USBD endpoint transfer completion event.

[Ronald Hoogenboom]

In my NRF52840 based device using NRF5 SDK 17.1.0, I notice that sometimes this loop in the nrfx usbd driver code of the SDK doesn't finish:

[modules/nrfx/drivers/src/nrfx_usbd.c: 1444]

/* There is a lot of USBD registers that cannot be accessed during EasyDMA transfer.
 * This is quick fix to maintain stability of the stack.
 * It cost some performance but makes stack stable. */
while (!nrf_usbd_event_check(nrfx_usbd_ep_to_endevent(ep)) &&
       !nrf_usbd_event_check(NRF_USBD_EVENT_USBRESET))
{
    /* Empty */
}

It seems the transfer-end EasyDMA hardware-event isn't generated reliably in all situations. This happens mostly with small transfers on interrupt-in-endpoints and only with some usb hosts (notably raspberry pi). This is a big problem, because this function is called from interrupt context (and therefore should actually not do such busy waiting).

Is this a known issue and is there a workaround? I tried to enter the loop conditionally only when transfer size is bigger than 4 or 8 bytes, which seems to fix this, but that causes other problems.  I tried to pad the 2-byte transfer to 8 bytes, but that doesn't change anything. I think it could work to skip the loop for interrupt endpoints, but this routine handles interrupt and bulk endpoints the same; there is no USBD register that keeps track of bulk vs. interrupt endpoints.

I know that NRF5 sdk is out of support, but my project was built with it and it isn't easy to move to NRF-Connect SDK.

[Hung Bui]

Hi Ronald, 
Please correct me if I'm wrong, what you are saying is that the hardware event ENDEPIN or ENDEPOUT didn't come as expected ? Could you check exactly which one ? 
Is there any chance that it's been cleared by something else and got stuck there  ? Are you able to reproduce the issue with an example in the nRF5 SDK ? This way we can be sure it's not something else with the application may cause the problem. 

As you know the nRF5 SDK is discontinued for a long time, but if you can reproduce with NCS then it's easier for us to request investigation/bug fix in NCS than on nRF5 SDK.