Re: [PATCH v2] xhci: fix runtime PM imbalance in USB2 resume

[Henry Lin <henryl@nvidia.com>]

>> USB2 resume starts with usb_hcd_start_port_resume() in port status
>> change handling for RESUME link state. usb_hcd_end_port_resume() call is
>> needed to keep runtime PM balance.

> For normal usb2 port resume the usb_hcd_end_port_resume() is called when resume
> has been signaled for long enough in xhci_handle_usb2_port_link_resume().
>
> This is also where driver directs the port to go from Resume state to U0.
> Port can't do this without driver directing it.
>
> If there's a failure during resume signaling (disconnect, reset, error) then
> stale resume variables are detected in xhci_get_port_status() and
> usb_hcd_end_port_resume() is called.

> I do now see a231ec41e6f6 ("xhci: refactor U0 link state handling in get_port_status")
> does change order of checking and clearing stale resume variables, but this should
> only happen if the first port state we read is a fully enabled functional U0 state after
> a failed resume.

> Could you expand a bit how this was detected?
We observed the racing issue when usb2 device-initiated resume occurs in system resume.
If usb2 host-initiated resume for system resume directs U0 before xhci_get_usb2_port_status()
see RESUME state, xhci_get_usb2_port_status() will not finish resume process in
xhci_handle_usb2_port_link_resume(). Its scenario is as follows:

1. System resume starts. All driver system resume callbacks get called in order. XHCI controller
    is resumed by xhci_resume().
2. USB2 root hub is resuming. hcd_bus_resume() is being executed.
3. Before xhci_bus_resume() is finished, XHCI driver receives a port status change event for 
    an USB2 port with RESUME link state in xhci_irq(). XHCI driver starts the process to resume
    HS port for device-initiated resume.
4. In xhci_bus_resume(), host-initiated resume (direct U0) is performed on the same port that is
    resuming in step 3 in below loop:

                if (bus_state->bus_suspended) {
                        spin_unlock_irqrestore(&xhci->lock, flags);
                        msleep(USB_RESUME_TIMEOUT);
                        spin_lock_irqsave(&xhci->lock, flags);
                }
                for_each_set_bit(port_index, &bus_state->bus_suspended,
                                 BITS_PER_LONG) {
                        /* Clear PLC to poll it later for U0 transition */
                        xhci_test_and_clear_bit(xhci, ports[port_index],
                                                PORT_PLC);
                        xhci_set_link_state(xhci, ports[port_index], XDEV_U0);
                }
5. Then, link state of the resuming port is observed as U0 in following
    xhci_get_usb2_port_status(). xhci_handle_usb2_port_link_resume() has
    no chance to get called on the resuming port.

Thanks,
Henry