How to identify Alsa eLements?

How to identify Alsa eLements?

[Tanjeff Moos]

Hi alsa-devel,

I'm programming a GUI for the mixer in my Focusrite Scarlett 6i6 
interface. The Alsa driver supports that functionality, i.e. I can 
control the mixer using alsamixer, for example. Now I'm writing a Qt GUI 
for it.

I wonder how I can reliably identify a mixer control element (e.g. the 
master volume control). Each element has an ID (struct snd_ctl_elem_id), 
consisting of multiple elements:
- numid (seems to change at each boot)
- iface
- device
- subdevice
- name (is always constant)
- index
The docs says I can identify an element by the combination of name and 
index.

My questions:
1) Are name and index enough to identify an element?
2) How can I obtain the index? The name and numid are shown by 'amixer
    contents', for example.

Kind regards, Tanjeff

Re: How to identify Alsa eLements?

[Takashi Sakamoto]

Hi,

On Thu, Jul 02, 2020 at 10:05:01PM +0200, Tanjeff Moos wrote:
> I'm programming a GUI for the mixer in my Focusrite Scarlett 6i6 interface.
> The Alsa driver supports that functionality, i.e. I can control the mixer
> using alsamixer, for example. Now I'm writing a Qt GUI for it.
> 
> I wonder how I can reliably identify a mixer control element (e.g. the
> master volume control). Each element has an ID (struct snd_ctl_elem_id),
> consisting of multiple elements:
> - numid (seems to change at each boot)
> - iface
> - device
> - subdevice
> - name (is always constant)
> - index
> The docs says I can identify an element by the combination of name and
> index.
> 
> My questions:
> 1) Are name and index enough to identify an element?

In ALSA control core, 'snd_ctl_find_id()' helper function[1] is used to
find control element set by the given information from userspace
application. You can see the algorithm supports two cases:

1. numid matching
2. the combination matching with:
  * iface
  * device
  * subdevice
  * name
  * index is within the range of set

As you know, the numid is not constant between every bootup since it's
assigned dynamically (see __snd_ctl_add_replace() helper function[2]).
Thus the way 2 is available in the case to hard-code in userspace
application.

> 2) How can I obtain the index? The name and numid are shown by 'amixer
>    contents', for example.

The amixer program uses 'snd_ctl_ascii_elem_id_get()' API in alsa-lib to
show identifier information. The API is programmed not to output the
index value if it equals to zero[3]. Potentially you can retrieve the
index value by a call of 'snd_ctl_elem_id_get_index()'.


Anyway, when using alsa-lib application for the purpose, you should pay
enough attention to which API is used since alsa-lib includes several
abstractions of API for control element set in each level:

 * Lower abstraction (snd_ctl_xxx)
 * Higher abstraction (snd_hctl_xxx)
 * Setup control interface (snd_sctl_xxx)
 * Mixer interface (snd_mixer_xxx)
   * Simple Mixer interface (snd_mixer_selem_xxx)

The configuration space of alsa-lib affects Setup control interface
and Mixer interface. On the other hand, it doesn't affect the
lower/higher abstraction. The amixer is a kind of application to use
'snd_hctl_xxx', 'snd_mixer_xxx', and 'snd_mixer_selem_xxx'.


When you'd like to communicate to kernel land implementation without any
effects of alsa-lib's configuration space. it's better to use the lower/higher
abstractions. As long as I've used, 'qashctl' in QasTools[4] is good GUI
application for this purpose. It's written with Qt5 and seems to be helpful
for your work in both of GUI programming and control elements handling.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound.git/tree/sound/core/control.c#n653
[2] https://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound.git/tree/sound/core/control.c#n338
[3] https://github.com/alsa-project/alsa-lib/blob/master/src/control/ctlparse.c#L110
[4] https://gitlab.com/sebholt/qastools


Regards

Takashi Sakamoto

Re: How to identify Alsa eLements?

[Takashi Sakamoto]

On Fri, Jul 03, 2020 at 09:34:20AM +0900, Takashi Sakamoto wrote:
> > My questions:
> > 1) Are name and index enough to identify an element?
> 
> In ALSA control core, 'snd_ctl_find_id()' helper function[1] is used to
> find control element set by the given information from userspace
> application. You can see the algorithm supports two cases:
> 
> 1. numid matching
> 2. the combination matching with:
>   * iface
>   * device
>   * subdevice
>   * name
>   * index is within the range of set

I sent PR[1] to update the documentation in alsa-lib.


[1] https://github.com/alsa-project/alsa-lib/pull/66

Regards

Takashi Sakamoto

Re: How to identify Alsa eLements?

[Tanjeff Moos]

Hi Takashi Sakamoto,


On 03/07/2020 02:34, Takashi Sakamoto wrote:
> Hi,
> 
> On Thu, Jul 02, 2020 at 10:05:01PM +0200, Tanjeff Moos wrote:

[...]

>>
>> My questions:
>> 1) Are name and index enough to identify an element?
> 
> In ALSA control core, 'snd_ctl_find_id()' helper function[1] is used to
> find control element set by the given information from userspace
> application. You can see the algorithm supports two cases:
> 
> 1. numid matching
> 2. the combination matching with:
>    * iface
>    * device
>    * subdevice
>    * name
>    * index is within the range of set
> 
> As you know, the numid is not constant between every bootup since it's
> assigned dynamically (see __snd_ctl_add_replace() helper function[2]).
> Thus the way 2 is available in the case to hard-code in userspace
> application.
Thanks for the clarification.

> 
>> 2) How can I obtain the index? The name and numid are shown by 'amixer
>>     contents', for example.
> 

[...]

> 
> Anyway, when using alsa-lib application for the purpose, you should pay
> enough attention to which API is used since alsa-lib includes several
> abstractions of API for control element set in each level:
> 
>   * Lower abstraction (snd_ctl_xxx)
>   * Higher abstraction (snd_hctl_xxx)
>   * Setup control interface (snd_sctl_xxx)
>   * Mixer interface (snd_mixer_xxx)
>     * Simple Mixer interface (snd_mixer_selem_xxx)
I find this quite confusing. If I could change a volume control using 
any of those interfaces, then I don't understand when to use which 
interface. I'm sure that there is good reasoning for each of them, but 
unfortunatly the documentation has very little information about these 
concepts.

Anyway, I will stick to the lower abstraction which serves my needs. In 
the worst case I will do more work than necessary ;-)

> 
> The configuration space of alsa-lib affects Setup control interface
> and Mixer interface. On the other hand, it doesn't affect the
> lower/higher abstraction. The amixer is a kind of application to use
> 'snd_hctl_xxx', 'snd_mixer_xxx', and 'snd_mixer_selem_xxx'.
So the controls offered by CTL/HCTL are determined by the driver? And 
SCTL, MIXER and MIXER_SELEM are influenced by user space config files?

> When you'd like to communicate to kernel land implementation without any
> effects of alsa-lib's configuration space. it's better to use the lower/higher
> abstractions. As long as I've used, 'qashctl' in QasTools[4] is good GUI
> application for this purpose. It's written with Qt5 and seems to be helpful
> for your work in both of GUI programming and control elements handling.
qashctl is indeed very helpful, thank you! As being said, I'll stick to CTL.

Thank you very much for your advice!

  - Tanjeff

Re: How to identify Alsa eLements?

[Takashi Sakamoto]

On Fri, Jul 03, 2020 at 08:16:26AM +0200, Tanjeff Moos wrote:
> > Anyway, when using alsa-lib application for the purpose, you should pay
> > enough attention to which API is used since alsa-lib includes several
> > abstractions of API for control element set in each level:
> > 
> >   * Lower abstraction (snd_ctl_xxx)
> >   * Higher abstraction (snd_hctl_xxx)
> >   * Setup control interface (snd_sctl_xxx)
> >   * Mixer interface (snd_mixer_xxx)
> >     * Simple Mixer interface (snd_mixer_selem_xxx)
>
> I find this quite confusing. If I could change a volume control using any of
> those interfaces, then I don't understand when to use which interface. I'm
> sure that there is good reasoning for each of them, but unfortunatly the
> documentation has very little information about these concepts.
 
So do I. I can imagine that the toughness of work to design good
abstraction for the control feature...

> Anyway, I will stick to the lower abstraction which serves my needs. In the
> worst case I will do more work than necessary ;-)
> 
> > The configuration space of alsa-lib affects Setup control interface
> > and Mixer interface. On the other hand, it doesn't affect the
> > lower/higher abstraction. The amixer is a kind of application to use
> > 'snd_hctl_xxx', 'snd_mixer_xxx', and 'snd_mixer_selem_xxx'.
> 
> So the controls offered by CTL/HCTL are determined by the driver? And SCTL,
> MIXER and MIXER_SELEM are influenced by user space config files?
 
Yes, as long as I know.

HCTL abstraction nearly equals to 'CTL + cache mechanism for control elements +
event notification callback'. However in some cases of addition/removal control
element set from user space[1], it hits assert and aborts programs with
panic.

SCTL/MIXER/MIXER_SELEM features includes extra filter logic for control
elements with the configuration. They're specialized to usual channel of
audio; e.g. stereo, surround sounds. It's functional as long as using usual
sound devices such as stereo speakers. On the other hand, they can handle
just a part of the channels when handling control elements for exceptional
multi-channel of audio for recording equipments. I guess that your case
is the latter.

> > When you'd like to communicate to kernel land implementation without any
> > effects of alsa-lib's configuration space. it's better to use the lower/higher
> > abstractions. As long as I've used, 'qashctl' in QasTools[4] is good GUI
> > application for this purpose. It's written with Qt5 and seems to be helpful
> > for your work in both of GUI programming and control elements handling.
> qashctl is indeed very helpful, thank you! As being said, I'll stick to CTL.
> 
> Thank you very much for your advice!

Quashctl is pretty good tool for the purpose as long as I know. I wish for
someone to develop similar functional tool with CUI or TUI...


[1] for example, below test program can abort pulseaudio when removing
added control element set:
https://github.com/alsa-project/alsa-lib/blob/master/test/user-ctl-element-set.c


Regards

Takashi Sakamoto