On Tue, 24 Apr 2018, Mathias Nyman wrote:
> >>> In this situation, the HCD_WAKEUP_PENDING(hcd) test in
> >>> hcd-pci.c:suspend_common() should prevent the controller from going
> >>> back into D3. The WAKEUP_PENDING bit gets set in
> >>> usb_hcd_resume_root_hub() and it doesn't get cleared until
> >>> hcd_bus_resume() runs.
> >>>
> >>
> >> I think xhci never calls usb_hcd_resume_root_hub() in xhci_resume() in this
> >> specific failing case
> >>
> >> xhci_resume() has a check:
> >> /* Resume root hubs only when have pending events. */
> >> status = readl(&xhci->op_regs->status);
> >> if (status & STS_EINT) {
> >> usb_hcd_resume_root_hub(xhci->shared_hcd);
> >> usb_hcd_resume_root_hub(hcd);
> >> }
> >>
> >> If the check fails, then WAKEUP_PENDING bit is not set, and runtime PM
> >> can suspend host controller again. when xhci driver finally gets to handle the interrupt
> >> the controller may be in D3 already
> >>
> >> This should only happen if xhci_resume() is called before xhci driver sees a pending interrupt,
> >> could be possible as xhci has interrupt moderation enabled.
> >
> > Then maybe that test should be removed. Calling
> > usb_hcd_resume_root_hub() for every wakeup shouldn't be too bad,
> > because there probably are not very many times when the controller gets
> > resumed without the root hub also being resumed.
> >
>
> The check was added to fix system suspend issue on a runtime suspended host:
>
> commit d6236f6d1d885aa19d1cd7317346fe795227a3cc
>
> xhci: Fix runtime suspended xhci from blocking system suspend.
>
> The system suspend flow as following:
> 1, Freeze all user processes and kenrel threads.
>
> 2, Try to suspend all devices.
>
> 2.1, If pci device is in RPM suspended state, then pci driver will try
> to resume it to RPM active state in the prepare stage.
>
> 2.2, xhci_resume function calls usb_hcd_resume_root_hub to queue two
> workqueue items to resume usb2&usb3 roothub devices.
>
> 2.3, Call suspend callbacks of devices.
>
> 2.3.1, All suspend callbacks of all hcd's children, including
> roothub devices are called.
>
> 2.3.2, Finally, hcd_pci_suspend callback is called.
>
> Due to workqueue threads were already frozen in step 1, the workqueue
> items can't be scheduled, and the roothub devices can't be resumed in
> this flow. The HCD_FLAG_WAKEUP_PENDING flag which is set in
> usb_hcd_resume_root_hub won't be cleared. Finally,
> hcd_pci_suspend will return -EBUSY, and system suspend fails.
Hmmm. I don't recall seeing this problem occur with ehci-hcd. But
then, I haven't tested it very much recently.
We could change to a different work queue, one that doesn't get
frozen. But there's no guarantee that the work items would run before
your step 2.3.2.
Maybe we can avoid step 2.1. I think there have been some recent
changes to the PM code in this area. There may be a flag you can set
that will prevent the PCI core from resuming the host controller.
Or maybe we can change step 2.3.1, so that the root hub's suspend
callback will first do a resume if the WAKEUP_PENDING flag is set.
That might be the most reliable approach.
Alan Stern
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