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
