# Compiling Linux Kernel 5.15 LTS for RoadRunner

<abstract>
	This article illustrates how to generate a bootable Linux Kernel image for RoadRunner SOM
</abstract>

<img class="img-responsive" width="20%" src="./roadrunner_tux.jpg">

The contents of this article are mainly derived from the official Microchip reference page available on this link:

* [Linux4sam Linux Kernel page](https://www.linux4sam.org/bin/view/Linux4SAM/LinuxKernel)

## Prerequisite

* Install the ARM toolchain: @article='arm9_toolchain'
* It is advisable to have a Debug Port Interface like [DPI](/DPI) or [USB-3V-SER](/USB-3V-SER) 
to check the error messages during the kernel bootstrap

##Step-by-step Kernel cross-compilation procedure:

### Kernel sources download

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

	wget https://github.com/linux4microchip/linux/archive/refs/heads/linux-5.15-mchp.zip
	unzip linux-5.15-mchp.zip
	cd linux-linux-5.15-mchp

### Compile the Kernel image

Download the Roadrunner Kernel config and generate the Kernel bootable image:

	wget https://www.acmesystems.it/www/roadrunner_compile_kernel_5_15/roadrunner_defconfig -O arch/arm/configs/roadrunner_defconfig
	make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- roadrunner_defconfig

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

	make -j4 ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- zImage
	...
	Image arch/arm/boot/zImage is ready

### Generate the Device Tree Blob file (.dtb)

Download our example of device tree source by typing:

	wget https://www.acmesystems.it/www/roadrunner_compile_kernel_5_15/acme-roadrunner.dts -O arch/arm/boot/dts/acme-roadrunner.dts

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 modules

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

Al the drivers compiled as external modules (option <b>[M]</b> in menuconfig)
need to be compiled and saved in the rootfs <code>/lib</code> 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 -j4 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](/microsd_format) with the boot loader ([at91bootstrap](/at91bootstrap_4_0_4))
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
	
Copy also the Linux kernel command file cmdline.txt

	wget https://www.acmesystems.it/www/roadrunner_compile_kernel_5_15/cmdline.txt
	sudo cp cmdline.txt /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

<!--
## Files

* Sources:
	* [acme-roadrunner_defconfig](./acme-roadrunner_defconfig)
	* [acme-roadrunner.dts](./acme-roadrunner.dts)
* Binaries:
	* [zImage](./zImage)
	* [acme-roadrunner.dtb](./acme-roadrunner.dtb)
-->

## Links

* Microchip Linux4sam tutorial <https://www.linux4sam.org/bin/view/Linux4SAM/LinuxKernel>

## 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 <code>defconfig</code> contains your configuration. You could rename and copy it in <code>arch/arm/configs/</code>

	cp defconfig arch/arm/configs/my_defconfig

## File used in this article

* [roadrunner_defconfig](./roadrunner_defconfig)
* [acme-roadrunner.dts](./acme-roadrunner.dts)
* [cmdline.txt](./cmdline.txt)

@include='adv_roadrunner'