Implementation of postcard-rpc using usb-gadget (OTG)

[dingari]

Allows Linux systems with access to a UDC supporting OTG gadget mode to run as a postcard-rpc server (device mode).

Adds usb-gadget dependency and requires --features usb-gadget

I added a small ping example in the examples directory.

I've tested this on a Radxa CM5 board with both Linux and macOS hosts with good results.

Note: I've left send_log_str and send_log_fmt unimplemented (I don't use them and I don't understand fully how they work), but would probably be easy to port the impl from one of the eusb servers.

[dingari]

I'm not really sure how the ci works here, but the server-usb-gadget example only builds (and makes sense) on Linux.

How can we tell the CI only to build that specific example on Linux, or somehow let rustc know not to attempt to link it?

[jamesmunns]

Hey @dingari, right now:

• ci.sh runs on win/mac/linux
• ci-stubs.sh runs on mac/linux

If you need this to only be run on linux, feel free to make a new ci-linuxonly.sh or something, and add a new step to https://github.com/jamesmunns/postcard-rpc/blob/main/.github/workflows/ci.yml.

[dingari]

Makes sense, thanks! Fwiw it may just be enough to do cargo check on that specific example with a linux target. At least the CI script is OK on my Macbook. Let's see how it fares on GA.

[dingari]

I rebased this on top of main (too bad missing the 0.12 release). I think this should be OK to merge in the next release?

P.s. I also filled in the send_log_* by referring to the eusb impl (although I feel like there's a lot of potential for reuse in that logic).

[jamesmunns]

Thanks @dingari! Let's see if CI is green now, happy to cut a 0.12.1 release once we have this ready.

[dingari]

Cool, I just added max_usb_frame_size to init to make it conform with the new changes in 0.12.0.

[dingari]

Hey @jamesmunns any idea on when a new release might be released? :)

[jamesmunns]

@dingari thanks for the reping! v0.12.1 is now released! 🎉