Raspberry Pi 4B & 5 baremetal blink example

[iCMDdev]

Currently just a draft for a baremetal Raspberry Pi 4B 64-bit blink example (-target aarch64-none-none-elf). I'll add the code soon - I'm making this draft public now to receive suggestions & feedback as I work on this example.

I already got Embedded Swift working on a 4B (blinking a led with Swift-MMIO), but I need to do some cleanup & better organize the code & build process. I'll upload it here after I do that (it will be in a Swift Package format).

I'm currently using llvm-objcopy as a last step to convert the resulted ELF file, with something like $(LLVMPATH)/llvm-objcopy -O binary kernel8.elf kernel8.img. From what I've seen, there doesn't seem to be any viable alternative, so users will need to install llvm with brew, since llvm-objcopy is not installed by default.

[iCMDdev]

This doesn't need to be necesarily specific to the 4B, I might as well adapt it to the 5B. Some register values might be different, especially because the 5's ACT (green) LED (and the whole GPIO) is not directly connected to the main processor, but via the RP1 chip, perhaps making it a bit harder to blink directly.

Still, one could blink another LED connected directly to the GPIO by changing the pin(currently hardcoded in Swift MMIO registers)

Edit: I think I'm partially wrong, It appears that the green (ACT) led is connacted to the always-on GPIO pin 9 (the BCM2712's GPIO) on the RPi 5 according to this.

Edit 2: Yes, the last edit is accurate. I got this working.

[iCMDdev]

That was faster than expected, I think it's ready for review now!

[iCMDdev]

boot.S and link.ld are borrowed from this bare-metal code example for Raspberry Pi 4 (CC0-licensed).

[iCMDdev]

I also got this working on a Pi 5! I managed to gather the relevant register details (adresses & necessary bit values).
Now, I'm wondering whether these should be 2 separate examples, or if I should merge them into one (and use compiler directives to specify which board is used).

[rauhul]

I think the example makes sense to add. However Im trying to get CI setup for this repo before adding additional examples, so avoid hosting broken code!

[iCMDdev]

However Im trying to get CI setup for this repo before adding additional examples

Make sense, alright!

so avoid hosting broken code!

I'm not sure what you mean by that, do you mean that this example is broken somewhere? I was able to build & run it just fine, but maybe that's on my machine only. Let me know if you had any problems if you tested this.

Either way, as you suggested, we should popstone this example and come back to it once we get CI integration. That's a good idea.

[rauhul]

I mean the examples silently breaking due to changes in the swift compiler or other example dependencies, e.g. the SDKs where we depend on a branch.

[iCMDdev]

I mean the examples silently breaking due to changes in the swift compiler or other example dependencies, e.g. the SDKs where we depend on a branch.

Oh, right, yeah. Great idea.

[iCMDdev]

Thanks for running the checks! I’m not sure how I missed installing make, I know I had it in the script… Maybe I pushed the change to the wrong branch.

Either way, I’ll fix it asap.

Edit: yes, I think that’s exactly what happened. Hopefully it’s fine now.

[rauhul]

almost good to merge, sorry about the delay.

[iCMDdev]

almost good to merge, sorry about the delay.

No worries, and happy new year! I made the changes you've requested and retested the workflow, it should work fine now.

[rauhul]

@iCMDdev theres a couple yml lint issues remaining, otherwise this is ready

[iCMDdev]

Thanks for pointing out!

Apparently I didn't notice them in my fork, though the test also failed there. Sorry about that, fixing it now.

[iCMDdev]

Seems fine now, can you run checks again?

[iCMDdev]

Thanks! 🎉
If you want me to change or add anything else, let me know.

[kubamracek]

@rauhul shall we merge?

[rauhul]

Ah yes please, sorry I thought we already merged it!

I think I got lazy making a nice commit message. The GitHub one wasn't great with all the intermediate commit messages.

Do you mind summarizing the overall change when merging?

[iCMDdev]

The GitHub one wasn't great with all the intermediate commit messages.

Yeah, sorry, I took a bit of advantage of the squash-on-merge policy. I think I should also provide more descriptive intermediate commit messages to replace the automatically-generated "Update [file]" ones - I'll try to do this more in the future.

Do you mind summarizing the overall change when merging?

Perhaps something like this would work (feel free to modify it, feedback always appreciated!):

Add Raspberry Pi 4B & 5 baremetal examples
Provide new examples demonstrating how to run Swift Embedded on the Raspberry Pi 4B and 5 single-board computers, in a baremetal fashion, without an underlying operating system. The provided examples showcase how to blink the onboard green LED using Swift MMIO.

[iCMDdev]

Thanks for merging! 🎉