Thanks for making this work! I'd given up on the LLVM backend completely due to showstopping issues I couldn't resolve, and had assumed it had vanished into the aether. Glad to know you found it useful!

Regarding the boot loader, have you seen my piface boot loader? http://cowlark.com/piface/ It sounds like what you have is way more sophisticated, as I never got SDRAM init working properly, but you never know --- there might be something useful there.

I was actually considering having another go at a VC4 compiler, this time with pcc; there's an OS I want to port. I'm really happy to know that I no longer have to!

Yeah I looked at it, my firmware mostly just aims at bringing up enough stuff to be able to boot ARM, which is essentially just:

  - Setting up exception vectors and enabling exceptions
  - Reclocking VPU from PLLC
  - UART initialization
  - SDRAM initialization
  - Copying an ARM blinker stub to 0x0
  - ARM power domain initialization
  - PLLB initialization
  - Enabling passthrough mapping for ARM
  - ARM AXI interface initilaization

I'm still trying to figure out what I'm missing in order to get it to work but I suspect it's related to not properly setting up the ARM PLL. I was going to port the RPi clock management driver from Linux but I don't have the time at the moment.

As far as the compiler goes, I think it works reasonably well, though I only tested it by compiling my own code with it, there may be things that cause it to error out (anything that involves the frame pointer like VLAs/some C++ features). Also code quality is not ideal since it still doesn't make use of conditional instructions (aside from conditional branches) and doesn't implement AnalyzeBranch to eliminate redundant branches.

I started cleaning up TableGen to turn multi-instruction asm prints into glue DAG in SelDAGtoDAG but I still have to do it for like 4 instructions, which is pretty much a requirement to have MC code emission.

It's been too long and I've forgotten too much about how LLVM works. The gcc port I linked to, BTW, claims to have VC4 binutils.

Random question: how did you do 64 bit arithmetic? I couldn't find any kind of add-with-carry or subtract-with-carry instruction. I was semi-resigning myself to have to do a compare-and-test as well as the add, which would have more than doubled the amount of work.

Also, nice work reverse engineering the ARM controller --- how did you get the info?

...hilariously, I've just checked my email, and I see a two-day old message from someone who's just got my previous attempt at a VC4 compiler working. This one was with gcc:

https://github.com/puppeh/vc4-toolchain

This is awesome! It seems like an important series of steps towards having a legitimately-open educational platform, rather than the somewhat asterisk-laden one that the Pi currently represents.

Great work. Very interested to see what you have going on the firmware side.

I outlined what I did in another comment, here's a log from my current firmware: http://crna.cc/vpu_bootlog.txt

I think I'm on the right track but I don't have ARM working yet, most likely due to clock misconfiguration. Can probably fix it when I have more time.

Sidenote, I wish #raspberrypi-internals was more active :(

Will you be getting this merged into LLVM upstream?

Don't think I can without first implementing MC emission and running LLVM unit tests on it.