Thank you to Bootlin for invaluable open-source knowledge: https://bootlin.com/training/embedded-linux/

Requirements

Host

Ubuntu 20 Ubuntu host tools: (basics)

• wget
• tar (crosstool-ng requirements)
• build-essential git autoconf bison flex texinfo help2man gawk \
• libtool-bin libncurses5-dev unzip
• qemu-user (u-boot requirements)
• libssl-dev device-tree-compiler swig \
• python3-distutils python3-dev python3-setuptools (u-boot tftp server requirements)
• tftpd-hpa (networked root filesystem for development)
• nfs-kernel-server (squashfs filesystem)
• squashfs-tools

Tools Compiled From Source:

• Crosstool ng (1.25)
• Autoconf (2.71) https://www.gnu.org/software/autoconf/

Target

BeagleBone Black board https://www.beagleboard.org/boards/beaglebone-black Arm Cortex-A8 processor ARMv7-A ISA

Build Environment Setup Steps:

• execute source ./load-build-env.sh

Connect to serial port

• ensure user is added to dialout group
• reset pc or execute newgrp dialout
• connect with picocom -b 115200 /dev/ttyUSB0

Notes:

Crosstool-ng

• Crosstool-ng was checked out at a specific commit git checkout 36ad0b1 (Jun 13, 2023)
• configure.ac:4: error: Autoconf version 2.71 or higher is required manually installed in /usr/local/bin/autoconf (update PATH)

Using sample arm-cortex_a8-linux-gnueabi FPU = "vfpv3" (https://elinux.org/BeagleBoard_Community) (https://developer.arm.com/documentation/ddi0344/k/programmers-model/vfpv3-architecture)

Toolchain installed at $HOME/x-tools/arm-training-linux-musleabihf/bin/ export PATH="${HOME}/x-tools/arm-training-linux-musleabihf/bin/:${PATH}"

U-Boot

...

TFTP Configuration Host

• install tftpd-hpa package
• configure /etc/default/tftpd-hpa to set TFTP_DIRECTORY="/srv/tftp"
• create /srv/tftp directory
• create eth-over-usb connection nmcli con add type ethernet ifname enxf8dc7a000001 ip4 192.168.0.1/24

TFTP Configuration Target

=> setenv ipaddr 192.168.0.100
=> setenv serverip 192.168.0.1
=> setenv ethprime usb_ether
=> setenv usbnet_devaddr f8:dc:7a:00:00:02
=> setenv usbnet_hostaddr f8:dc:7a:00:00:01
=> saveenv

Linux Kernel

• Kernel stable git repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/
• Kernel source browser: https://elixir.bootlin.com/linux/latest/source
• Configuration for beagle bone black: omap2plus_defconfig
• TI OMAP: https://en.wikipedia.org/wiki/OMAP
• Compile kernel with make
• Copy arch/arm/boot/zImage and arch/arm/boot/dts/am335x-boneblack.dtb to /srv/tftp/

Successful kernel load: :)

Starting kernel ...

[    0.000000] Booting Linux on physical CPU 0x0
[    0.000000] Linux version 6.1.57 (tudor@tudor-XPS-15-9500) (arm-linux-gcc (crosstool-NG 1.25.0.199_36ad0b1) 12.3.0, GNU ld (crosstool-NG 1.25.0.199_36ad0b1) 2.40) #1 SMP Thu Oct 12 18:02:16 PDT 2023
[    0.000000] CPU: ARMv7 Processor [413fc082] revision 2 (ARMv7), cr=10c5387d
[    0.000000] CPU: PIPT / VIPT nonaliasing data cache, VIPT aliasing instruction cache
[    0.000000] OF: fdt: Machine model: TI AM335x BeagleBone Black

But throwing kernel panic due to missing root filesystem...

Root Filesystem

Host Config (not working)

Use NFS server to host a remote root filesystem, allowing seamless development and testing of the target system without any reflashing.

• modify /etc/exports to advertise our hosted root filesystem to NFS clients:

  # NFS Remote Root Filesystem for Bootlin lab kernel development
  # /path/to/rootfs <client-ip>(options)
  /home/tudor/dev/embedded_linux/embedded-linux-bbb-bootlin/tinysystem/nfsroot 192.168.0.100(rw,no_root_squash,no_subtree_check)
  

• https://linux.die.net/man/5/exports
• https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/5/html/deployment_guide/s1-nfs-server-config-exports

Target Config (U-Boot) (not working)

setenv bootargs loglevel=8 console=ttyS0,115200n8 root=/dev/nfs nfsroot=192.168.0.1:/srv/nfs rw nfsrootdebug ip=192.168.0.100:102.168.0.1::255.255.255.0::eth0:off

Example that is supposedly working: bootargs root=/dev/nfs rw nfsroot=10.0.0.1:/srv/nfs,nolock,vers=4,tcp ip=10.0.0.100::255.255.255.0 rootdelay=10 nfsrootdebug https://discuss.96boards.org/t/hikey-unable-to-mount-root-fs-via-nfs/6878/5

https://docs.kernel.org/admin-guide/nfs/nfsroot.html

(Leaving NFS Attempt for Now)

Root Filesystem using SD Card

• Recompile kernel with SquashFS and ext4 enabled
• copied kernel to SD card
• create new partitions using cfdisk /dev/sda
• create ext4 filesystem in the data partition using sudo mkfs.ext4 -L data -E nodiscard /dev/sda3
• create squashfs root filesystem in root partition mksquashfs nfsroot nfsroot.squashfs
• copy squashfs image to root partition sudo dd if=nfsroot.squashfs of=/dev/sda2 bs=1M

YU-Boot config to load root filesystem from SD card

• Kernel command-line parameters: https://docs.kernel.org/admin-guide/kernel-parameters.html
• set root=/dev/mmcblk0p2 in bootargs variable
  • setenv bootargs loglevel=8 console=ttyS0,115200n8 root=/dev/mmcblk0p2 rootfstype=squashfs rw rootwait
  • saveenv
• load kernel from SD card boot partition
  • setenv bootcmd 'load mmc 0:1 0x81000000 zImage; load mmc 0:1 0x82000000 am335x-boneblack.dtb; bootz 0x81000000 - 0x82000000'

Linux Userspace Startup Config

• Mount /proc virtual filesystem mount -t proc nodev /proc (mount -t type device dir)
  • https://man7.org/linux/man-pages/man5/proc.5.html
• Mount /sys virtual filesystem mount -t sysfs nodev /sys
  • https://man7.org/linux/man-pages/man5/sysfs.5.html

TODO:

• (10/17/23) Fix bootup errors about rcS and "can't access tty" error