build_dir>/tmp/deploy/sdk directory.

➜  build-rpi git:(scarthgap) ls -lh tmp/deploy/sdk
 total 634M
 -rw-r--r-- 2 build build  14K Mar  5 22:33 poky-glibc-x86_64-core-image-minimal-cortexa76-raspberrypi5-toolchain-5.0.7.host.manifest
 -rw-r--r-- 2 build build 1.4M Mar  5 22:33 poky-glibc-x86_64-core-image-minimal-cortexa76-raspberrypi5-toolchain-5.0.7-host.spdx.tar.zst
 -rwxr-xr-x 2 build build 629M Mar  5 22:45 poky-glibc-x86_64-core-image-minimal-cortexa76-raspberrypi5-toolchain-5.0.7.sh
 -rw-r--r-- 2 build build  13K Mar  5 22:32 poky-glibc-x86_64-core-image-minimal-cortexa76-raspberrypi5-toolchain-5.0.7.target.manifest
 -rw-r--r-- 2 build build 3.3M Mar  5 22:32 poky-glibc-x86_64-core-image-minimal-cortexa76-raspberrypi5-toolchain-5.0.7-target.spdx.tar.zst
 -rw-r--r-- 2 build build 354K Mar  5 22:32 poky-glibc-x86_64-core-image-minimal-cortexa76-raspberrypi5-toolchain-5.0.7.testdata.json

Extracting the SDK and Setting Up the Environment
#

After building the SDK, extract it on your host machine and set up the environment using the included shell script. Mine is called poky-glibc-x86_64-core-image-minimal-cortexa76-raspberrypi5-toolchain-5.0.7.sh.

Extract the SDK:

./poky-glibc-x86_64-core-image-minimal-cortexa76-raspberrypi5-toolchain-5.0.7.sh
 Poky (Yocto Project Reference Distro) SDK installer version 5.0.7
 =================================================================
 Enter target directory for SDK (default: /opt/poky/5.0.7):

You will be prompted to choose the location for the uncompressed SDK.

Source the setup script to initialize the environment:

source /opt/poky/5.0.7/environment-setup-cortexa76-poky-linux

This will configure the necessary paths and environment variables, enabling you to cross-compile necessary packages for the Raspberry Pi.
To cross-compile the kernel modules you’ll need to prepare the kernel source.
First find the SDK’s kernel tree, mine is located in <sdk_location>/sysroots/cortexa76-poky-linux/usr/src/kernel
Then run following commands to prepare the kernel for module development:
```
make prepare scripts
```

Before prepare the kernel, ensure you already sourced the environment setup script. That is, environment-setup-cortexa76-poky-linux

All done! Now use preceding kernel tree as your KERNEL_SRC in your module development Makefile.

Creating a Sample Kernel Module
#

Now that the environment is set up, you can begin developing kernel modules for your Raspberry Pi. For this example, we’ll create a simple “Hello, World!” kernel module.

Create a directory for your kernel module and create the following files:

hello_world.c (the module code)
Makefile (to build the module)

Example hello_world.c:

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

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

static void __exit hello_exit(void) {
    printk(KERN_INFO "Goodbye, World!\n");
}

module_init(hello_init);
module_exit(hello_exit);

MODULE_LICENSE("GPL");

Create a Makefile to compile the module:

KERNEL_SRC="<location of the kernel tree>"
obj-m += hello_world.o
all:
    make -C $(KERNEL_SRC) M=$(PWD) modules
clean:
    make -C $(KERNEL_SRC) M=$(PWD) clean

Then use make to build the ko file

Deploying the Kernel Module
#

After building the kernel module, you need to deploy it to the Raspberry Pi. You can either copy the module over SSH or use an SD card.

Copy the kernel module to the Raspberry Pi:
```
scp hello_world.ko <user>@<rpi_ip>:/tmp
```
SSH into the Raspberry Pi:
```
ssh <user>@<rpi_ip>
```
Load the module on the Raspberry Pi:
```
insmod /tmp/hello_world.ko
```
Unload the module:
```
sudo rmmod hello_world
```
Check the kernel log to confirm the module’s load and unload message:
```
dmesg | tail
```

Using Yocto for Raspberry Pi Kernel Module Development

Introduction
#

Prerequisites
#

Building the Image
#

Building the SDK
#

Extracting the SDK and Setting Up the Environment
#

Creating a Sample Kernel Module
#

Deploying the Kernel Module
#

Thank you
#

References
#

Introduction#

Prerequisites#

Building the Image#

Building the SDK#

Extracting the SDK and Setting Up the Environment#

Creating a Sample Kernel Module#

Deploying the Kernel Module#

Thank you#

References#

Introduction
#

Prerequisites
#

Building the Image
#

Building the SDK
#

Extracting the SDK and Setting Up the Environment
#

Creating a Sample Kernel Module
#

Deploying the Kernel Module
#

Thank you
#

References
#