USB: CDC adds set line coding callback

[xiongyihui]

Is your feature request related to a problem? Please describe.

Want to create a USB to UART application, but no API to find out when the baud rate is changed

Describe the solution you'd like
May provide an API to register callbacks similar to USB HID callbacks

Describe alternatives you've considered
Or add a weak function to cdc_acm_class_handle_req, then we can override it.

[jfischer-no]

Or add a weak function to cdc_acm_class_handle_req, then we can override it.

This approach is not acceptable and not necessary.
Please use uart_line_ctrl_get from UART API, see samples/subsys/usb/cdc_acm

[xiongyihui]

uart_line_ctrl_get() is OK if the USB CDC is opened once. But if the USB CDC is opened with a different baud rate at the second time, we don't know it.

[jfischer-no]

yes you have to poll it periodically.

[xiongyihui]

Polling it periodically is not a good way. Maybe poll it when having new data to receive or to send.
But I still prefer an event-driven way to do that.

[jfischer-no]

But I still prefer an event-driven way to do that.

Well the UART API does not provide that and usually baudrate does not change in the middle of a communication. To place an app hook in the processing of the control or class request or make it weak for a specific and not generic reason it not acceptable. Actually it is a bad API extension and will always make a problem during internal stack rework or redesign. It would also cause unnecessary latency and security issues. It is also not comparable wit HID because CDC ACM is self-contained implementation with clearly defined user interface (UART API).

[xiongyihui]

OK. I'd try it in my branch anyway.