Added some stuff to README.
[rpi-open-firmware.git] / README.md
1 # Minimal Raspberry Pi VPU firmware
2 This is a small firmware for RPi VPU (VideoCore4) versions 1/2/3 that is capable of initializing UART, VPU PLL (PLLC) 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.
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4 This has been tested on RPi1 Model B (Hynix PoP DDR), RPi 2 Model B and RPi 3 Model B (both Elpida DDR).
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6 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.
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8 All Broadcom headers are licensed under 3-Clause BSD License while the rest of this is under GPLv2+. See `LICENSE` for more information.
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10 ## Building
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12 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`.
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14 ## Technical Details
15 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.
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17 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).
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19 **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.
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21 ## Issues
22 * PLL rate on ARM is slow, it's a bit annoying. Need to tweak the PLL rate later.
23 * Code that prints SDRAM capacity is a bit wrong, I need to fix it, but it makes no functional difference.
24 * 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.
25 * It only maps a small amount of memory at the moment. You can tweak it in `arm_loader.c` if you want.
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27 ## Does/Will it boot Linux?
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29 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.
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31 ## Thanks To
32 * **Herman Hermitage** for his VC4 documentation and for helping me with working out suitable ARM PLL configurations.
33 * **Julian Brown** for reviewing my code and for his awesome VC4 toolchain.
34 * **phire** for reviewing my code.
35 * **Broadcom** for their header release.
36 * Various other people not mentioned here.
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