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Date:   Tue, 30 Apr 2019 10:17:06 -0400 (EDT)
From:   Alan Stern <>
To:     "Tangnianyao (ICT)" <>
cc:, <>,
        <>, <>
Subject: Re: [RFC] Question about reset order for xhci controller and pci

On Tue, 30 Apr 2019, Tangnianyao (ICT) wrote:

> On 2019/4/29 22:06, Alan Stern wrote:
> Hi, Alan
> > On Mon, 29 Apr 2019, Tangnianyao (ICT) wrote:
> > 
> >> Using command "echo 1 > /sys/bus/pci/devices/0000:7a:02.0/reset"
> >> on centos7.5 system to reset xhci.
> >>
> >> On 2019/4/26 11:07, Tangnianyao (ICT) wrote:
> >>> Hi,all
> >>>
> >>> I've meet a problem about reset xhci and it may be caused by the
> >>> reset order of pci and xhci.
> >>> Using xhci-pci, when users send reset command in os(centos or red-hat os),
> >>> it would first reset PCI device by pci_reset_function. During this
> >>> process, it would disable BME(Bus Master Enable) and set BME=0, and
> >>> then enable it and set BME=1.
> >>> And then it comes to xhci reset process. First, it would send an
> >>> endpoint stop command in xhci_urb_dequeue. However, this stop ep command
> >>> fails to finish. The reason is that BME is set to 0 in former process and
> >>> xhci RUN/STOP changes to 0, and when BME is set to 1 again, RUN/STOP doesn't
> >>> recover to 1.
> >>> I've checked BME behavior in xhci spec, it shows that "If the BME bit is set to 0
> >>> when xHC is running, the xHC may treat this as a Host Controller Error, asserting
> >>> HCE(1) and immediately halt(R/S=0 and HCH=1). Recovery from this state will
> >>> require an HCRST." It seems that the stop ep command failure is reasonable.
> >>> Maybe I've missed something and please let me know.
> > 
> > Your email subject says "Question about...".  What is the question?
> > 
> Sorry I didn't descibe it clearly.
> When sending a reset command, now the reset order is first BME and then xhci.
> BME reset would make xhci controller stop, resulting in xhci reset failure,
> because it can't finish stop ep command in xhci_urb_dequeue.
> I'm not sure if this situation is in expectation.

Probably it isn't.

> > Also, given that your question concerns what happens when you write to
> > /sys/bus/pci/..., perhaps you should consider mailing it to some PCI
> > maintainers as well as to the USB maintainers.
> > ok, I will mailing it to PCI maintainer as well.
> > Perhaps the reset was not meant to be used the way you are doing it.  
> > A more conservative approach would be to unbind xhci-hcd from the 
> > device before doing the reset and then rebind it afterward.
> > 
> > Alan Stern
> >
> >
> > .
> >
> I think this approach not work. When reset BME, xhci controller is stopped and
> can't recover even BME is set enable later. According to xhci spec, it's appropriate.
> When rebind it afterward, with xhci stop, xhci driver would consider the
> xhci controller already died and it fails to work again.
> To recover xhci, now I rmmod xhci-pci.ko and then insmod it again.

That's basically what I meant.  "rmmod xhci-pci" does an unbind, and 
"insmod xhci-pci.ko" does a rebind.

So long as you do the unbind before the reset and the rebind after the
reset, you should be okay.  Perhaps you misunderstood what I wrote; it 
sounds like you're doing the reset first and then the unbind.

Alan Stern

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