[alsa-devel] ioctl request code for specific alsa control

Peter Wurmsdobler peter at wurmsdobler.org
Fri May 16 11:16:34 CEST 2008


Thanks for your help.

>> What ioctl request code would I have to pass in order to reach this
>> control's put/get methods?
> snd_ctl_elem_write() (alsa-lib/src/control/control.c)
> snd_ctl_hw_elem_write() (alsa-lib/src/control/control_hw.c)
> snd_ctl_ioctl() (linux/sound/core/control.c)
> snd_ctl_elem_write_user()
> snd_ctl_elem_write()
> kctl->put()
This is the path in the alsa world. I have followed this code as well, 
and it uses the alsa control element as payload into the ioctl.

I have taken a more pragmatic and certainly less orthodox one:

         ioctl(mixerFd, SOUND_MIXER_WRITE_VOLUME, &leftright);

will eventually call the put method of a kernel control 
pmic_control_pb_vol with the name "Master Playback Volume".

The function snd_mixer_oss_build_input() appears to be responsible for 
creating this link, by looking through a list of words such as "Master", 
"CD", etc in conjunction with expressions such as "%s Playback Volume". 
It probably will then try to match the names of the alsa kernel controls 
and set the put function pointer if appropriate.

Consequently, if I rename the control responsible for the output, 
pmic_control_op_sw, to something for which a ioctl exists, e.g. "CD", 
then it works as a pseudo volume control and I can use:

         ioctl(mixerFd, SOUND_MIXER_WRITE_CD, &output);

This approach is admittedly a hack, but it works. This ioctl will result 
in the call of the fake CD control and its put method.

If I had renamed the the output control "Master Output Switch", I could 
not have used a volume control and hence I would not know what ioctl 
request code to use.


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