[alsa-devel] Jack sensing in snd_usb_audio ?

[Takashi Iwai]

On Tue, 18 Oct 2016 15:33:42 +0200,
Felipe Ferreri Tonello wrote:
> 
> Hi Takashi
> 
> On 18/10/16 13:07, Takashi Iwai wrote:
> > On Wed, 12 Oct 2016 18:15:04 +0200,
> > Bastien Nocera wrote:
> >>
> >> On Wed, 2016-10-12 at 18:06 +0200, Clemens Ladisch wrote:
> >>> Bastien Nocera wrote:
> >>>> On Wed, 2016-10-12 at 14:43 +0200, Clemens Ladisch wrote:
> >>>>> Bastien Nocera wrote:
> >>>>>> "
> >>>>>> A change of state in the audio function is most often caused by
> >>>>>> a
> >>>>>> certain event that takes place. An event can either be user-
> >>>>>> initiated
> >>>>>> or device-initiated. User-initiated jack insertion or removal
> >>>>>> is a
> >>>>>> typical example of a user-initiated event.
> >>>>>> "
> >>>>>
> >>>>>
> >>>>> There are not many USB Audio 2.0 devices, and I'm not aware of
> >>>>> any
> >>>>> that actually implements this.
> >>>>
> >>>>
> >>>> I guess I would see whether there are events if I captured the USB
> >>>> traffic even without special handling/turning on a feature in the
> >>>> drivers, right?
> >>>
> >>>
> >>> Most devices do not even have the status endpoint (see "lsusb -v").
> >>> To check what events arrive, you can add logging to the
> >>> snd_usb_mixer_interrupt() function.
> >>
> >> I'm guessing it doesn't support it then (see attached log)
> > 
> > So this looks like a HID, not from the audio device class.
> > It's an oft-seen implementation.
> > 
> >> I also checked the input device output when plugging in something, with
> >> evtest, and no feedback either.
> > 
> > Then at first you need to hack a HID driver to support this device.
> > It'll create an input device, and then we'll need to find some way to
> > couple the given input device and the audio device.  We can parse the
> > sysfs device path to figure out, but I'm not sure what's the best way
> > to tell it to applications.
> > 
> 
> Why not use similar API as a normal ALSA card? This will enable jack
> detection by default in applications using kcontrol interface.

Are you suggesting to create another sound card object by a HID
driver?  This would be even less useful.  Then you'll have two
individual sound cards in the end that have no connection between
them.


Takashi