Proposal: Fixed len size on FD

[David-OConnor]

Proposal: In FD mode, All variable-length arrays are prepended with a 1-byte length field

Current behavior: They're prepended with a bit-aligned field that changes length depending on maximum array length.

Why:
Variable-length arrays are common in DroneCAN messages, including fundamental types defined in uavcan.protocol. These introduce bit-level alignment. Bit-level alignment, vice byte is not user-friendly to develop for for these reasons:

• Computer registers are byte-aligned
• Programming languages like C and Rust have good byte-level APIs, but poor or non-existant bit-level APIs
• ICs and wire protocols (other than DC's) almost always use byte-level alignment
• Common message view protocols, like in DroneCAN GUI, print bytes.

Overall, implementing and debugging byte-aligned protocols is much easier than bit-aligned ones.

Bit-level alignment can save bandwidth in some cases, but often these arrays use byte-aligned variables, like uint8.

This doesn't come up in many cases in non-FD mode, since variable-length arrays are often placed at the end of payloads. In non-FD Dronecan, the len field is removed in this cases due to tail optimization. In FD mode, these len fields introduce bit-level alignment, even when the message is otherwise byte-aligned.

Is this a breaking change?

Maybe. I don't think that means we should dismiss it; rather now is a good time to implement this. Here's why:

• Few devices currently support FD mode. This is likely to change in the near future
• Ardupilot currently doesn't support FD mode in official releases; this will change in the next release if I understand correctly
• FD mode's implementation currently is already a breaking change by removing tail optimization
• The Dronecan spec page currently doesn't discuss FD implementation details like the removed TCO

For basic/BX CAN, I don't think we should make this change, because it would break many devices, and is not worth forcing the change. For FD, I think this is an ideal time to do it, with a narrowing window.

[tridge]

I'd like @bugobliterator to comment as well, but I don't think we should be changing the CANFD format now. It is already working well and well tested