I'm running an Intel Apollo Lake SoM (Celeron N3350E) which I want to use as a USB gadget with the functionFS gadget driver. I have created two bulk endpoints for sending and receiving data. The hardware and cabling is only USB 2.0 capable. In one test case the receive side of the SoM is slowed down deliberately (200ms sleep between reads) while the host PC tries to send as fast as possible. This setup leads to send timeouts on every second transmission on the host PC. I believe this is an issue with the USB 2.0 LPM feature, more specifically hardware LPM done by the host USB controller. I have tested different USB descriptors and the issue is gone when removing the USB_LPM_SUPPORT flag from the USB 2.0 extension descriptor (actually removing only USB_BESL_SUPPORT seems to suffice). Also the issue occurs only on some newer PCs and adding a hub (doesn't matter if 2.0 or 3.0 capable) makes the issue go away. I can reproduce this issue with a Windows 10 (1909) host running an Intel B360 chipset. I use libusb v1.0.24 with the WinUSB driver on the host side to send data on the bulk endpoint. See the attached dwc3 trace and registers. It was created using the current 5.12.1 kernel version. It shows multiple WakeUp [U0] events in short succession but never any event showing different link states than U0. The host is doing 8 transmissions of 16 bytes to the device, but the device only receives 4 of these transmissions. The first transmissions always succeeds while the next one will timeout on the host. I believe this is because the device is currently not ready to receive new data. But instead of sending the data when the delay on the receive side is over the request never finishes and times out after 1 second (or even longer when I increase the timeout value). Is the USB 2.0 LPM extension even supposed to work with the dwc3 controller? I can work around this issue currently by downgrading the device to USB 2.0 only (setting bcdUSB to 0x0200). But I believe USB 3.0 capable device must support LPM, so this issue might come up again when having USB 3.0 capable hardware.
Attachment:
dwc3_trace.tar.gz
Description: dwc3_trace.tar.gz
