[alsa-devel] UCM/DAPM - saving power when idle?

Liam Girdwood lrg at ti.com
Thu Nov 1 16:26:16 CET 2012

+ Mark and Arun,

On 01/11/12 10:39, David Henningsson wrote:
> I don't deal much with ASoC, so sorry if I'm missing something obvious here...
> When a certain use case is activated, that would then activate some DAPM
> widgets, causing the codec to consume more power. So, we should essentially
> avoid this unless we're actually playing back/recording. But, why isn't there
> a "disable all use cases" command, or possibly an "idle" command? (That,
> essentially, e g pulseaudio should set whenever playback/recording stops)

> Or; the option would be that the codec is actually powered down, but only
> activates itself when there is a stream; but this looks like it could cause
> problem with loopback paths (e g listening to FM radio, where there are no
> streams passing through CPU/main memory).

DAPM will only power on the associated widgets if there is a valid source -> 
sink path and the stream is active (e.g. we set a path and perform a playback 
or capture). So if pulseaudio sets a playback use case and then keeps the 
playback open (even when playing silence) DAPM will keep the widgets enabled.

DAPM also handles the sidetone or FM radio task as described above too by 
detecting the source -> sink between two widgets that are not enabled by the 
CPU PCM devices (e.g. not DACs or ADCs, etc) and will then power this path.

Fwiw, There is nothing stopping us adding a low power idle UCM modifiers for 
each use case atm with the current UCM code base. I would imagine a single 
"idle" modifier could be used by most of the devices on a UCM verb, but there 
may need to be additional idle modifiers to deal with any audio artifacts 
generated by some devices when powering ON/OFF.


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