lowRISC
[usbdev] Bit stuffing violations at packet end are undetected
Description
This is a minor, perfective improvement.
A bit stuffing violation has occurred if the received packet data concludes with 6 '1' bits and no additional '0' bit before the EOP. See '7.1.9 Bit Stuffing' in the protocol specification.
This is not detected by usb_fs_rx and reported to software diagnostically because the EOP is treated in the same manner as a '0' bit (ie. transition on DP and DN).
The data is still received, and if such a condition actually occurred on the USB it would be the fault of the host controller/hub. DV does attempt to inject such a bit stuffing violation to check that the DUT ignores the transaction (OUT packet), which it does not. USBDEV should strictly ignore this packet.
The only risk of functional failure would be if the USB host controller chose this very specific form of bit stuffing violation to try to cancel a DATA transaction that could not be repeated after a lost/corrupted ACK from the USB device (8.6.4 Corrupted ACK Handshake).
