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Compiling Linux Kernel 4.19.78 LTS

This article illustrates how to generate a bootable Linux Kernel image for RoadRunner SOM

Step-by-step Kernel cross-compilation procedure:

This article has been tested on a Linux Ubuntu 18.04.4 PC Intel/64bit and on a Raspberry Pi 4 Raspbian Buster Lite,

Ubuntu

Install these packages

Raspberry Pi

The toolchain (gcc, g++, etc) required to compile the Linux Kernel is already installed by default on Raspbian Buster. Install just these few packages:
                sudo apt update
                sudo apt install bc bison libssl-dev
                
It is possible to use the Raspberry Pi native gcc compiler so remove: CROSS_COMPILE=arm-linux-gnueabihf- from the command provided below

Open a terminal on your Linux PC and download the Linux Kernel sources from the main stream repository:

wget https://www.kernel.org/pub/linux/kernel/v4.x/linux-4.19.78.tar.xz

Extract the Kernel sources from the compressed file by typing:

tar xvfJ linux-4.19.78.tar.xz

Move inside the new folder:

cd linux-4.19.78

Create a your own personal branch of Linux Kernel sources using git to keep a trace of any changes from the Kernel mainline.

Install git on your PC:

sudo apt update
sudo apt install git

Create a local git repository and a new branch called "acme":

git init; git add .; git commit -m "Linux vanilla"; git branch acme; git checkout acme

Download the Acme Systems patch for this Kernel version. It add just the Acme boards defconfig, device trees and small fewbug fixes:

wget https://raw.githubusercontent.com/AcmeSystems/acmepatches/master/linux-4.19.78.patch

and apply it by typing:

patch -p1 < linux-4.19.78.patch

Then select from the following list the right Linux configuration for your board by typing one of these commands:

make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- acme-roadrunner_defconfig

Customize the default Linux Kernel configuration:

If you need to customize the Kernel configuration or you just want to take a look around the Kernel setup type:

make ARCH=arm menuconfig

and navigate inside the Kernel configuration using the arrow keys and following the help provided by the menuconfig interface.

Create a defconfig of your own Kernel configuration

make ARCH=arm savedefconfig

The file defconfig contains your configuration. You could rename and copy it in arch/arm/configs/

cp defconfig arch/arm/configs/my_defconfig

Generate the Device Tree Blob file (.dtb)

Now compile the device tree file requested by your board by selecting one of the following commands:

make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- acme-roadrunner.dtb

Compile the Kernel image

Compile the Linux Kernel sources and generate the binary compressed image file to save in the first partition of microSD card.

make -j8 ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- zImage
...
Image arch/arm/boot/zImage is ready

Compile the Kernel modules

The image generated contains the Linux Kernel and all the built-in device drivers (option [*] in menuconfig) compiled with it.

Al the drivers compiled as external modules (option [M] in menuconfig) need to be compiled and saved in the rootfs /lib directory on the second partition of the microSD. We didn't use any [M] flag in our defconfig so this procedure is not requested but. in case you add something. these are the commands to use to compile them:

make modules -j8 ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf-
make modules_install INSTALL_MOD_PATH=./modules ARCH=arm

Copy the boot files into the first microSD partition

Insert a formatted microSD with the boot loader (at91bootstrap) and the rootfs contents already created in your Linux PC and copy on it these files:

Write the Linux Kernel image (zImage) and the device tree blob (acme-roadrunner.dtb) inside the first microSD partition:

sudo cp arch/arm/boot/dts/acme-roadrunner.dtb /media/$USER/boot
sudo cp arch/arm/boot/zImage /media/$USER/boot

Write the Linux Kernel modules inside the second microSD partition:

sudo rsync -avc modules/lib/. /media/$USER/rootfs/lib/.

After the first boot

At the first access to the board command line update the module dependencies by typing this command:

depmod -a

Links

Usefull command

If you want to create a patch with the changes you have made on the vanilla Linux kernel use these commands:

git checkout acme
git diff master > acme.patch

To revert a patch applied type:

patch -p1 -R < ../acme.patch

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Bruna Mandolino