[alsa-devel] Standard mixer control names

[Takashi Iwai]

At Mon, 23 Feb 2009 17:47:00 +0100,
Lennart Poettering wrote:
> 
> On Mon, 23.02.09 10:11, Takashi Iwai (tiwai at suse.de) wrote:
> 
> > > ALSA is making that very hard to implement something like this because
> > > every driver seems to wrap input/output selection differently.
> > > 
> > > On one card I have only has a couple of cswitches
> > > (snd-es1371). The same one has an enum "Mic Select". Another card has
> > > an enum "Input Source", but no cswitches (a HDA chip). The
> > > "ControlNames.txt" file in the kernel seems to suggest that there is an
> > > element "Capture Source".
> > 
> > That's because "Capture Source" can't work for multiple (sub)devices
> > with the mixer abstraction of alsa-lib, per design.
> > "Input Source" was born as a workaround (still found in many places
> > in the driver code).  Maybe we should update ControlNames.txt as well.
> > 
> > > For playback it seems that some cards have a a headphone switch, and
> > > others a headphone slider (which i guess makes sense).
> > > 
> > > Now, the question, how should I implement this?
> > > 
> > > For playback the handling is easy as long as there is only one element
> > > to deal with, but what about capture? One option would be to simply
> > > go by cswitch and nothing else. Or go by "Input Source" and nothing
> > > else. Or combine some form. Now I'd of course prefer if the drivers
> > > get fixed to use a single element naming scheme only. Is there any
> > > chance to get that? And which one would that be?
> > 
> > I rarely believe this will be ever "fixed" in the driver side
> > completely.  We may improve a bit, but not the whole stuff.
> > It's no good idea to have a restriction in the driver code because
> > the control API is just for generic purpose, not only about mixers.
> > And, many embedded devices love to have specific unique control names
> > just for their purpose...
> 
> Hmm, so this won't get fixed.
> 
> From an application pov, how am I supposed to use the current
> abstraction? What algorithm should I then pick for PulseAudio? How
> should I compile the list of possible outputs and inputs? And if an
> item is selected from that list, how am I supposed to activate the
> entry?

I feel we need another meta data to manage / group the mixer elements.
It'll be likely based on the per-driver database, or implemented as a
module.  We have already a framework for mixer module, so it's
feasible.  Where it's the best choice is another question, though.

But, as now, it's still a vaporware.  So, let's assume to solve in
PA.

> For inputs: should I simply compile a list of all elements that have a
> cswitch plus all items from "Mic Select" plus all items from "Input
> Source"? And if an item is selected the logic would be like this: if a
> cswitch is selected we simply activate it, deactivating all others. If
> an item from "Mic Select" is selected we activate it in the enum and
> set the cswitch for "Mic" if there is any. And if an item from "Input
> Source" is seleced we activate it in the enum and set the cswitch for
> "Capture" if there is any. And we ignore "Capture Source". Is that a
> good idea?

In alsa-lib mixer abstraction, there is no "Capture Source" indeed.
It's always expanded to individual cswitches.  "Input Source" is never
cswitches, OTOH.  So they are basically exclusive.

The "Capture Volume" and "Capture Switch" are basically independent
from the sources.  They are a kind of "master" contorl for inputs.

Also, for inputs, there are also some exceptional mixer elements such
as "Mic Boost".  They may work both inputs & outputs, though.

> For outputs: If there is a Headphone element we assume we can use the
> "Master" and "Headphone" elements to switch between "Line-Out" and
> "Headphone". Is that assumption correct?

At least, "Headphone" control only affects the headphone output.
Ditto for "Speaker".  It's just for speaker output.

But, whether "Master" controls both HP and line-out depends on the
driver (rather codec chip).  It sucks, but because of a long history
background...


Takashi