msperl / rpi-open-firmware

Open source VPU side bootloader for Raspberry Pi.

Minimal Raspberry Pi VPU firmware

This is a small firmware for RPi VPU (VideoCore4) versions 1/2/3 that is capable of initializing VPU PLL (PLLC), UART, SDRAM and ARM itself. It's intended to be used instead of stock bootcode.bin on RPi's SD card. You need to have UART to see anything meaningful as far as output goes.

This has been tested on RPi1 Model B (Hynix PoP DDR), RPi 2 Model B and RPi 3 Model B (both Elpida DDR).

If you want to contact me because you're interested in contributing, you can message kristina on Freenode, but I would suggest talking in #raspberrypi-internals instead. Any pull requests are appreciated.

All Broadcom headers are licensed under 3-Clause BSD License while the rest of this is under GPLv2+. See LICENSE for more information.

Building

You need Julian Brown's VC4 toolchain to build this (https://github.com/puppeh/vc4-toolchain) as well as a arm-none-eabi toolchain. You can tweak the paths to it in CROSS_COMPILE in Makefile (for VC4) and for ARM in arm_chainloader/Makefile. After you've done it, run buildall.sh and you should have a blob in build/bootcode.bin.

Technical Details

The firmware is split into two parts, a VC4 part and and ARM part. The VC4 part initializes PLLC and moves VPU over to it, and then brings up UART. It then performs SDRAM initialization, making SDRAM available at 0xC0000000 (uncached alias). The ARM loader will do ARM initialization and then copy the ARM bootloader that's embedded in it to the alias. It will then map it to 0x0 in ARM's memory space and start ARM. The code under arm_chainloader is what will run on the ARM.

The current makefiles in the ARM part of it aim at RPi1 (ie. ARMv6) but they can be changed to ARMv7 if you want to build it for a newer model. I tested it on all RPi models and it works without any issues as far as I can tell (ARM can access peripherals and memory just fine as AXI supervisor).

Beware: This doesn't handle SMP at the moment so if you run this on RPi2 and above, all ARM cores will start executing the ARM bootloader code at the start which could cause problems. I will fix this soon.

Issues

• PLL rate on ARM is slow, it's a bit annoying. Need to tweak the PLL rate later.
• Code that prints SDRAM capacity is a bit wrong, I need to fix it, but it makes no functional difference.
• At the moment the ARM side of the bootloader doesn't do anything aside from printing a line to UART. I'm going to add an SDHOST driver to load the next stage bootloader to it at some point.
• It only maps a small amount of memory at the moment. You can tweak it in arm_loader.c if you want.

Does/Will it boot Linux?

Eventually maybe. Since start.elf is responsible for clock and power management (all registers in the cpr block), these drivers will have to be rewritten on ARM (or even on the open source VC4) to have any meaningful peripherals working properly (for example HDMI/DMA/Pixel Valve/Whatever). UART and GPIOs do work though.

Thanks To

• Herman Hermitage for his VC4 documentation and for helping me with working out suitable ARM PLL configurations.
• Julian Brown for reviewing my code and for his awesome VC4 toolchain.
• David Given for his initial LLVM project which I used as the base for my LLVM toolchain before moving to GCC.
• phire for reviewing my code.
• Broadcom for their header release.
• Various other people not mentioned here.