linux-kernel - Re: [PATCH 1/8] PM: Opportunistic suspend support.

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Message-ID: <AANLkTilCi3v0UrV_UG6nBhQt8oA_6Xs_EpI0lYrtqBV_@mail.gmail.com>
Date:	Tue, 25 May 2010 17:36:38 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Len Brown <len.brown@...el.com>,
	Pavel Machek <pavel@....cz>,
	Randy Dunlap <rdunlap@...otime.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Tejun Heo <tj@...nel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Nigel Cunningham <nigel@...onice.net>,
	Ming Lei <tom.leiming@...il.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	linux-doc@...r.kernel.org
Subject: Re: [PATCH 1/8] PM: Opportunistic suspend support.

2010/5/25 Dmitry Torokhov <dmitry.torokhov@...il.com>:
> On Tue, May 25, 2010 at 04:54:34PM -0700, Arve Hjønnevåg wrote:
>> 2010/5/25 Dmitry Torokhov <dmitry.torokhov@...il.com>:
>> > On Tue, May 25, 2010 at 04:13:35PM -0700, Arve Hjønnevåg wrote:
>> >>
>> >> You are missing the point. There are no event pending. The kernel
>> >> reported the key down event, it was handled, but the keypad driver is
>> >> still scanning to see if the user presses another key,
>> >
>> > Employ reasonable timeout.
>>
>> Timeout for what? Stop trying to suspend altogether, stop scanning for
>> key changes, or a more "reasonable" poll interval?
>
> Stop trying to suspend for a fraction of a second to see if user wants
> to press another key.
>

In other words, try to suspend several times a second until it succeeds.

>>
>> >
>> >> or releases the
>> >> currently held key.
>> >>
>> >
>> > Userspace consumer should wait for the key release and retract "busy"
>> > once event is received and handled.
>> >
>>
>> No it should not. User-space does not know if the key is coming from a
>> keypad driver that needs to actively scan the matrix while keys are
>> pressed.
>
> OTOH nor does kernel driver know whether system suspend should be
> blocked while it is scanning.

Yes it does, it cannot deliver wakeup keys if it suspends.

> What should happen in I press KEY_SUSPEND?
> Do we always want to wait till user releases it?

If KEY_SUSPEND is part of the keypad matrix, then yes.

>
>>
>> >> >
>> >> > 2. Wait a tiny bit after last application notified you that it finished
>> >> > processing events.
>> >> >
>> >> > So basically the difference is that with in-kernel suspend blockers,
>> >> > there is a tiny window where we haven't started the suspend yet but are
>> >> > about to the driver has a chance to prevent entire system from starting
>> >> > sleep.
>> >>
>> >> No, the difference is that if a driver needs to prevent suspend for an
>> >> extended period of time, you don't have user space continuously
>> >> polling to see if it can suspend.
>> >
>> > Why would a driver, on its own, prevent suspend for extended periods of
>> > time? I think that the decision should originate from userspace, kernel
>> > is here just to serve the requests.
>> >
>>
>> A driver prevents suspend if suspend would prevent it from working.
>> For instance, the gpio keypad matrix code prevents suspend when a key
>> is help down, since it has to activly scan the keypad for changes.
>> Only no-keys-pressed versus one-or-more-keys-pressed can be detected
>> with an interrupt.
>>
>> >>
>> >> >
>> >> > Without the blocker we may start suspending and will stop midcycle. We
>> >> > may be even better off in the end since we could leave some devices
>> >> > still powered down after aborting system-wide suspend.
>> >> >
>> >>
>> >> That does not sound right.
>> >
>> > Why doesn't it? If a device implements runtime PM it may chose remain in
>> > powered-down mode even if system is awake.
>> >
>>
>> If the device implements runtime PM it should already be powered-down
>> if it is not in use.
>
> It could have been in use but userspace-initiated suspend accelerated it
> entering low power state.
>

So you are suggesting we should repeatedly try to suspend the whole
system to accelerate runtime PM powering down devices? Why not power
the device down as soon as it is not in use?

-- 
Arve Hjønnevåg
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