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

Alan Stern
2014-05-13T14:45:39Z

On Tue, 13 May 2014, Rafael J. Wysocki wrote:

> Hi All,
>
> We've discussed that at length here:
>
> http://marc.info/?t=139950469000003&r=1&w=4
>
> but I'm starting a new thread to refresh things a bit.
>
> This is about adding a mechanism allowing us to avoid runtime-suspended
> devices during system suspend. The reason why it has to touch the PM core
> is because that needs to be coordinated across the device hierarchy.
>
> The idea is to add a new device PM flag and to modify the PM core as follows.
>
> - If ->prepare() returns a positive number for a device, that means "this
> device is runtime-suspended and you can leave it like that if you do the
> same for all of its descendants".
>
> - If that happens, the PM core sets the new flag for the device in
> question *if* the device is indeed runtime-suspended *and* *if*
> the transition is a suspend (and not hibernation, for example).
> Otherwise, it clears the flag for the device. All of that happens in
> device_prepare().
>
> - In __device_suspend() the PM core clears the new flag for the device's
> parent if it is clear for the device to ensure that the flag will only
> be set for a device if it is also set for all of its descendants.

There's nothing to prevent a runtime-suspended device from being
resumed in between the ->prepare() and ->suspend() callbacks. (Ulf
mentioned this too.)

Therefore it makes little sense to check the device's runtime status in
device_prepare(). The check should be done in __device_suspend().

> - PM core skips ->suspend/late/noirq and ->resume/early/noirq for all devices
> having the flag set - so the flag can be called "direct_complete" as it
> causes the PM core to go directy for the ->complete() callback when set.
>
> - The ->complete() callback has to check direct_complete if ->prepare()
> returned a positive number previously and is responsible for further
> handling of the device.
>
> That is introduced by patch [2/3].
>
> To simplify things slightly it is helpful to move the invocation of
> pm_runtime_barrier() from __device_suspend() to device_prepare(), but still
> under pm_runtime_get_noresume() beforehand (patch [1/3]).

If the check is moved to __device_suspend() then the barrier can remain
where it is now.

> Patch [3/3] shows how this can be used by adding support for it to the ACPI
> PM comain.
>
> Thanks!

Aside from this one matter, everything seems pretty good.

Alan Stern