Re: [RFC][PATCH 0/3] PM / sleep: Avoid resuming runtime-suspended devices during system suspend

Alan Stern
2014-05-13T15:25:07Z

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