[alsa-devel] Softvol controls

[Takashi Iwai]

At Tue, 4 Dec 2007 16:42:55 +0100 (CET),
Jaroslav Kysela wrote:
> 
> On Fri, 30 Nov 2007, Takashi Iwai wrote:
> 
> > At Fri, 30 Nov 2007 00:28:59 +0100,
> > Lennart Poettering wrote:
> > > 
> > > On Wed, 21.11.07 12:22, Takashi Iwai (tiwai at suse.de) wrote:
> > > 
> > > > >    Now, is there a way I can detect for a mixer element if it is
> > > > >    "real" or if it is implemented only for the "plughw" PCM? As it
> > > > >    seems the "hw:0" mixer device does contain "virtual" tracks, isn't that a
> > > > >    bit strange? Shouldn't the virtual mixer tracks be available only
> > > > >    in a mixer device "plughw:0"? That at least would be symmetric to
> > > > >    the PCM case, wouldn't it?
> > > > 
> > > > Check snd_ctl_elem_info_is_user().  All softvol controls are
> > > > implemented as user elements.
> > > 
> > > Hmm, how am I supposed to get the snd_ctl_elem_info_t from a
> > > snd_mixer_elem_t or snd_mixer_selem_id_t?
> > 
> > No, there is no API to get the id mapping.
> > And I guess we can't do it because there is no 1:1 mapping between
> > ctl_elem and mixer_selem.  It's N:1.
> 
> I don't think that application should know about this mapping.
> 
> I think that we have to provide API giving a mixer control element for 
> opened PCM handle, otherwise applications might use hacks like 
> suggested snd_ctl_elem_info_is_user() checks.

I basically agree that this isn't about the mixer API, but in general,
it depends on what apps require.  The sound system like pulseaudio is,
of course, a program that wants the detailed controls over the lower
system.

> > > I figure there is no useful documentation or even example how this is
> > > supposed to work? Hmm, is there any real documentation available which
> > > describes the relation of ctl, hctl, mixer and smixer at all? For the
> > > uninitated the whols structure looks overly complex and redundant.
> > 
> > Yes, it's overly complex.  The mixer abstracion is what I'd really
> > love to clean up, maybe better to write from scratch.
> 
> I think that we might remove only 'mixer' and simplify initalization from 
> the user side, but each time I tried to think about an optimal mixer 
> interface, I ended with the current 'simple mixer API'.

Well, it's partly about API but rather its implementation.  I see
little reason to have layering like below as now:

	ctl -> hctl -> mixer -> module -> app

Another part of problem is the API design.  The current API is good
for old hardwares like SB16.  But, this abstraction breaks too often
the simplicity of control API itself for other hardwares.


> > I guess PA could use ctl API better than mixer API because it requires
> > only certain elements like Master or PCM.  You can simply take
> > "Master Playback Control" with MIXER iface for master volume and
> > "Master Playback Switch" for master mute switch.  Of course, you'll
> > take care of number of channels or value range, but it's also same for
> > mixer API, too.
> 
> I don't agree here. The simple mixer layer should be used, because it 
> covers at least some abstraction. In my recent changes, we have 
> possibility to use python for fast prototyping of simple mixer backends 
> (see alsa-lib/modules/mixer/simple/python directory how fast with minimal 
> code can be backend implemented). Unfortunately, main problem will be
> probably the work to cover all cards.

Again, it's the question of "what kind of app".  The python is great,
but it's not for "sound systems" like pulseaudio at all...


Takashi