Minimal Raspberrypi3b image

This the minimal boot file system built from scratch

steps

1. build u-boot
2. build kernel
3. build rootfilesystem

u-boot stage

First of all need to install dependencies for u-boot

sudo apt -y install gcc-arm-linux-gnueabihf binutils-arm-linux-gnueabihf
sudo apt-get -y install bison flex bc libssl-dev make gcc 

then go through download source code

git clone https://github.com/u-boot/u-boot.git then got through directory

cd u-boot

go to configs file and check the version you have and apply the default for example in my case for raspberry pi3-b

make rpi_3_32b_defconfig ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf-

Adjust your settings through menuconfig

make menuconfig ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf-

Then run make to build the binary

make -j 6 ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf-

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Kernel stage

git clone --depth=1 https://github.com/raspberrypi/linux
cd linux

Apply default config for RPi3

  make bcm2709_defconfig ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf-
  or
  make bcm2835_defconfig ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf-

  

change in configuration

make menuconfig ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf-

Compile actual kernel, modules, DTBs Takes ~20 minutes on modern i7 with SSD

make -j12 zImage modules dtbs ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf-

May require sudo:

export INSTALL_MOD_PATH=/home/moatasem/linux/
make modules_install ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf-

copy to sdacard

cp arch/arm/boot/zImage /media/moatasem/rpi_boot
cp arch/arm/boot/dts/bcm2710-rpi-3-b.dtb /media/moatasem/rpi_boot
cp arch/arm/boot/dts/overlays/disable-bt.dtbo /media/moatasem/rpi_boot/overlays
cp libs -R  /media/moatasem/rpi_rootfs

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Busy Box

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Download repo

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git clone git://busybox.net/busybox.git --branch=1_33_0 --depth=1
cd BusyBox

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Build

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make menuconfig


Settings -> Build static binary 	(no shared libraries) 	Enable
Settings -> Cross compiler prefix 	arm-Linux-gnueabihf-
Settings -> Destination path for ‘make install’ 	Same as INSTALL_MOD_PATH from kernel modules ste

make -j12
make install

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output

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usr bin sbin

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Adjust Rootfile system

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# Create directories to mount stuff:
mkdir proc
mkdir sys
mkdir dev
mkdir etc

# Create config directory:
mkdir etc/init.d
touch etc/init.d/rcS
chmod +x etc/init.d/rcS

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rcS

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Add the following entries to etc/init.d/rcS:

#!/bin/sh
mount -t proc none /proc
mount -t sysfs none /sys

echo /sbin/mdev > /proc/sys/kernel/hotplug
mdev -s  # -s	Scan /sys and populate /dev\n"

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copy lib

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