[pulseaudio-discuss] kcontrol implementation for jack detection

[David Henningsson]

On 01/18/2012 06:35 PM, Tanu Kaskinen wrote:
> On Wed, 2012-01-11 at 17:03 +0100, David Henningsson wrote:
>> With 3.3 merge window opened, and the new jack detection interface
>> scheduled to go in, maybe it's time to talk about the implementation as
>> well.
>>
>> My draft patch is here:
>> http://kernel.ubuntu.com/git?p=diwic/pulseaudio.git;a=commit;h=1f239a9966fe7e43d3c9cb37fedabf31ff7d706e
>>
>> Fetching the jack state is done through the alsa-lib hctl interface,
>> whereas the existing mixer stuff uses the alsa-lib smixer interface,
>> which layers on top of the hctl interface. Right now I've tried to reuse
>> the existing mixer code, which leads to stuff such as:
>>
>>       if (fdl->hctl)
>>           err = snd_hctl_poll_descriptors_revents(fdl->hctl,
>> fdl->work_fds, fdl->num_fds,&revents);
>>       else
>>           err = snd_mixer_poll_descriptors_revents(fdl->mixer,
>> fdl->work_fds, fdl->num_fds,&revents);
>>
>> Looks a bit copy-pasted, but I think it is the easiest solution at the
>> moment. It's just five or so places anyway.
>> The other option would be to try to port the entire mixer stuff to use
>> the hctl interface instead of the smixer interface, but I'm not sure I
>> can foresee how much trouble that actually would be, and if we would run
>> in to any regressions.
>>
>> Thoughts?
>
> The current approach of using hctl and smixer side by side looks ok to
> me. A couple of questions:
>
> Supposedly the simple mixer interface is significantly simpler in some
> way. If we moved to hctl entirely, do you know roughly how much there
> would be extra complexity? A lot, a little or are the two interfaces
> about the same? Do you know in what ways the simple interface is
> simpler? If there's no explosion of unnecessary code resulting from
> moving to hctl, I'd file a wishlist bug. Otherwise we can just forget
> about it.

I'm not completely sure but after a short look I think we should remain 
at using smixer for the mixers. They got stuff like reading tlv and 
turning it into dB information; and by doing that manually, we would 
have to adapt if ALSA ever wanted to change that stuff.

I'll see if I can clean up my patch a bit, to make it ready for merging 
upstream.

> Have the alsa developers said anything about adding the jack stuff to
> the smixer interface?

No, this stuff is quite new and nobody's been requesting it anyway. But 
I'd say probably not - the jacks are not mixers.

@Colin, I just had a quick look at libsalsa at seems to implement the 
hctl interface. So that should be okay.

-- 
David Henningsson, Canonical Ltd.
http://launchpad.net/~diwic