Libre Computer Design Stack

Low Level Booting

Each platform boots from an onboard ROM which is built into the SoC.
On power-on, the ROM code will scan SPI or MMC devices for secondary bootloader based on a hardware defined configuration.
Secondary bootloader software is platform-specific and does not work across platforms.
Hardware defined configuration may be controlled via switches on specific platforms.
Once the secondary bootloader loads, it provides UEFI abstraction for operating system images, isos, and network capability.
We call the secondary bootloader "firmware".

Firmware initializes board subsystems.
Firmware loads boot customization environment configuration boot.ini.
Firmware scans for bootable media based on environment configuration.
Firmware loads scripts, override device-trees, and/or UEFI applications into memory and runs them.
Firmware provides device-tree or override device-tree to UEFI application.
UEFI application is typically GRUB, systemd-boot, or Linux.

Libre Computer boards designed after 2018 have embedded the "firmware" on SPI NOR.
They will boot to the UEFI graphical firmware interface layer on power on.
There is a switch on these boards to disable the onboard firmware should an user decide to run custom firmware.
Legacy boards designed prior to 2018 do not have SPI NOR so they do not boot to graphical interface.
The "firmware" must be flashed correctly to an MMC device (SD or eMMC) so that the bot ROM can load it.
If not flashed correctly, the board will not do anything on bootup or may appear to not power on in the case of ROC-RK3399-PC Renegade Elite.