[alsa-devel] how to mandate the use of PCM plugin?

Colin Guthrie gmane at colin.guthr.ie
Mon Sep 27 19:57:04 CEST 2010

'Twas brillig, and Stas Sergeev at 27/09/10 05:48 did gyre and gimble:
>   Hello.
> 27.09.2010 04:58, Mark Brown wrote:
>>> Is it possible to somehow mandate the
>>> use of a PCM plugin? For example, the
>>> PC-Speaker.conf defines the use of softvol
>>> plugin for "default" and "front" devices.
>>> It is still possible for the software to bypass
>>> it by the use of "hw", and some software
>>> does exactly that.
>>> Is it possible to somehow make some
>>> plugin a mandatory?
>> What is the actual problem you are trying to solve here?  It's not
>> possible to *mandate* anything to all users, if nothing else someone
>> with administrative access can always rebuild whatever part of the
>> system is trying to restrict them.
> What I am trying to solve, is the problem
> that people have with my snd-pcsp driver.
> Pulseaudio uses "hw" to bypass some pcm
> plugins, and then people are left with the
> non-functional softvol control, and they blame
> my driver.

FWIW, PA does not actually use "hw" to open the device in most cases. It
actually uses front, surround51 etc. etc.

The volume control pipeline can be summed up here if you're not already
familiar with it:

which explains how the various mixer elements are used.

It is quite common however for incorrect dB data to be presented from

This can be rectified by calculating proper dB information and fixing up
the driver. Some details can be found here:

And today, one of the Canonical guys published his version of a tool to
help here:

Hope this helps.



Colin Guthrie

Day Job:
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Open Source:
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  PulseAudio Hacker [http://www.pulseaudio.org/]
  Trac Hacker [http://trac.edgewall.org/]

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