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