linux-kernel - Re: Attempted summary of suspend-blockers LKML thread

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Message-ID: <alpine.DEB.2.00.1008060106260.30564@asgard.lang.hm>
Date:	Fri, 6 Aug 2010 01:07:47 -0700 (PDT)
From:	david@...g.hm
To:	Brian Swetland <swetland@...gle.com>
cc:	paulmck@...ux.vnet.ibm.com,
	kevin granade <kevin.granade@...il.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	pavel@....cz, florian@...kler.org, stern@...land.harvard.edu,
	peterz@...radead.org, tglx@...utronix.de, alan@...rguk.ukuu.org.uk
Subject: Re: Attempted summary of suspend-blockers LKML thread

On Thu, 5 Aug 2010, Brian Swetland wrote:

> On Thu, Aug 5, 2010 at 6:01 PM,  <david@...g.hm> wrote:
>> On Thu, 5 Aug 2010, Brian Swetland wrote:
>>> On Thu, Aug 5, 2010 at 5:16 PM,  <david@...g.hm> wrote:
>>>>>>
>>>>>> So for an mp3 playback, does an Android suspend between data fetches?
>>>>>
>>>>> It can if the latency is long enough (which is why I point out low
>>>>> power audio which is usually high latency).  For low latency (system
>>>>> sounds, etc) 10-25ms between buffers it's not practical to fully
>>>>> suspend but we will go to the lowest power state in idle if possible.
>>>>
>>>> the playback is able to continue even with all the clocks stopped? that
>>>> surprises me. I would hav expected it to be able to sleep while playing
>>>> audio, but not do a full suspend.
>>>
>>> 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?
>
> While the cpu (and the bulk of the system) is suspended, it's not
> uncommon for some peripherals to continue to operate -- for example a
> cellular radio, gps, low power audio playback, etc.  Details will vary
> depending on the SoC and board design.  It's not so much a different
> suspend mode (the system is still suspended), just a matter of whether
> a peripheral can operate independently (and if it is lower power for
> it to do so).

this helps, but isn't quite what I was trying to ask.

on a given piece of hardware, does suspend always leave the same 
peripherals on, or do you sometimes power more things down than other 
times when suspending?

David Lang