MIPS M4K port

[LukeGary462]

I would like to begin developing a MIPS M4K port for CMRX to target the Microchip PIC32 MIPS based MCUs.
The PIC32MX series does not have an MMU per se, but there is support through the CP0 for separate user and kernel address spaces. Unlike the PIC32MZ series which does have a full-fledged MMU.
Any guidelines or advice regarding this port would be appreciated! I am not sure how quickly I will be able to complete this, but CMRX seems to be an RTOS that solves a lot of problems I have ran into in the past and it is a project I would love to be a part of!

[ventZl]

Welcome! MMU is indeed not a requirement for CMRX to operate. Actually, we address environments which don't have MMU rather are only equipped with limited memory protection facilities. They don't even have to be able to perform mapping.

I am not familiar with MIPS but after briefly checking datasheet for PIC32MX, it seems it has FMT which should do the job.

Following text assumes your familiarity with the MIPS architecture and SDKs.

There is generic documentation on what belongs to a port here: https://cmrxrtos.org/documentation/abstract_machine/

Then there is a "template" for headers in porting layer in this directory: https://github.com/ventZl/cmrx/tree/master/src/os/kernel/arch which could serve as a starter for what you have to implement.

And you can use recently-developed Linux hosted port as an example, bulk of it is here: https://github.com/ventZl/cmrx/tree/master/src/os/arch/linux

You can always ask questions. No question is stupid to be asked! Docs above were created in a hope to be useful but may need some more comments or be incomplete, if you get stuck or something is not clear, just ask.

As for porting itself, during the Linux hosted port, it turned out it can be done in three stages which make porting effort easier:
0. Make yourself at least somewhat familiar with CMRX environment if you didn't yet. It will be orders of magnitude easier to port something you know than something you only guess how it is working.

1. First make thread switcher and basic syscalls working. This implies that kernel / user mode separation will be at least somewhat working. Seeing two threads being swapped by the kernel will motivate you to continue. Fortunately, kernel will build and mostly run if you keep bunch of the "abstract machine" API empty returning nothing or just errors.
   (You'll probably have to create MIPS variant of aux_systick as SysTick is ARM-specific)
2. Next make RPC call/return working. This is a bit tricky step to be done on its own so it is good if kernel itself is already working by now. You won't bother about memory protection at this stage yet.
3. Add support for remaining missing stuff such as signals
4. Make memory isolation working

Every of these four stages will be good enough to be pushed as PR. It doesn't matter that port is not complete yet.

CMRX source tree also contains short (and long-neglected) integration test suite. This suite can help you to figure out if basics are right in your port. These are documented here: https://github.com/ventZl/cmrx/blob/master/HIL_TESTING.md#kernel-integration-tests

This will be an iterative process. It is possible that you will find that solution chosen in some of steps won't work with latter steps and that's OK. If you check the Linux port, you'll find out that I reworked some stuff multiple times and that it is not entirely done yet - MPU implementation is still missing.

[LukeGary462]

Thanks for the tips! Would you prefer correspondence as things progress with the port on this issue or in the discussions section?

[ventZl]

Discussions have the advantage of higher visibility. Information stored there can be used by others later. Also, questions asked there may get answered by others too.

I hope discussions are set up so that anyone can create a topic there.