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

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Date:	Fri, 11 Jun 2010 10:02:19 -0500
From:	James Bottomley <James.Bottomley@...e.de>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	David Brownell <david-b@...bell.net>, tytso@....edu,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Neil Brown <neilb@...e.de>,
	Brian Swetland <swetland@...gle.com>,
	Felipe Balbi <felipe.balbi@...ia.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Florian Mickler <florian@...kler.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	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>
Subject: Re: [linux-pm] suspend blockers & Android integration

On Fri, 2010-06-11 at 10:46 -0400, Alan Stern wrote:
> On Fri, 11 Jun 2010, James Bottomley wrote:
> 
> > On Thu, 2010-06-10 at 21:21 -0700, David Brownell wrote:
> > > Do we at least have a clean way that a driver can
> > > reject a system suspend?  I've lost track of many
> > > issues, but maybe this could be phrased as a QOS
> > > constraint:  the current config of driver X needs
> > > clock Y active to enter the  target system suspend
> > > state, driver's suspend() method reports as much.  Then the entry to
> > > that system state gets blocked
> > > if the clock isn't enabled.
> > 
> > So in QoS modifications to android patches, the answer is "yes" ...
> > except that the current android patch set didn't actually have this type
> > of wakelock in it.
> > 
> > Android wants an idleness suspend block (or pm qos constraint) that a
> > driver can set to prevent the system idleness power govenor from
> > dropping into a power state too low for the driver, so in USB terms this
> > would prevent the states that shut down the clock.  For android, it
> > prevented shutdown of an internal i2c bus.
> > 
> > The one thing that does look difficult is that these power constraints
> > are device (and sometimes SoC) specific.  Expressing them in a generic
> > way for the cpu govenors to make sense of might be hard.
> 
> Doesn't the clock framework already handle this sort of thing?

Well, there are two elements to "this sort of thing":

     1. Allow a driver to request that a given clock not be turned off.
     2. Make the cpuidle governors aware of a pending "don't turn off X
        clock source" so they can keep the system in a state where the
        clock doesn't get powered down.

As far as I can tell from the code, neither currently exists at the
moment.

James


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