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From:   Henry Lin <henryl@nvidia.com>
To:     Mathias Nyman <mathias.nyman@linux.intel.com>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>
CC:     Mathias Nyman <mathias.nyman@intel.com>,
        "linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] xhci: fix runtime PM imbalance in USB2 resume
Thread-Topic: [PATCH v2] xhci: fix runtime PM imbalance in USB2 resume
Thread-Index: AQHYKhd7AopZ8pn1EUiip/DzNcJli6yoz4qAgAF/L3U=
Date:   Tue, 1 Mar 2022 10:28:49 +0000
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References: <20220225055311.92447-1-henryl@nvidia.com>
 <20220225071506.22012-1-henryl@nvidia.com>
 <2ef7da52-d8ad-05ca-bcb6-06bd6bb6f9d3@linux.intel.com>
In-Reply-To: <2ef7da52-d8ad-05ca-bcb6-06bd6bb6f9d3@linux.intel.com>
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>> USB2 resume starts with usb_hcd_start_port_resume() in port status=0A=
>> change handling for RESUME link state. usb_hcd_end_port_resume() call is=
=0A=
>> needed to keep runtime PM balance.=0A=
=0A=
> For normal usb2 port resume the usb_hcd_end_port_resume() is called when =
resume=0A=
> has been signaled for long enough in xhci_handle_usb2_port_link_resume().=
=0A=
>=0A=
> This is also where driver directs the port to go from Resume state to U0.=
=0A=
> Port can't do this without driver directing it.=0A=
>=0A=
> If there's a failure during resume signaling (disconnect, reset, error) t=
hen=0A=
> stale resume variables are detected in xhci_get_port_status() and=0A=
> usb_hcd_end_port_resume() is called.=0A=
=0A=
> I do now see a231ec41e6f6 ("xhci: refactor U0 link state handling in get_=
port_status")=0A=
> does change order of checking and clearing stale resume variables, but th=
is should=0A=
> only happen if the first port state we read is a fully enabled functional=
 U0 state after=0A=
> a failed resume.=0A=
=0A=
> Could you expand a bit how this was detected?=0A=
We observed the racing issue when usb2 device-initiated resume occurs in sy=
stem resume.=0A=
If usb2 host-initiated resume for system resume directs U0 before xhci_get_=
usb2_port_status()=0A=
see RESUME state, xhci_get_usb2_port_status() will not finish resume proces=
s in=0A=
xhci_handle_usb2_port_link_resume(). Its scenario is as follows:=0A=
=0A=
1. System resume starts. All driver system resume callbacks get called in o=
rder. XHCI controller=0A=
    is resumed by xhci_resume().=0A=
2. USB2 root hub is resuming. hcd_bus_resume() is being executed.=0A=
3. Before xhci_bus_resume() is finished, XHCI driver receives a port status=
 change event for =0A=
    an USB2 port with RESUME link state in xhci_irq(). XHCI driver starts t=
he process to resume=0A=
    HS port for device-initiated resume.=0A=
4. In xhci_bus_resume(), host-initiated resume (direct U0) is performed on =
the same port that is=0A=
    resuming in step 3 in below loop:=0A=
=0A=
                if (bus_state->bus_suspended) {=0A=
                        spin_unlock_irqrestore(&xhci->lock, flags);=0A=
                        msleep(USB_RESUME_TIMEOUT);=0A=
                        spin_lock_irqsave(&xhci->lock, flags);=0A=
                }=0A=
                for_each_set_bit(port_index, &bus_state->bus_suspended,=0A=
                                 BITS_PER_LONG) {=0A=
                        /* Clear PLC to poll it later for U0 transition */=
=0A=
                        xhci_test_and_clear_bit(xhci, ports[port_index],=0A=
                                                PORT_PLC);=0A=
                        xhci_set_link_state(xhci, ports[port_index], XDEV_U=
0);=0A=
                }=0A=
5. Then, link state of the resuming port is observed as U0 in following=0A=
    xhci_get_usb2_port_status(). xhci_handle_usb2_port_link_resume() has=0A=
    no chance to get called on the resuming port.=0A=
=0A=
Thanks,=0A=
Henry=0A=