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

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Message-ID: <AANLkTimWqFkDWvGwVGWD94488gs+CXgpB9juwM5G11ud@mail.gmail.com>
Date:	Thu, 5 Aug 2010 18:22:56 -0700
From:	Brian Swetland <swetland@...gle.com>
To:	david@...g.hm
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, 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).

Brian
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