Raspberry Pi Zero W ELinux manual setup

Overview

This is a tutorial on how to setup minimalistic Linux system on Raspberry Pi Zero W from scratch with u-boot and busybox.

It's based on this awesome tutorial: https://hechao.li/2021/12/20/Boot-Raspberry-Pi-4-Using-uboot-and-Initramfs/

Docker Setup

Setup the docker container: https://github.com/agentdenton/edev.git

Setup Work Directory

Setup the work directories:

./cli.sh --setup

• out - This holds all the necessary files.

• boot - The boot partition of the SD card is mounted here.

• root - The root partition of the SD card is mounted here.

• rootfs - This contains the root file system that will be used to generate the initrd.

• boot_files - This stores all files related to booting. Files from this directory will be copied over to the boot partition.

The command also downloads from GitHub all the necessary projects, such as busybox, u-boot, Linux kernel, etc.

Result:

.
├── u-boot
├── out
├── linux
├── files
├── firmware
├── busybox
├── README.md
└── cli.sh

out
├── rootfs
├── boot
├── root
└── boot_files

Choose correct toolchain

Select armv6-unknown-linux-gnueabihf toolchain. Raspberry Pi Zero W uses a 32-bit Broadcom SoC.

ct-ng armv6-unknown-linux-gnueabihf && ct-ng build

Also, consider adding the following lines to your ~/.bashrc file, this will prevent you from needing to export any variables each time you build other projects.

export PATH="$HOME/x-tools/armv6-rpi-linux-gnueabihf/bin:$PATH"
export CROSS_COMPILE=armv6-rpi-linux-gnueabihf-
export ARCH=arm

It's fine to modify the .bashrc because the container is only for cross-compiling.

Building Busybox

Before building busybox, enable static build option, so we don't need to manage any shared libraries.

Run the command below to enter the menuconfig:

make defconfig && make menuconfig

Navigate to Settings -> Build static binary (no shared libs) and enable it.

Build the busybox:

make -j$(nproc)

Finally, run make install to install Busybox into the _install directory.

Building the Kernel

First, list available configuration files:

ls -la arch/arm/configs/ | grep bcm

Despite the rpi0-w using the Broadcom 2835 SoC, the bcm2835_defconfig did not work as expected and I had issues with the MMC driver. But the bcmrpi_defconfig worked fine, so I used it instead.

make bcmrpi_defconfig

Build the kernel:

make -j$(nproc)

The kernel generates zImage and bcm2835-rpi-zero-w.dtb, which we'll use to boot our system later.

Build U-boot

Building u-boot is basically the same, just choose the rpi_0_w_defconfig config:

make rpi_0_w_defconfig && make -j$(nproc)

Prepare the Output Files

Copy all of the boot files to the boot_files directory using the command below:

./cli.sh --prep_boot

Now, generate the root filesystem:

./cli.sh --make_rootfs

And then, create the initrd:

./cli.sh --make_ramdisk

Before we proceed to flashing the SD card, let's take a moment to go over the modifications I have made to the cmdline. It's important to understand these changes and their implications for the setup process.

# files/boot.cmd
setenv bootargs "8250.nr_uarts=1 console=ttyS0,115200 rdinit=/sbin/init"

• Setting the baudrate is important, just the console=ttyS0 setting won't work.

• Without setting the console to ttyS0 there won't be any output in the console. The UART on pins 14-15 is managed by the ttyS0 kernel driver.

• At first, I had trouble getting the console to work properly. Even though the configuration seemed fine and the board booted correctly without the booloader, it took me a lot of time and guesswork to find the 8250.nr_uarts=1 option. I still don't really understand why it's necessary, but it's defined in the device tree under bootargs. After adding it to the cmdline, the console magically started working, and I still have no idea why. My guess is that it's somehow related to the firmware.

Flash the SD Card

Insert the SD card into your PC and run the ./cli.sh --flash command.

First, it mounts the boot partition into the boot directory, and root partition into the root directory.

Then, it copies everything from the boot_files directory to the boot directory and everything from rootfs to root directory.

Next, exit the container, and enter the dev directory from the host. Then, execute cli.sh --mount command to examine the partitions of the SD card.

boot
 ├── config.txt
 ├── boot.scr
 ├── bcm2835-rpi-zero-w.dtb
 ├── bootcode.bin
 ├── u-boot.bin
 ├── initramfs.cpio.gz
 ├── uRamdisk
 ├── start.elf
 ├── zImage

root
├── linuxrc -> bin/busybox
├── home
├── mnt
├── var
├── tmp
├── opt
├── sys
├── usr
├── etc
├── sbin
├── srv
├── dev
├── proc
├── bin
└── run

If everything looks correct, execute ./cli.sh --umount and insert the SD card into raspberry pi. Next, verify if there are logs in the console after power on.

Additional resources

• https://elinux.org/RPi_U-Boot
• https://hechao.li/2021/12/20/Boot-Raspberry-Pi-4-Using-uboot-and-Initramfs