Hi,
(before anything, thanks for actually following the steps for bug
reporting. Much, much appreciated)
Sebastian von Ohr <vonohr@xxxxxxxxxxx> writes:
> 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
It's like the host is trying to go down to lower LPM states every 100ms:
irq/13-dwc3-236 [000] d..1 71.363262: dwc3_event: event (00000401): WakeUp [U0]
irq/13-dwc3-236 [000] d..1 71.363315: dwc3_event: event (00000401): WakeUp [U0]
irq/13-dwc3-236 [000] d..1 71.363423: dwc3_event: event (00006084): ep1out: Transfer In Progress [0] (SIm)
> 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
it could be that LPM is disabled on older xHCI revisions. Mathias,
anything you can add here?
> (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.
could you share a dump of your descriptors? It could be that the wake-up
latencies are incorrect which tricks the host into trying to go down to
lower LPM states too frequently.
> 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
yes, it should be supported :-)
> 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.
correct, LPM is mandatory for USB3.0+. From the traces, we don't see any
errors, though. Everything looks fine.
Mathias, I have an old memory from my time at Intel when we discussed
this very fact. I can't remember what was the conclusion, but is there
anything the peripheral can do to tell the host to *not* enter U1/U2 so
frequently? If my memory doesn't fail me, I don't think there is
anything at all for the peripheral to do, right? Other than not
supporting U1/U2, which means STALLing the relevat SetFeature
requests. Do you know if that would still be a certifiable solution?
cheers
--
balbi
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