Wakeup requests during a system sleep transition

Alan Stern
2005-11-08T19:43:07Z

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A question has come up recently in private conversations concerning remote
wakeup requests arriving while a system sleep transition is in progress.
How should they be handled?

There are at least three possibilities:

(a) Carry out the wakeup request (i.e., resume the requesting device)
and then abort the system sleep when the device's parent is told
to suspend and it sees that its children aren't all suspended.

(b) Carry out the wakeup request, then sometime later try to recover
by re-suspending the device that was just resumed, and continue
the sleep transition.

(c) Ignore the wakeup request and proceed with the sleep transition.

(a) is very simple and doesn't require any extra code, since drivers
should already be checking to make sure that all the children are
suspended before suspending their devices. When the sleep transition is
aborted, we can just return -ERESTARTSYS and force the user to try again
-- exactly the same as would happen with any other interrupted system
call.

(b) is not a good solution, IMO. It involves a pretty hairy recovery
scheme that's not well defined. It runs the risk of leaving some devices
unsuspended while suspending their parents, or leaving data structures out
of sync with the actual hardware state. Furthermore I don't see any
advantage in making the kernel do the re-suspend over making userspace
restart the entire sleep transition, as in (a).

(c) seems like a reasonable approach. It does, however, have a couple of
problems. First, drivers can't (currently) tell the difference between an
overall system sleep and a device-specific runtime suspend. Since we
certainly _don't_ want to ignore device-wakeup requests during a runtime
suspend, there's no way for a driver to ignore them during a system sleep
transition.

The second thing to consider about (c) is that ignoring a wakeup request
might not make it go away. It will depend on the exact nature of the
hardware and the type of request of course, but there's a good chance that
the request will persist until the system finally goes to sleep. At that
point the request will immediately wake the system right back up. The
overall effect is the same as with (a) -- the system is awake at the end
-- but you've wasted a lot of time and effort to get there.

Now it's true that sometimes there will be no choice. Once tasks are
frozen, there's no way to resume a device since doing so requires a
process context. Then we'd be forced to adopt (c). However there are
situations where tasks are _not_ frozen during a sleep transition.
Suspend-To-RAM on PPC, for example. What should happen in such cases?