Re: [Qemu-devel] [PATCH v2 4/4] audio: paaudio: ability to specify stream name

[Maxim Levitsky]

On Wed, 2019-08-28 at 10:53 +0100, Daniel P. Berrangé wrote:
> On Wed, Aug 28, 2019 at 12:43:49AM +0200, Zoltán Kővágó wrote:
> > On 2019-08-27 07:42, Gerd Hoffmann wrote:
> > > On Mon, Aug 26, 2019 at 09:59:04PM +0200, Kővágó, Zoltán wrote:
> > > > This can be used to identify stream in tools like pavucontrol when one
> > > > creates multiple -audiodevs or runs multiple qemu instances.
> > > 
> > > Hmm, can we create an useful name automatically, without yet another
> > > config option?
> > > 
> > > Useful choices could be the device name (usb-audio, ...) or the device
> > > id (whatever -device id=xxx was specified on the command line).
> > 
> > I'm afraid this is not going to work with the current architecture: due
> > to mixeng even if you have multiple devices, they'll be mixed to a
> > single stream and the audio backend will only see this one mixed stream.
> >  As a workaround we could do something like concat all device names or
> > ids, but I don't like that idea.
> > 
> > Alternatively we could use the id of the audiodev instead, and no more
> > problems with mixeng.  However, with mixeng off (implemented in my next
> > patch series) suddenly soundcards will have suddenly end up as different
> > streams.  (This can be worked around by creating multiple audiodevs,
> > like what you have to use now to get multiple streams from pa, so this
> > is probably a smaller problem.)
> > 
> > Currently I'm leaning for the audiodev's id option, unless someone
> > proposes something better.
> 
> Using the audiodev id is not a good idea. If you have multiple QEMU's
> on your host, it is highly likely that libvirt will have assigned
> the same audiodev id to all of them.  Using the vm name would be ok,
> but only if you assume that each gust only has a single audio device.
> 
> Using a combination of vm name + audidev id is going to be unique
> per host, but not especially friendly as a user visible name. It
> would be ok as a default, but I'd think we should let the mgmt app
> specify stream name explicitly, so that something user friendly
> can be set.
No, no!
It seems that pulseaudio has a name for each connection, and a name for each
steam within that connection.

The suggestion is that we use the VM name for the connection,
(which will be unique per VM usually, at least the user can make it be so)
and then use the audiodev id for each stream. Of course for multiple VMs,
the audiodev ids will be the same, but this is all right since you can
always distinguish them that the streams come from different VMs.

Also note that this thing is cosmetic from the correctness point of view,
that is pulse-audio internally has no problem with duplicate IDs.

The thing is useful mostly for tweaking the output streams in the pavucontrol,
where the names will allow you to easily know which steam is which.

Speaking of this, there is another minor feature missing, which is volume level
feedback from pavucontrol to the guest. That means that if you change the 
volume of the
stream in pavucontrol, the guest will not be aware of this, and still think 
that the
volume is the same.
But for 'tweaking' I was mostly thinking about assigning the input/output 
streams to
different sound devices, which pavucontrol can do and does work very well.

Best regards,
        Maxim Levitsky