• Turn off the laptop.
• To invoke Recovery mode, you hold down the ESC and Refresh keys and poke the Power button.
• At the Recovery screen press Ctrl-D (there's no prompt - you have to know to do it).
• Confirm switching to developer mode by pressing enter, and the laptop will reboot and reset the system. This takes about 10-15 minutes.

Note: After enabling developer mode, you will need to press Ctrl-D each time you boot, or wait 30 seconds to continue booting.

• After booting into developer mode, hold Ctrl and Alt and poke the T key. This will open up the crosh shell.
• Type shell to get into a bash shell.
• Type sudo -i to become root. (sudo su doesn't seem to be available and this is functionally the same)
• Then type this to enable USB booting:

crossystem dev_boot_usb=1 dev_boot_signed_only=0

• Reboot the system to allow the change to take effect.

Note: I'm doing the build from my Orange Pi 800 with the same RK3399 so no cross compile environment is required.

root #parted /dev/sdc mklabel gpt
root #parted -a optimal /dev/sdc unit mib mkpart Kernel 1 65
root #parted -a optimal /dev/sdc unit mib mkpart Root 65 100%

Depthcharge requires some specific parameters to be set. These signal the bootloader the presence of a valid kernel partition:

root #cgpt add -i 1 -t kernel -S 1 -T 5 -P 15 /dev/sdc

Create a filesystem on the root partition and mount it:

root #mkfs.ext4 /dev/sdc2
root #mount /dev/sdc2 /mnt/sdcard
</mnt>

Grabbed latest arm64 stage3 and extracted to /mnt/sdcard: [[https://distfiles.gentoo.org/releases/arm64/autobuilds/20240128T234815Z/stage3-arm64-openrc-20240128T234815Z.tar.xz]]

Also grabbed the latest portage snapshot as it doesn't appear its in the stage 3 and extracted to /var/db/repos/gentoo: [[https://distfiles.gentoo.org/snapshots/portage-latest.tar.bz2]]

I copied my make.conf from my Orange Pi 800 as they both have the same rk3399 SOC.
<code>
# These settings were set by the catalyst build script that automatically
# built this stage.
# Please consult /usr/share/portage/config/make.conf.example for a more
# detailed example.
COMMON_FLAGS="-march=armv8-a+crc+crypto -mtune=cortex-a72.cortex-a53 -mfix-cortex-a53-835769 -mfix-cortex-a53-843419"
CFLAGS="${COMMON_FLAGS}"
CXXFLAGS="${COMMON_FLAGS}"
FCFLAGS="${COMMON_FLAGS}"
FFLAGS="${COMMON_FLAGS}"

# WARNING: Changing your CHOST is not something that should be done lightly.
# Please consult https://wiki.gentoo.org/wiki/Changing_the_CHOST_variable before changing.
CHOST="aarch64-unknown-linux-gnu"

# NOTE: This stage was built with the bindist Use flag enabled

# This sets the language of build output to English.
# Please keep this setting intact when reporting bugs.
LC_MESSAGES=C.utf8
USE="-X gpm alsa -udev pulseaudio"
ACCEPT_LICENSE="-* @FREE @BINARY-REDISTRIBUTABLE"
LLVM_TARGETS="AArch64"
VIDEO_CARDS="panfrost"

Copy over resolv.conf:

cp --dereference /etc/resolv.conf /mnt/sdcard/etc/

Setup binds:

root #mount --types proc /proc /mnt/sdcard/proc
root #mount --rbind /sys /mnt/sdcard/sys
root #mount --make-rslave /mnt/sdcard/sys
root #mount --rbind /dev /mnt/sdcard/dev
root #mount --make-rslave /mnt/sdcard/dev
root #mount --bind /run /mnt/sdcard/run
root #mount --make-slave /mnt/sdcard/run

Chroot in:

root #chroot /mnt/gentoo /bin/bash
root #source /etc/profile
root #export PS1="(chroot) ${PS1}"

emerge --ask gentoo-sources

I stole the config from https://git.alpinelinux.org/aports/plain/testing/linux-gru/config I edited the config text and added the gentoo specific options required:

< # Linux/arm64 6.6.2 Kernel Configuration
---
> # Linux/arm64 6.6.13-gentoo Kernel Configuration
5c5
< CONFIG_CC_VERSION_TEXT="gcc (Alpine 13.2.1_git20231014) 13.2.1 20231014"
---
> CONFIG_CC_VERSION_TEXT="gcc (Gentoo 13.2.1_p20240113-r1 p12) 13.2.1 20240113"
7294a7295,7312
> 
> #
> # Gentoo Linux
> #
> CONFIG_GENTOO_LINUX=y
> CONFIG_GENTOO_LINUX_UDEV=y
> CONFIG_GENTOO_LINUX_PORTAGE=y
> 
> #
> # Support for init systems, system and service managers
> #
> CONFIG_GENTOO_LINUX_INIT_SCRIPT=y
> # CONFIG_GENTOO_LINUX_INIT_SYSTEMD is not set
> # end of Support for init systems, system and service managers
> 
> # CONFIG_GENTOO_KERNEL_SELF_PROTECTION is not set
> CONFIG_GENTOO_PRINT_FIRMWARE_INFO=y
> # end of Gentoo Linux

Compiling the kernel:

make
make dtbs

• https://archlinuxarm.org/platforms/armv8/rockchip/samsung-chromebook-plus
• https://wiki.gentoo.org/wiki/Samsung_Chromebook_Plus
• https://wiki.gentoo.org/wiki/Creating_bootable_media_for_depthcharge_based_devices
• https://git.alpinelinux.org/aports/tree/testing/linux-gru/
• https://sites.google.com/a/chromium.org/dev/chromium-os/how-tos-and-troubleshooting/mainline-kernel-on-kevin-chromebook-plus
• https://github.com/hexdump0815/imagebuilder/blob/main/systems/chromebook_gru/readme.md