xhci: fix USB3 device initiated resume race with roothub autosuspend

xhci: fix USB3 device initiated resume race with roothub autosuspend

A race in xhci USB3 remote wake handling may force device back to suspend
after it initiated resume siganaling, causing a missed resume event or warm
reset of device.

When a USB3 link completes resume signaling and goes to enabled (UO)
state a interrupt is issued and the interrupt handler will clear the
bus_state->port_remote_wakeup resume flag, allowing bus suspend.

If the USB3 roothub thread just finished reading port status before
the interrupt, finding ports still in suspended (U3) state, but hasn't
yet started suspending the hub, then the xhci interrupt handler will clear
the flag that prevented roothub suspend and allow bus to suspend, forcing
all port links back to suspended (U3) state.

Example case:
usb_runtime_suspend() # because all ports still show suspended U3
  usb_suspend_both()
    hub_suspend();   # successful as hub->wakeup_bits not set yet
==> INTERRUPT
xhci_irq()
  handle_port_status()
    clear bus_state->port_remote_wakeup
    usb_wakeup_notification()
      sets hub->wakeup_bits;
        kick_hub_wq()
<== END INTERRUPT
      hcd_bus_suspend()
        xhci_bus_suspend() # success as port_remote_wakeup bits cleared

Fix this by increasing roothub usage count during port resume to prevent
roothub autosuspend, and by making sure bus_state->port_remote_wakeup
flag is only cleared after resume completion is visible, i.e.
after xhci roothub returned U0 or other non-U3 link state link on a
get port status request.

Issue rootcaused by Chiasheng Lee

Cc: <stable@vger.kernel.org>
Cc: Lee, Hou-hsun <hou-hsun.lee@intel.com>
Reported-by: Lee, Chiasheng <chiasheng.lee@intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20191211142007.8847-3-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>