Date: Fri, 6 Aug 2010 10:22:26 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: david@lang.hm, Brian Swetland <swetland@google.com>,
        kevin granade <kevin.granade@gmail.com>,
        Arve Hj?nnev?g <arve@android.com>,
        Matthew Garrett <mjg59@srcf.ucam.org>,
        "Rafael J. Wysocki" <rjw@sisk.pl>,
        Arjan van de Ven <arjan@infradead.org>,
        linux-pm@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
        pavel@ucw.cz, florian@mickler.org, stern@rowland.harvard.edu,
        peterz@infradead.org, tglx@linutronix.de, alan@lxorguk.ukuu.org.uk
Subject: Re: Attempted summary of suspend-blockers LKML thread
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Reply-To: paulmck@linux.vnet.ibm.com
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On Fri, Aug 06, 2010 at 01:30:48PM +0100, Mark Brown wrote:
> On Thu, Aug 05, 2010 at 06:01:24PM -0700, david@lang.hm wrote:
> > On Thu, 5 Aug 2010, Brian Swetland wrote:
> 
> >> Obviously not all clocks are stopped (the DSP and codec are powered
> >> and clocked, for example), but yeah we can clock gate and power gate
> >> the cpu and most other peripherals while audio is playing on a number
> >> of ARM SoC designs available today (and the past few years).
> 
> > does this then mean that you have multiple variations of suspend?
> 
> > for example, one where the audio stuff is left powered, and one where it  
> > isn't?
> 
> This was the core of the issue I was raising in the last thread about
> this (the one following the rename to suspend blockers).  Essentially
> what happens in a mainline context is that some subsystems can with
> varying degress of optionality ignore some or all of the instruction to
> suspend and keep bits of the system alive during suspend.
> 
> Those that stay alive will either have per subsystem handling or will be
> outside the direct control of the kernel entirely (the modem is a good
> example of the latter case in many systems - in terms of the software
> it's essentially a parallel computer that's sitting in the system rather
> than a perhiperal of the AP).

This underscores a basic difference between servers and these embedded
devices.  When you suspend a server, it is doing nothing, because servers
rely very heavily on the CPUs.  In contrast, many embedded devices can
perform useful work even when the CPUs are completely powered down.

							Thanx, Paul
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