PXE network boot is very useful for Retro-gaming OS such like Batocera, Recalbox, RetroPie, Lakka, etc., on Raspberry Pi. In this way, you do not need to buy any microSD/SSD/USB-HDD, you can dynamically switch to another OS image without flashing a new microSD/SSD/USB-HDD, and you can also dynamically add/remove/change game roms on network storage and play it right away. A similar but much simpler approach can be found on Batocera NAS setup, which will require you to dedicate and flash a tiny image (4GB is enough) for /boot folder and mount the entire /userdata (containing emulators and game roms) or just the ROM folder /userdata/roms from network storage (NFS or Samba) using either Wifi or LAN.
This is an advanced tutorial, you are assumed to be familiar with basic Linux working principles, SSH, and know how to manipulate files, folder and work with shell scripts. The principle can apply to any other RetroGaming OS including Recalbox, Lakka, RetroPie, etc. as well as other Raspberry Pi devices, accordingly.
Before we start, we first need to understand Batocera's standard boot sequence. For Raspberry Pi, after powering on, the eeprom boot program will initialize hardware clock and other peripherals to prepare for boot (RPi4 boot sequence). Then, it will search for boot device according to the BOOT_ORDER specified in eeprom. After found, it will load and run bootcode.bin which looks forconfig.txt. The following two lines in config.txt tell the bootloader program to load the Linux kernel in boot/linux and mount the Initial Root Directory in boot/initrd.gz (1st root file-system, ~400KB gzipped CPIO archive) as ramfs (RAM file-system, all changes will be reverted upon reboot) and run /init:kernel=boot/linuxinitramfs boot/initrd.gz
At the end of running /init in the /boot/initrd.gz , it will mount /boot/boot/batocera (2nd root file-system, ~850MB SquashFS) as the new root and switch root into it. Finally, it will read /boot/batocera-boot.conf to mount the data partition that contains all roms/bios/saves/screenshots/etc onto /userdata . This is where the NAS setup method can instruct Batocera to mount data partition from NFS/Samba storage other than internal storage by modifying the sharedevice field in /boot/batocera-boot.conf. However, you will still need to flash a small Batocera image to provide the /boot directory, but you can use Wifi for hosting NAS.
The steps for PXE network boot need to be done on mainly 2 or 3 sides, server side, client side (Raspberry Pi) and router side (if your home router cannot host NAS)
On the client side (change boot order to PXE network boot if no microSD is found):
1. SSH into Raspbian OS or open a terminal directly
2. (Optional) update firmware, sudo apt update && sudo apt install rpi-eeprom --upgrade
3. Go into firmware directory, cd /lib/firmware/raspberrypi/bootloader/stable/
4. Extract boot configuration from the latest stable firmware, rpi-eeprom-config pieeprom-2021-07-06.bin > bootconf.txt
5. Edit bootconf.txt, vi bootconf.txt ; change BOOT_ORDER to at least 0x21, preferably 0x00654321 (refer to RPi4 boot order for your own preference), add or change to BOOT_ORDER=0x00654321
6. Generate updated EEPROM image, rpi-eeprom-config --out netboot-pieeprom-2021-07-06.bin --config bootconf.txt pieeprom-2021-07-06.bin
7. Flash the updated EEPROM image to EEPROM, rpi-eeprom-update -d -f netboot-pieeprom-2021-07-06.bin
On the router side (if you use router to host DHCP dhcp-boot info and use another machine to host NAS):
1. SSH into your home router
2. edit DHCP config file, vi /etc/dnsmasq.conf, append or modify into the line, dhcp-boot=bootcode.bin,192.168.1.2 (where 192.168.1.2 is your NAS server IP address), this allows DHCP server to tell RPi4 where is your TFTP server (for PXE network boot) during DHCP offer
3. restart DHCP service, service dnsmasq restart (you might need to manually kill and relaunch the dnsmasq process)
Take note that some routers does not allow SSH access or they host DHCP service in another way, then you have to figure out yourself or use your NAS server to host DHCP (if so, you will have two DHCP servers on your home intranet as every router host DHCP service, you might need to disable DHCP service on your router). Some other routers may have a DHCP settings page which allows you to specify PXE boot options, then you can do it in a nicer way.
On the server side:
This is the main and most complicated part. You need to do 4 things: A) download, mount, and extract the Batocera image; B) host /boot folder on TFTP server for PXE boot; C) host /rootfs and /boot on NFS (or Samba); D) modify configuration files and initrd.gz in /boot.
A. You have already downloaded Batocera image for RPi (either official image from Batocera Official Download , or pre-made game pack images from Arcade Punks, etc.)
A1. extract the gzip file, gunzip batocera-rpi4-32-20210920.img.gz
A2. mount it as a loop device, losetup -vP /dev/loop0 batocera-rpi4-32-20210920.img
A3. mount its boot and data partitions:mkdir -p /mnt/boot /mnt/rootfsmount /dev/loop0p1 /mnt/bootmount /dev/loop0p2 /mnt/rootfs
A4. copy out the two folders for NFS mounting (this step is optional since you can modify and host NAS on mounted loop devices directly, however it will be super slow when accessing files); you can use any folder name other than /nfs, mkdir -p /nfs && cp -rfPp /mnt/boot /mnt/rootfs /nfs/ && chmod 777 /nfs /nfs/*
B. host /nfs/boot folder on TFTP server
B1. install TFTP server, apt install tftpd-hpa
B2. edit TFTP config file to set TFTP root directory, vi /etc/default/tftpd-hpa, change the line into TFTP_DIRECTORY="/nfs/boot"
B3. start TFTP server, service tftpd-hpa restart
C. host /nfs/boot and /nfs/rootfs on NFS (or Samba) server
C1. install Network File System kernel server, apt install nfs-kernel-server
C2. edit export config, vi /etc/exports, append or modify into the 2 lines:/nfs/rootfs *(rw,sync,no_subtree_check,no_root_squash,nohide)/nfs/boot *(rw,sync,no_subtree_check,no_root_squash,nohide)
C3. restart NFS serverservice rpcbind restartservice nfs-kernel-server restart
D. modify files in /nfs/boot
D1. Create a backup and edit/nfs/boot/cmdline.txt to add or change into these settings, dev=192.168.1.2:/nfs/boot root=/dev/nfs nfsroot=192.168.1.2:/nfs/rootfs ip=dhcp
D2. disable auto-resize partition, vi /nfs/boot/batocera-boot.conf, set autoresize=false
D3. unpack initrd.gz into a temporary folder:mkdir -p /tmp/initrd && cd /tmp/initrdzcat /nfs/boot/boot/initrd.gz | cpio -iv
D4. modify files inside /tmp/initrd as shown afterwards
D5. backup and repack initrd.gz:cp /nfs/boot/boot/initrd.gz /nfs/boot/boot/initrd.gz.bakfind . | cpio -ov -H newc | gzip -9 >/nfs/boot/boot/initrd.gz
D4. Modifying /tmp/initrd:
The reason why PXE network boot does not work on Batocera right now is because the latest aarch64-version busybox (for early-stage file-system access) does not support NFS properly. So we need to find a statically-linked mount program that works in early stage. An example is to get it from Ubuntu for RPi. You can download "Ubuntu Server 20.04.3 LTS", use losetup to mount its boot folder as before and find its initrd (it's in /<mount-point>/boot/initrd.img ), extract the LZ4-compressed CPIO archive (cd /tmp/initrd2 && lz4 -dc /<mount-point>/boot/initrd.img | cpio -iv) into some temp folder (/tmp/initrd2). Find the mount program that can mount nfs:
root@dell:/tmp/initrd2# file bin/nfsmountbin/nfsmount: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, interpreter /lib/klibc-unVzPS-prFh5518UkFjYOJInn9c.so, BuildID[sha1]=bc89c69698500f3b17c10a6ff8718e162fbd5bd3, stripped
Copy out both bin/nfsmount and its dependency library /lib/klibc-unVzPS-prFh5518UkFjYOJInn9c.so into /tmp/initrd/bin and /tmp/initrd/lib respectively.
Now, we need to edit the init script, /tmp/initrd/init:
- in the do_mount() function, add the following nfsmount command after the 1st mount command failed, if nfsmount -o ro "${1}" /boot_root; then return 0; fi
- in "# read the parameters" section, add reading nfsroot parameter inside the case statement, nfsroot=*) nfsroot=${param#nfsroot=};;
- before "# moving current mounts" section, add the following lines to do nfsmount if nfsroot is specified:if test -n "${nfsroot}"then mkdir -p /new_root/userdata nfsmount -o rw "${nfsroot}" /new_root/userdata rm -f /new_root/etc/init.d/S11sharefi
The last rm -f /new_root/etc/init.d/S11share line is to prevent Batocera from remounting /userdata according to /nfs/boot/batocera-boot.conf, because the built-in busybox mount program does not work for NFS.
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