[alsa-devel] Getting a notification of snd_kcontrol value change, defined in codec, in machine driver

[Giedrius Trainavičius]

I would like to 'hook into' controls defined in a codec driver from my
machine driver - so I don't really have access to how these controls get
created in the first place.

Would it be possible to iterate in my machine driver through all the
controls defined by the codec, memorize the original 'put' callback,
override it with my own which performs the task I need and calls the
original put callback?

On Sun, Jul 8, 2018 at 4:00 AM, Takashi Sakamoto <o-takashi at sakamocchi.jp>
wrote:

> Hi,
>
> On Jul 8 2018 08:10, Giedrius Trainavičius wrote:
>
>> What would be the best way in machine driver to learn about changes of
>> alsa
>> mixer control values? The controls are defined in the codec driver.
>>
>> The intended use case would be to use the current volume setting for the
>> codec on the board to forward to a dedicated microcontroller for visual
>> feedback.
>>
>> I am aware that this could be bridged over via a user space program, but I
>> will have to forward the current PCM stream format to the MCU anyway from
>> hw_params, so ideally I would like all of this functionality contained in
>> the machine driver.
>>
>
> By assigning a function to  'struct snd_kcontrol.put', in-kernel drivers
> can receive any notification as a callback at changing value array of a
> control element set including handled conntrol element. When calling
> snd_ctl_new(), passing proper data for an argument of 'struct
> snd_kcontrol_new'.
>
> I note that for userspace applications, ALSA control core supports
> notification mechanism for this purpose by typical I/O handling in UNIX
> environment. With alsa-lib, at first, call 'snd_ctl_subscribe_events()',
> then listen to file descriptor for opened ALSA control character device (or
> call 'snd_ctl_wait()'). When any events are emitted, the applications can
> read data of 'struct snd_ctl_event' from the descriptor and handle the
> notification by parsing the data.
>
>
> Regards
>
> Takashi Sakamoto
>