Re: Handling complex matrix mixers in ALSA

[Takashi Sakamoto <o-takashi@sakamocchi.jp>]

On Fri, Jul 12, 2024 at 06:48:09PM +0900, Asahi Lina wrote:
> > For example, assuming the matrix mixer has 34 destination and 66
> > sources, they can be expressed by 34 control elements with 66 array
> > elements. A single snd_kcontrol structure can provide them, as long as
> > they have the same nature. The control elements are identified by index
> > value.
> 
> It took me a while to understand what you meant here, but I think I get
> it: Using a single snd_kcontrol for the entire mixer, with 34 indexed
> elements each taking 66 array values, right?

This is rough example for the above idea.

```
static int info_cb(struct snd_kcontrol *kctl, struct snd_ctl_elem_info *einfo)
{
    einfo->type = SNDRV_CTL_ELEM_TYPE_INTEGER;
    einfo->count = 66;                  // up to 128 for integer value array.
    einfo->value.integer.min = 0;       // note long type, varies between
                                        // ILP32/LP64 data models.
    einfo->value.integer.max = 256;     // ditto.
    einfo->value.integer.step = 1;      // ditto.

    return 0;
}

static int get_cb(struct snd_kcontrol *kctl, struct snd_ctl_elem_value *uval)
{
    unsigned int index = snd_ctl_get_ioff(kctl, &info->id);

    // Any operation according to the index of control elements in the set.

    return 0;
}

static int put_cb(struct snd_kcontrol *kctl, struct snd_ctl_elem_value *uval)
{
    unsigned int offset = snd_ctl_get_ioff(kctl, &info->id);

    // Any operation according to the index of control elements in the set.

    return 0;
}

int add_control_elements(struct my_data *my_data)
{
    // Emulate AK4396.
    // 20 * log10(x/255) (dB)
    // Here, x is written value.
    //
    // Some examples in:
    // https://github.com/alsa-project/alsa-lib/blob/master/test/user-ctl-element-set.c.
    static const SNDRV_CTL_TLVD_DECLARE_DB_LINEAR(range_tlv, -4813, 0);
    
    static const struct snd_kcontrol_new template = {
        . iface = SNDRV_CTL_ELEM_IFACE_MIXER,
        .device = 10,
        .subdevice = 20,
        .name = "multiplexer",
        .index = 0,
        // I guess any operation on ARC USB does not control the hardware directly unlike RC and
        // ARC, thus SNDRV_CTL_ELEM_ACCESS_VOLATILE is not required.
        .access = SNDRV_CTL_ELEM_ACCESS_READWRITE | SNDRV_CTL_ELEM_ACCESS_TLV_READ,
        .count = 34,        // The total number of control elements in the set.
        .info = info_cb,
        .get = get_cb,
        .put = pub_cb,
        .tlv.p = &range_tlv,
    };
    
    struct snd_kcontrol *kctl = snd_ctl_new1(&template, my_data);
    err = snd_ctl_add(my_data->card, kctl);
    ...
}
```

> How do these kinds of controls show up in alsamixer and other userspace
> mixer tools? Are they usable at all, or just with low-level access via
> amixer/alsactl?

These control elements can be enumerated and operated as usual, just
have the sequential value in index (and numid) field in element
identifier.

```
$ amixer -c 0 controls
...
numid=16,iface=MIXER,name='multiplexer',device=10,subdevice=20
numid=17,iface=MIXER,name='multiplexer',index=1,device=10,subdevice=20
numid=18,iface=MIXER,name='multiplexer',index=2,device=10,subdevice=20
numid=19,iface=MIXER,name='multiplexer',index=3,device=10,subdevice=20
numid=20,iface=MIXER,name='multiplexer',index=4,device=10,subdevice=20
numid=21,iface=MIXER,name='multiplexer',index=5,device=10,subdevice=20
numid=22,iface=MIXER,name='multiplexer',index=6,device=10,subdevice=20
numid=23,iface=MIXER,name='multiplexer',index=7,device=10,subdevice=20
numid=24,iface=MIXER,name='multiplexer',index=8,device=10,subdevice=20
numid=25,iface=MIXER,name='multiplexer',index=9,device=10,subdevice=20
...
```

Of cource, these is no integrations in any type of alsa-lib mixer API
abstractions. I use Quasmixer in quastools project to operate them.

* https://gitlab.com/sebholt/qastools


Regards

Takashi Sakamoto