Re: [Qemu-devel] [PATCH 0/2] Make mixer emulation configurable at runtime

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

>   Only pulse and the spice audio backends set VOICE_VOLUME_CAP.
> 
>   Impact on users isn't obvious to me.

It offloads volume control to the audio backend, i.e. it will be done by
pulseaudio/spice-client instead of mixemu.

> Questions (not just for you, Bandan):
> 
> 1. Why is CONFIG_MIXEMU off by default?

malc wants it this way because mixemu eats cpu time.

> 2. Why do we bother providing these devices when CONFIG_MIXEMU off?
> 
>    Why would anyone want hda audio devices without a mixer?  Why
>    wouldn't anyone who wants hda audio devices also want CONFIG_MIXEMU
>    enabled?

It'll actually work just fine with HDA.  The guest figures there is no
volume control and will fallback to do volume control in software then.
Net effect is that the audio sample processing to change volume is done
by the guest instead of mixemu, so you don't actually save cpu time by
turning off mixemu ...

I think AC97 has broken volume control with CONFIG_MIXEMU=n, because the
guest tries to use the non-functional volume controls.

So, yes, for serious sound support you want CONFIG_MIXEMU=y.

cheers,
  Gerd