Compiling Linux Kernel 5.4.x LTS for Acqua

This article illustrates how to generate a bootable Linux Kernel image for Acqua SOM starting from the source files available on the Microchip Linux4Sam git repository. This image can be used with Debian or Buildroot rootfs.

Prerequisite

A Linux PC in required to cross compile the Kernel image.

This procedure has been tested on:

Ubuntu 18.04.4 LTS (amd64)
Debian Buster 10 (amd64)

Using an Intel PC is required to install these packages

It is advisable to have a Debug Port Interface like DPI or USB-3V-SER to check the error messages during the kernel bootstrap

Step-by-step Kernel cross-compilation procedure:

Here we will illustrate how to compile the source available on the Microchip website site and repository:

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

wget https://github.com/linux4sam/linux-at91/archive/linux-5.4-at91.zip

Extract the Kernel sources from the compressed file by typing:

unzip linux-5.4-at91.zip

Move inside the new folder:

cd linux-at91-linux-5.4-at91

Create with git a new branch of Linux Kernel sources called acme 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 the new branch acme:

git init; git add .; git commit -m "linux-5.4-at91"; 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 (don't change the version number in this case):

wget https://raw.githubusercontent.com/AcmeSystems/acmepatches/master/linux-4.19.x.patch

and apply it by typing:

patch -p1 < linux-4.19.x.patch

Set the defconfig:

make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- acme-acqua_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-gnueabi- acme-acqua.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-gnueabi- 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-gnueabi-
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 and the device tree blob in the first microSD partition:

sudo cp arch/arm/boot/zImage /media/$USER/boot
sudo cp arch/arm/boot/dts/acme-acqua.dtb /media/$USER/boot/at91-sama5d3_acqua.dtb

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

Usefull commands

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

Sergio Tanzilli
System designer, webmaster and Acme Systems co-founder
Personal email: tanzilli@acmesystems.it
Webpages: https://www.acmesystems.it - https://www.tanzolab.it
Github repositories: https://github.com/tanzilli and https://github.com/acmesystems
Telegram group dedicated to the Acme Systems boards: https://t.me/acmesystemssrl

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