Re: [RFC] virtual master control for HD-audio

[Takashi Iwai <tiwai@suse.de>]

At Fri, 21 Dec 2007 20:37:09 +0100 (CET),
Jaroslav Kysela wrote:
> 
> On Thu, 20 Dec 2007, Takashi Iwai wrote:
> 
> > Hi,
> > 
> > another thing I'd like to push into the next kernel is the virtual
> > master volume control.  As I posted in earlier posts, it adds virtual
> > controls that have several slave controls with the same types,
> > e.g. Front, Surround, Center, LFE, etc.  Then these are adjusted
> > simultaneously via the master control.
> > 
> > It'd be appreciated if some one can test the patches with HD-audio h/w
> > that has no master control yet (e.g. some STAC codecs).
> > 
> > Note that this won't add the master volumes if there are no real
> > volume controls.  Some codecs have really no volume control, and this
> > won't help for such devices.
> > 
> > Two (and one for driver) patches will follow after this.
> 
> NAK from my side. I am convinced that this code can be implemented in the 
> user space even without any daemon just in the mixer abstract layer using 
> standard control elements and using eventually new user controls to store 
> data for virtual mixer controls.

A user-space implementation of virtual mixer elements would be far
more complicated than the simplistic kernel-space patch.  I've
considered it many times, even tried to implement it, but got that
conclusion.  You'll see obviously the following difficulties:

1. Many user-space virtual elements

Each slave control element needs a virtual element (eventually a
user-space one) because we need to keep both raw and virtual values to
handle saturation.  That is, the same number of additional controls
would be added.  Significant for 7.1 outputs.

2. Easy incosistency between virtual and raw elements

Even if the mixer abstraction hides the virtual elements, both
virtual and raw elements are exposed on control API.  This can cause
the value-inconsistency between them quite easily, because many apps
access directly via control API (even some mixer apps), and they
likely change only raw values.  The similar situation is for kernel
OSS emulation.

3. Complicated configuration

The requirement of virtual master controls is very much dependent on
the hardware.  In the case of HD-audio, it depends on codec chip
types, and even on the preset model chosen via PCI SSID or a module
option.  Implementing such a complex conditional in alsa-lib
configuration is a clear overhead.

4. More resource requirement

Clearly a user-space solution requires more layers in alsa-lib, which
is invoked by each process, and even more memory footprint than the
kernel solution, not only the additional complexity.


... and think again what is the benifit of the user-space solution in
this case, in comparison with the demerits above.

If you could implement the same feature on user-space with less amount
of codes, I'd happily take it.  But, I'm sure it's very hard.


Takashi