cdcacm: send RX flow control only for the first time when limit is crossed

[michallenc]

Summary

Current implementation sends RX flow control activation every time the limit is crossed when receive packet is read by driver. This may result in repetitious sends of RX flow control signal when some packets are left on the bus or in FIFO.

This change ensures RX flow control is send only when it is not active (priv->iactive is false). It also removes break statement from while loop when watermark is used. The driver should activate RX flow and read the rest of the packet instead of discarding it.

I am not entirely sure if this is the correct way to implement this, should be carefully checked before merged (cc to @gregory-nutt who implemented the flow control to CDC ACM).

Impact

CDC ACM device driver.

Testing

Tested on SAM E70 based MCU.

[michallenc]

Moving to draft so it wont get merged accidentally before we agreed (or discard) this change.

[acassis]

@michallenc our modification seems correct to me. There is an example application (apps/example/flowc) that you can use to force the CDC/ACM device to its transfer limits and that could be used to confirm that everything is working as expected. Greg implemented it because a company was using the CDC/ACM to transfer data to their printer and it was losing data. After this implementation the issue was fixed.

[michallenc]

@acassis Yes, the driver in current state is working as expected (tests with our application were also fine). The question is whether we should send the flow control status once or every time (both will work but not sure what is preferable).

[acassis]

@michallenc good question! The advantage of keep sending it is because if for some reason the other side missed the first one they could catch the next ones. Suggest: test on Linux with debug enabled to see how it is done on Linux. You will need a board that supports USB Gadgets subsystem (I think BBB and Raspberry Pi has OTG and support it)