Re: xhci_reset_endpoint() doesn't reset endpoint

[Michal Necasek]

On 12/14/2016 3:28 PM, Mathias Nyman wrote:
> On 14.12.2016 12:58, Michal Necasek wrote:
> > prior to the endpoint reset. SetFeature(CLEAR_HALT) resets the toggle
> > on the device, but not on the host. But we know for a fact that the
> > device sends a packet (with data toggle 0) which the host USB stack
> > never sees, and a data toggle mismatch explains that quite well.
> >
> > We are using USBFS to talk to the printer, but that shouldn't matter
> > much. I will note that the available documentation<1> explicitly says
> > that USBDEVFS_RESETEP and USBDEVFS_CLEAR_HALT both reset the data
> > toggle. That is indeed the case for the Linux EHCI driver but not
> > xHCI. Both of the USBFS IOCTLs call into xhci_reset_endpoint() which
> > does nothing.
>
> This is very likely the case.
 Thanks for confirming that.

> xhci can not reset the host side of the endpoint unless it really is
> halted.
> xhci 4.6.8:
>
> "If the endpoint is not in the Halted state when an Reset Endpoint  Command
> is executed -The xHC shall reject the command and generate a  Command
> Completion Event with the Completion Code set to Context State Error."
 Right, the Reset Endpoint is limited to halted endpoints, not even 
stopped ones. It's really only useful for error cleanup.

> Normal halt/stall case is that xhci receives a STALL from the device,
> and immediately resets the endpoint (clears toggle, host side) then
> propagates the HALT status to usb core.
> USB core then sends SetFeature(CLEAR_HALT) to the device which will
> reset the
> toggle for the device side of the endpoint, and host and device toggles
> will be in sync.
>
> After this xhci_endpoint_reset() is called by usb core to inform xhci
> that the
> endpoint was reset, but currently we don't do anything in it.
 OK, that's what I thought was happening.

> If SetFeature(CLEAR_HALT) is called without endpoint actually being
> HALTED we can not
> reset it from xhci. we should issue a config endpoint command to reset
> the host side
> toggle, as mentioned in xhci 1.0 120814 as a last note:
>
> "Note: The Reset Endpoint Command may only be issued to endpoints in the
> Halted state.
> If software wishes reset the Data Toggle or Sequence Number of an
> endpoint that isn't
> in the Halted state, then software may issue a Configure Endpoint
> Command with the Drop
> and Add bits set for the target endpoint. that is in the Stopped state."
>
> There was a case with a scanner we believed had the same issue, and we
> tried to
> resolve it by issuing the configure endpoint command in
> xhci_endpoint_reset() but
> if I remember correctly It did not resolve the case and code never got
> anywhere.
 Ah, that's interesting. I'm not surprised that we weren't the first 
ones running into this. The scenario is a bit obscure but not very 
device specific.

> I might have some really old implementation somewhere for this, at least
> there is
> a really old and outdated hack at
>
>
> git://git.kernel.org/pub/scm/linux/kernel/git/mnyman/xhci.git
> ep_reset_halt_test
> https://git.kernel.org/cgit/linux/kernel/git/mnyman/xhci.git/log/?h=ep_reset_halt_test
>
>
> which really is quite a hack, and based on 3.19 kernel so it's probably
> only useful
> as an Idea to base a real solution on.
 Thanks, we'll have to try it out. It looks like the right approach to me.

 In the meantime, I did a little survey of other operating systems. I 
know Windows can do it (reset the data toggle for non-halted endpoint), 
because the printer works just fine on a Windows host, using both Intel 
xHCI drivers (Windows 7) and Microsoft xHCI drivers (Windows 10). And 
that is true whether the printer is attached to the host or passed 
through to a VM. I do not yet fully understand how Windows does it.

 Solaris can't do it. FreeBSD can I believe, but the FreeBSD method is 
very heavy-handed (they just reset everything in sight that's remotely 
related to the endpoint).

 I also looked at Apple's xHCI driver <1> (not new, but the most recent 
one published) and found that they do exactly what the xHCI 1.1 spec 
suggests, doing a dummy Configure Endpoint command which drops and 
re-adds the endpoint without explicitly changing its state. There are 
even some interesting comments: "State is not halted, so the quiesce 
endpoint did not clear the toggles. This will clear the toggles", and 
"Now do configure endpoint with both add and drop flags set."


    Thanks,
       Michal

1:

https://opensource.apple.com/source/IOUSBFamily/IOUSBFamily-560.4.2/AppleUSBXHCI/Classes/AppleUSBXHCIUIM.cpp.auto.html