Hello Matt,
After an exhausting search for this issue, we could finally identify the root cause for this issue:
The performance drop is caused by the delay when NAK occurs in _usb_khci_atom_tr
case 0x0a: // NAK
res = KHCI_ATOM_TR_NAK;
if (retry)
_time_delay(delay_const * (pipe_desc_ptr->G.NAK_COUNT - retry));
break;
This NAK will occur when the host send out the request when the USB sticks is not ready to handle the request yet. So it depends on the interval of host sending the request and how fast the USB stick can handle the request.
The interval of host sending the request is different on MQX 4.0 and MQX 4.2; the interval of MQX 4.2 is small than MQX 4.0, so for some USB stick, it can handle the request so fast then it replies a NAK. And once the NAK occurs, the _time_delay would cause a huge delay before next transfer could begin, then thoughput drops a lot.
Actually both MQX 4.0 and MQX 4.2 have the same issue, but the NAK occurs more frequent on MQX 4.2 than MQX 4.0. That's why MQX 4.0 seems to have better performance than MQX 4.2.
You can remove this _time_delay and set USBCFG_DEFAULT_MAX_NAK_COUNT to (3000) to avoid such a huge delay to improve the performance on both MQX 4.0 and MQX 4.2.
In summary, comment the line _time_delay(delay_cont * (pipe_desc_ptr->G.NAK_COUNT - retry)); in khci.c file and also modify in usb_cnfg.h file tje USBCFG_DEFAULT_MAX_NAK_COUNT to 3000 instead of 150.
I hope this can help you,
Best Regards,
Isaac
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