linux-kernel - Re: [linux-pm] suspend blockers & Android integration

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Date:	Mon, 7 Jun 2010 17:57:19 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Matthew Garrett <mjg59@...f.ucam.org>,
	Vitaly Wool <vitalywool@...il.com>, Neil Brown <neilb@...e.de>,
	tytso@....edu, Peter Zijlstra <peterz@...radead.org>,
	Brian Swetland <swetland@...gle.com>,
	Felipe Balbi <felipe.balbi@...ia.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Florian Mickler <florian@...kler.org>,
	James Bottomley <James.Bottomley@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux PM <linux-pm@...ts.linux-foundation.org>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [linux-pm] suspend blockers & Android integration

On Sun, Jun 6, 2010 at 5:01 PM, Alan Stern <stern@...land.harvard.edu> wrote:
> On Sun, 6 Jun 2010, Matthew Garrett wrote:
>
>> The difference between idle-based suspend and opportunistic suspend is
>> that the former will continue to wake up for timers and will never be
>> entered if something is using CPU, whereas the latter will be entered
>> whenever no suspend blocks are held. The problem with opportunistic
>> suspend is that you might make the decision to suspend simultaneusly
>> with a wakeup event being received. Suspend blocks facilitate
>> synchronisation between the kernel and userspace to ensure that all such
>> events have been consumed and handld appropriately.
>
> Remember that suspend takes place in several phases, the first of which
> is to freeze tasks.  The phases can be controlled individually by the
> process carrying out a suspend, and there's nothing to prevent you from
> stopping after the freezer phase.  Devices won't get powered down, but
> Android uses aggressive runtime power management for its devices
> anyway.
>
> If you do this then the synchronization can be carried out entirely
> from userspace, with no need for kernel modifications such as suspend
> blockers. And since Android can reach essentially the same low-power
> state from idle as from suspend, it appears that they really don't need
> any kernel changes at all.
>

I don't think this is true. If you stop after the freezer phase you
still need all the suspend blockers that are held until user-space
consumes an event, otherwise it never gets consumed since user-space
is frozen.

-- 
Arve Hjønnevåg
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