1.13. PreBootLoader images (PBL)

Traditionally barebox generates a raw uncompressed binary. PBL is an effort to create self extracting compressed images instead. This helps on some boards where storage space is sparse. Another usecase of PBL is on SoCs on which the ROM code loads the initial bootloader to (limited) SRAM. With self extracting binaries, more binary space becomes available.

PBL is available for ARM and MIPS. It can be enabled in make menuconfig with the [*] Pre-Bootloader image option.

The user visible difference is that with PBL support barebox.bin is no longer the final binary image, but instead arch/$ARCH/pbl/zbarebox.bin. Use the barebox-flash-image link which always points to the correct image.

1.13.1. Technical background

Normal object files are added to the make system using obj-y += file.o. With PBL the build system recurses into the source directories a second time, this time all files specified with pbl-y += file.o are compiled. This way source code can be shared between regular barebox and PBL. A special case is lwl-y += file.o which expands to obj-y when PBL is disabled and to pbl-y when PBL is enabled.

HINT: for getting an overview over the binaries, disassemble barebox.bin (make barebox.S) with or without PBL support and also disassemble the PBL (make arch/$ARCH/pbl/zbarebox.S)