Crossing emails again... On Tue, 13 May 2014, Rafael J. Wysocki wrote: > > There's nothing to prevent a runtime-suspended device from being > > resumed in between the ->prepare() and ->suspend() callbacks. > > I'm moving the barrier from __device_suspend() to device_prepare(), so there > shouldn't be surprise resumes in that time frame. A wakeup request from the hardware can cause a runtime resume, even if most threads are in the freezer: Not all kernel threads get frozen. One of the unfrozen threads could respond to the wakeup request by calling pm_runtime_resume(). Some runtime PM callbacks are marked as IRQ-safe and can run directly within an interrupt handler. > > Therefore it makes little sense to check the device's runtime status in > > device_prepare(). The check should be done in __device_suspend(). > > If we do the barrier in device_prepare(), then I'm not sure what mechanism > would cause the device to resume. See above. A wakeup request can arrive after the barrier has finished. > If there is one, the whole approach is in danger, because ->prepare() has to > check if devices are runtime-suspended and has to be sure that their status > won't change after it has returned 1. ->prepare() cannot guarantee in all cases that a device will remain in runtime suspend. Fortunately, it doesn't need to. In fact (as I mentioned sometime before), it doesn't even need to check whether the device currently is runtime suspended -- it suffices to know that _if_ the device is runtime suspended _then_ it has the proper settings for system suspend. Regardless, status changes cannot cause a problem. If the device does get runtime-resumed after ->prepare(), it will remain that way when __device_suspend() runs. The device can't be runtime-suspended again, because device_prepare() does pm_get_noresume(). Therefore, if the device is still runtime-suspended when __device_suspend() runs, we can be sure that its status and state are still the same as when ->prepare() ran. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
