[alsa-devel] USB Audio initialization race

Clemens Ladisch clemens at ladisch.de
Fri Sep 13 22:55:43 CEST 2013

Alan Stern wrote:
>> 1) The first interface is probed, which might contain the mixer controls
>> 2) snd_card_register is called, which sends a signal to userspace
>> 3) PulseAudio probes the device and notices that it has no PCMs, so it
>> ignores the device
>> 4) The second interface is probed, which adds the PCMs
>> 5) snd_card_register is again called, which is a no-op since the card
>> already exist
>> 6) User is unhappy because his plugged in sound card did not show up in
>> PulseAudio.
>> So, assuming all this is right, it seems like we need some type of
>> callback from the usb driver when all the interfaces for the sound card
>> has been probed, so we can call snd_card_register at that point instead.
>> Thoughts?
> The driver probably doesn't know when all the interfaces have been
> probed.  Maybe it would be better to send a signal to userspace each
> time snd_card_register is called, even if nothing is done.

New devices will result in a signal because they result in new device

> Alternatively, when the first interface is probed, the driver could
> claim all the other interfaces belonging to the same association.

The driver already does this.


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