Lk

LK (Little Kernel) loads a boot.img file, that was directly written to the Android boot partition of the device (without any partitioning, just the file directly; this is not the postmarketOS boot partition, see partition layout). This file consists of the postmarketOS Linux kernel and initramfs, which we flash to that partition during the installation. Flashing instructions are passed to LK with the fastboot command (abstracted in pmbootstrap flasher).

Notably LK does not allow to boot a different kernel from an SD card (without hacks like chainloading another bootloader that is stored in boot.img, which potentially increases the boot time and is a whole lot of effort to implement). LK is licensed under the MIT license, without copyleft, meaning that vendors are not required to release the source code of their LK bootloader forks. In practice, they end up being proprietary - we don't get the source code and can't make changes.