On Thu, Oct 06, 2016 at 10:42:14AM -0400, Alan Stern wrote: > On Wed, 5 Oct 2016, Lukas Wunner wrote: > > On Wed, Oct 05, 2016 at 01:54:01PM -0500, Bjorn Helgaas wrote: > > > On Wed, Oct 05, 2016 at 10:45:22AM -0400, Alan Stern wrote: > > > > In short, Pierre's USB host controller doesn't send wakeup signals from > > > > runtime suspend, because the firmware limits the runtime-suspend state > > > > to D0 and the controller can't issue PME# from the D0 state. In this > > > > situation we would prefer to avoid suspending the controller at all, > > > > rather than have it go into runtime suspend and then stop working. > > > > As Alan has correctly pointed out below, there are PCI devices which > > do not support PME but should still be runtime suspended, e.g. because > > they have some non-standard mechanism to sideband signal wakeup or > > because they can detect when they need to resume even if they're in > > a low-power state. > > > > AFAIUI, this device should not be runtime suspended at all because it > > doesn't generate a PME interrupt and thus stays suspended forever. > > No, as Oliver said, the device can generate a PME# signal. It just > can't do so from D0, and the firmware doesn't allow it to go into a > deeper power-savings state. Okay. > > > The PCI core doesn't allow runtime PM by default. Rather it calls > > pm_runtime_forbid() when the device is added (see pci_pm_init(), called > > indirectly from pci_device_add()). PCI drivers need to explicitly call > > pm_runtime_allow(), typically from their ->probe hook. > > No, pm_runtime_allow() is generally called by userspace, via writing > to the .../power/control file in sysfs. Most drivers do not use it; it > is a policy mechanism. And drivers can't use it to _enforce_ anything, > since the user can always override the setting. Okay, I stand corrected. > > > If this xHC cannot signal wakeup, it shouldn't allow runtime PM in the > > first place. Simple as that. > > > I still think this belongs in the PCI core -- except for the difficulty > of determining whether a device can use a non-PME method for wakeup > signalling. If that issue has a good solution then the PCI core could > call pm_runtime_get_noresume() for devices that are capable of > generating wakeup signals but not in any D-state that the firmware will > allow for runtime suspend. The PCI core already calls pm_runtime_get_sync() before invoking the ->probe hook of a driver (see local_pci_probe()). Drivers need to explicitly release a runtime ref to allow their device to suspend. For xhci-pci, this seems to happen in usb_hcd_pci_probe(): if (pci_dev_run_wake(dev)) pm_runtime_put_noidle(&dev->dev); So you could either modify the if-condition if you want to change the behaviour for XHCI devices only, or if you want to change it in general, add something like this to pci_dev_run_wake(): /* PME capable in principle, but not from the intended sleep state */ if (dev->pme_support && !pci_pme_capable(dev, pci_target_state(dev))) return false; I've briefly looked over the callers of pci_dev_run_wake() and the above seems safe but you should double-check them. Best regards, Lukas -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
