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

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Date:	Tue, 13 May 2014 10:45:38 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
cc:	Linux PM list <linux-pm@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Aaron Lu <aaron.lu@...el.com>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kevin Hilman <khilman@...aro.org>,
	Ulf Hansson <ulf.hansson@...aro.org>
Subject: Re: [RFC][PATCH 0/3] PM / sleep: Avoid resuming runtime-suspended
 devices during system suspend

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

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