Implement coprocessor access instructions

[spcan]

Implements coprocessor basic access instructions (MCR, MRC, MCRR, MRRC) that compile to a single instruction under --release profiles

[spcan]

CI tests:

• rustfmt fails but functions are formatted the same as the rest of the assembly functions
• stm32 target tests fail due to 3 probes being present: Need to repeat test

[adamgreig]

Thanks for the RP! What's the longer term plan, since these methods are all pub(crate)? Do you imagine adding further coprocessor-related functionality in cortex-m? Should the methods be publicly available?

[spcan]

The aim of these is to expose a common method to interact through the ARM coprocessor interface. Separate libs can then implement the coprocessor driver's using these functions.

For example, I'm using it currently to interact with the LPC55S69 math coprocessor

[adamgreig]

If you want separate libs to later use these functions, don't they need to be pub instead of pub(crate)?

[spcan]

Yeah, good catch, that was there on the test library, forgot to remove it

[spcan]

Ok, it's now failing to compile because const ASM operands are unstable, needing #![feature(asm_const)], can this be enabled in the crate, or should this PR wait until ASM const is stabilized??

This is getting stabilized soon as per #93332

[jannic]

This is getting stabilized soon as per #93332

Likely in rust 1.82.0.

[spcan]

Hello

With the stabilization of Rust 1.82 asm_const has been stabilized. This should be enough to merge these instructions into cortex-m without breaking, although there is the caveat of a new minimum Rust version now (1.82).

Additionally, with the release of the new RP2350 chip, there is now another mainstream processor with coprocessors, which adds some necessity to the access to coprocessor instructions.

I would like some input from somebody in the team of how to proceed at this point, if I should close this PR and open a new one, keep this PR or maybe spin this functionality into a different crate.

Many thanks

[thejpster]

which adds some necessity to the access to coprocessor instructions

The rp2350-hal uses inline assembler to make use of the instructions. Nicer APIs would be better though.