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Doxygen docs at https://kovirobi.github.io/comms-ccf/.

Comms-CCF

This is a simple communication layer based on COBS, CBOR and FNV-1A.

Can be used to implement other channels on top, e.g. RPC, logging, tracing etc.

None of the ideas are new here, but it's still useful to have them together like this. And it's more focused on embedded than some other libraries. (At the same time, the code size could perhaps be smaller, I used C++ templates and they can expand to a fair amount of code. Especially when using no optimisation [-O0] instead of debug optimisation [-Og].)

Status: Proof of concept

Currently there is a working demo in FreeRTOS-Demo/ which should be possible to port to other platforms. There is some Python support in python/comms_ccf/ which can connect to the demo and issue RPC commands.

This is proof-of-concept because not all of the layers below are filled in properly:

• Python layer could use IPython for a more friendly REPL.
• RPC
  • The Python type hints only do simple types, doing nested types such as tuples is not yet done.
  • Allow functions returning void values
  • More complicated RPC is not done (e.g. asynchronous functions, functions returning data over several packets).
  • Storing values (e.g. objects) across function calls is not done -- probably not necessary though.
  • Proper error values for issues with the RPC.
  • Encoding Python types more compactly (e.g. int/list of ints rather than string).
• Other protocols such as logging or streaming sensor results not yet demonstrated.
  • Basic logging done.
  • Deferred format logging to avoid pulling in printf (and compare space difference).
  • Only sending pointers to rodata string constants.
  • Streaming events to plot, e.g. ADC readings (though maybe random data for qemu).
  • Streaming trace events.
• CBOR
  • The API needs improving to be able to avoid doing all the decoding on the stack as a return value. For example, read/write memory doesn't need to materialise all of the bytes on the stack, it could use an iterator for decode, and a span for encode.
  • Supporting structs is not currently done. The constructor could probably be used as an RPC function, so constructing could be done, but destructing is more tricky. One solution for POD structs is to infer fields from default constructor, see https://github.com/Mizuchi/ForeachMember. Or have a way for users to define serialization of their struct.
• The circular buffer has a few TODOs to improve it.
• The COBS layer could have input & output iterators, which could then be used to have views-like interfaces and make defining the CCF layer neater.

Demo & Resource use

There is a demo in the FreeRTOS-Demo/ folder, with detailed information about where the flash/memory use comes from.

A summary of the resurce use is copied below from the FreeRTOS-Demo/README.md file.

Summary	.text	.bss	.data
No Comms-CCF	11.45KiB	3.54KiB	356B
Add Comms-CCF, no RPC handlers	13.14KiB (+1.68Ki)	5.62KiB (+2.09Ki)	356B (no change)
Add version RPC call	14.05KiB (+932)	5.64KiB (+24)	356B (no change)
Add add call	14.77KiB (+744)	5.67KiB (+28)	356B (no change)
Add sub call, similar to add	14.83KiB (+60)	5.70KiB (+28)	356B (no change)
Add greet call	15.13KiB (+304)	5.72KiB (+24)	356B (no change)
Add readm_mem and write_mem calls	16.52KiB (+1.39Ki)	5.78KiB (+56)	356B (no change)
Add test log task	20.63KiB (+4.11Ki)	6.86KiB (+1.09Ki)	356B (no change)
Inline vtable	20.61KiB (-28)	6.89KiB (+24)	356B (no change)
Deferred formatting	19.69KiB (-936)	6.86KiB (-24)	356B (no change)

Acknowledgements

With much gratitude to Robert McGregor and Jamie Wood, who have shown me that writing something like this can be easy and fun. And showing the utility in writing something like this compared to continuing on with the standard "write a CLI over UART" method.

Their method was different, using a Python program to parse and compile a DSL for describing the RPC calls. This allowed that to be used easily with C, and also support more features and fewer workarounds to the template limitations. I used C++ templates to continue my learning of them.

Expected use

The expected use of this is between a microcontroller and a host-machine, communicating over e.g. UART or similar (byte) serial channel. The aim is to make a few things easy:

• Controlling the embedded device (alternative to serial command-line, with support for history, tab-completion, all on the host side);
• In the same vein, automating that control for testing, without having to write command-line parsers (which potentially have to deal with the lack of flow-control [e.g. ready-to-send/clear-to-send] signals);
• Multiplexing the channel for different streams, e.g. different logging components.

This impacts design in a couple of ways:

• CPU is much faster than the transport, so we can do checksumming as we copy the received byte

  Note: This breaks down with SEGGER_RTT, specifically I found that using JLink + SEGGER_RTT doesn't do flow control on the host TX side (where it could/should), so for now I recommend setting the down buffer size to be the maximum packet size (e.g. -DBUFFER_SIZE_DOWN=256 from the default 16). On the other hand, the up buffer size has a default of 1024B, you could reduce that to 256B so still saving overall space.

  • TODO: Maybe this can be avoided with PyOCD?

• Channel can be connected/disconnected, so need a framing;

• Channel isn't 100% error free, due to glitches during connect/disconnect; but is considered reliable otherwise;

• Channel delivers packets in order, and has low latency (rather, the product of latency and speed is small, so negligible amount of data sent but not yet received) so no need to have multiple packets in flight at once;

• These two mean we can assume a FIFO model, with maybe the level above implementing ack/retry, but a simple check is fine, no need for error-correcting codes;

• Packets are received from an interrupt handler, that needs to queue them for deferred processing.

  Specifically it might need to enqueue a packet while some packets are being processed. Not seeing the newly added packet is fine (the interrupt will notify it to try again), but it cannot use a mutex because that would leave the interrupt being blocked on the thread (probably a crash but at least a priority inversion);

• Packets are sent from multiple threads, but processed by a single transmit task which might be busy (UART isn't so fast after all) so need a transmit queue also

Transport wire format

The format on the wire is the following, little endian network byte order for the checksum because that is what I'm used to writing.

ID: u8	data: u8[]/CBOR	checksum: u32

The ID is a channel ID/tag, the data is a variable length of bytes (the length is given by the framing layer), and the checksum is the FNV-1A checksum, over both the ID and data in that order.

This isn't the traditional CRC-32 (Ethernet variant) because FNV-1A uses fewer bytes for the same throughput (no precomputed table), is very easy to implement and is pretty good. There are better algorithms for absolute throughput and randomness (say SipHash, Murmur2), but these have usually been written for throughput on a desktop with vector extensions, and their code does end up being a little bigger.

Given the use case here of communicating between two trusted parties, the choice of FNV-1A seems good. It does operate on a byte byte basis, which does slow it down but also means we don't impose length/padding requirements on the messages.

Further, the data might be encoded as CBOR (depending on the ID). For example if the ID is for a serial channel, unstructured bytes are fine. But for logs, structured data can be useful (and not just for the metatdata). For RPC, structured data is necessary.

I chose CBOR because it is easy to encode/decode into a compact format, supports many types (more than I end up using).

Tweaks/defines

• DEFERRED_FORMATTING (or host-side formatting) means you can avoid doing printf on the microcontroller, which might save some code space.