Comment 19 for bug 1095315

On 01/04/2013 09:19 PM, Clemens Ladisch wrote:
> David Henningsson wrote:
>> this patch resolves the issue for him. It also leaves a message
>> in the log:
>> "snd-usb-audio: probe of 1-1.1:1.1 failed with error -5"
>
> I'd guess this is for an interface not claimed by the driver but
> without an explicit QUIRK_IGNORE_INTERFACE entry.

My patch causes this error because the quirk fails with an error, which
is later on turned into a -EIO. Any quirk_ignore_interface entry will
therefore be ignored. Using the quirk_ignore_interface entry *instead
of* my patch seems like an interesting option, but...

Looking at the code again, it seems like we have a more generic race
problem, unless I'm missing something:

  - the same ALSA sound card can contain more than one USB interface.
  - snd_card_register is called whenever a USB interface finishes probing.
  - snd_card_register fires off uevents, causing stuff to happen such as
volume restore (alsactl) and PulseAudio starts probing the device
  - so when userspace starts accessing the device, the device is not
fully functional, because not all USB interfaces have yet been probed
  - as a result, volumes might not get restored (if all mixer controls
do not belong to the first interface), and PulseAudio thinks the device
is not capable of all the stuff it actually can do (if all pcm streams
do not belong to the first interface).

I guess this is probably not a problem normally because the kernel is
faster than userspace, but it is revealed here because switching
configurations takes longer time than probing another interface.

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