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Bug: CDC_Control_FS() in usbd_cdc_if.c is empty

Question asked by Scarpi.Giuseppe.002 on May 25, 2017
Latest reply on May 27, 2017 by Scarpi.Giuseppe.002

I've experienced random and unexplainable problems with CDC communication over USB, while using CubeMX-generated code.


The PC was always able to enumerate the COM port, but sometimes it was unable to open it, (Windows reports a "wrong parameter" error).


After a thorough research and the use of an USB sniffer, I think I've identified the problem.


The issue comes from a wrong answer to the PC's request GET_CODING_LINE.


When the PC issues this request, it expects a payload containing line speed in bps, parity, stop bits and databits.


CDC_Control_FS() in usbd_cdc_if.c should fill the payload with correct data, but in facts its body is empty. There a switch listing the various CDC messages, but the case statements do nothing.


(...and so on...)

For this reason, the payload contains random data that, in most cases, prevent the communication to work.


Here is an example of a request that the sniffer captured.



The body of the function is classified as "user code", but leaving it empty is misleading.

I wasted a lot of time to track down this problem.

I understand that CDC code is provided as an example, yet there is not even a simple #warning to help developers avoiding this pitfall.

I think this should be fixed as soon as possible.