Why not

So, to get it out of the way, here's a dot point list of reasons you're probably better on sticking with the bootloader you already use on x86:

• Linux can take a few seconds to initialise, and you could even "DOS" your system by plugging in badly behaving hardware (e.g. I have a wireless keyboard dongle that confuses Linux when the keyboard isn't turned on, and can hang things for 60 seconds).
• Kexec can only load "linux" or "multiboot" files. You won't be able to run memtest or MS-DOS, not at least out of the box. You also can't "chainload" EFI files from it.
  • Similarly, while it probably could, it does not manage your EFI bootloader (e.g. via efibootmgr).
• Petitboot (currently, at least) only has an ncurses UI. A graphical UI is probably possible, but I'm not aware of one that exists. You're stuck with serial console or text mode video+keyboard.
• Petitboot (so far) doesn't seem to support saving settings. That appears to be somewhat hardware specific and the code isn't there for x86.
• Petitboot relies on the Linux kernel to discover and mount filesystems. This means that you may run into situations where your Petitboot image may not have the drivers / modules, or new enough code, to read a filesystem. (This is noticeable with EXT4 - really old kernels don't like the feature flags on really new file systems).
• Petitboot, on x86, still needs another bootloader to run.

When it makes sense

If you haven't read my Jetfire post, I'll summarise it - the bootloader in flash could only understand the Linux kernel format up to Linux 2.6.21. Any newer kernel would result in the system trapping in the BIOS debugger. I ended up using a 2.6.21 kernel to jump into a 6.x kernel built into it, and then look for bootable systems on disk.

Similarly, I've got an old Sophos XG-125 Rev 2 router that's "just" a normal x86-64 EFI PC. But, when using GRUB2, even the latest version, I can't boot any Linux kernel newer than 6.12 - they all just hang before earlyprintk can kick in. Weirdly, it /can/ boot a Linux kernel built as an EFI stub file, so I can build a Linux system that goes and finds the real system on disk.

So, from my experiments, petitboot as an "intermediate" shines when you find yourself with these kinds of situations:

• Your firmware or existing bootloader /can/ boot Linux but not the Linux you want (Jetfire, Sophos XG-125R2).
• Your firmware can't boot from your storage device (the HP Microserver G8 can't boot from an HP SAS HBA, and I couldn't get GRUB2 to work from a USB drive).
• You use a "storage abstraction" system (e.g. LVM, md, BTRFS, ZFS et al), and don't want to rely on complex bootloader replication (e.g. writing the MBR to every disk, or scripts to update the ESP or /boot partition on every disk) or "disjointed" bootloader set ups (e.g. /boot on a USB drive)
• You want to have some kind of recovery system (since Petitboot is just running on Linux, you can use whatever is available on the image) available before the OS is even loaded.

Buildroot config

If you already have a Buildroot config / project for your device, I would recommend "forking" it. Since we're making a system that is /just/ the bootloader, we want to keep build times and image sizes small.

Our requirements are as follows:

• One single file (i.e. a Linux kernel with embedded initramfs)
  • BR2_LINUX_KERNEL=y
  • BR2_TARGET_ROOTFS_INITRAMFS=y
  • # BR2_TARGET_ROOTFS_TAR is not set
• Build for your system (in my case, x86-64 EFI)
  • BR2_x86_64=y
  • BR2_ROOTFS_OVERLAY="<< YOUR PATH >>/board/petitboot/rootfs_overlay/" - Place some files strategically on the filesystem
  • BR2_ROOTFS_POST_BUILD_SCRIPT="<< YOUR PATH >>/board/common/fix_setsid.sh << YOUR PATH >>/board/common/usr_blkid.sh" - Run a couple of fix ups before the image is completed.
  • BR2_LINUX_KERNEL_USE_CUSTOM_CONFIG=y
  • BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE="<< YOUR PATH >>/board/<< BOARD NAME >>/linux_petitboot.config"
  • BR2_PACKAGE_ACPID=y - Optional - Handle ACPI signals from the system firmware (e.g. power button press)
• (Recommended) Keep everything small
  • BR2_LINUX_KERNEL_LZMA=y
  • BR2_TARGET_ROOTFS_CPIO_LZMA=y
  • BR2_OPTIMIZE_S=y
• With petitboot installed
  • BR2_ROOTFS_DEVICE_CREATION_DYNAMIC_EUDEV=y - This probably should be set automatically...
  • BR2_PACKAGE_PETITBOOT=y
  • BR2_PACKAGE_UTIL_LINUX_BINARIES=y - Required by udev for blkid
  • BR2_PACKAGE_KEXEC_ZLIB=y - Optional.
• Display petitboot on the serial console and the primary display TTY:
  • BR2_PACKAGE_PETITBOOT_GETTY_PORT="tty0 ttyS0" - I would leave your "main" console out of this option to start with to ensure you can log into the system to debug the Buildroot config.
• (Optional) Run getty on an alternative TTY:
  • BR2_TARGET_GENERIC_GETTY=y
  • BR2_TARGET_GENERIC_GETTY_PORT="tty2" - Set this to your "main" console until you're happy that Petitboot is working well enough, then disable the previous option or set this one to an alternative TTY.
• (Optional) Housekeeping:
  • BR2_PER_PACKAGE_DIRECTORIES=y - Builds each package with only it's requirements. Allows building packages in parallel.
  • BR2_TARGET_GENERIC_HOSTNAME="x86-64-petitboot"
  • BR2_TARGET_GENERIC_ISSUE="x86-64 EFI Petitboot"

The above should be all you need in your buildroot config.

Linux Kernel config

Ensure you enable all the needed drivers / options (e.g. Block devices, SCSI, PCI / USB, AHCI, partitions, filesystems, MD) for the kernel to find and mount your boot data.

On top of that, we need:

• kexec:
  • CONFIG_KEXEC=y
  • CONFIG_KEXEC_FILE=y - (If supported by your arch)
• A quiet console:
  • CONFIG_CMDLINE="earlyprintk=efi console=tty2 panic=5" - I would recommend you keep console=ttyxxx set to your main console until you know your Buildroot config is working enough to log in. panic=5 will restart the system if the kernel panics.
• File locking (for udev):
  • FILE_LOCKING=y - Should be enabled by default, and is hidden unless EXPERT=y
• (Optionally) A TTY on the graphics output:
  • CONFIG_DRM=y
  • CONFIG_DRM_CLIENT_LOG=y
  • CONFIG_DRM_AST=y - To suit your hardware
  • CONFIG_FB=y
  • CONFIG_FB_EFI=y - To suit your hardware
  • CONFIG_FB_SIMPLE=y
  • CONFIG_USB_HIDDEV=y

File system config

So with your kernel and Buildroot config "done", we have a few more tiny tweaks to do:

#!/bin/bash

echo [fix_setsid] Ensuring setsid is a symlink to busybox...

rm -fv "$1"/usr/bin/setsid
ln -v -s /bin/busybox "$1"/usr/bin/setsid

#!/bin/bash

echo [usr_blkid] Creating symlink for /usr/sbin/blkid...

ln -v -s -f /sbin/blkid "$1"/usr/sbin/blkid

#!/bin/sh

case "$1" in
        start|restart)
                stty -F /dev/ttyS0 38400
                ;;
        stop)
                echo ''
                ;;
        *)
                echo "Usage: $0 {start|restart}"
                exit 1
esac