[alsa-devel] USB Audio initialization race

Alan Stern stern at rowland.harvard.edu
Fri Sep 13 22:39:28 CEST 2013

On Fri, 13 Sep 2013, David Henningsson wrote:

> I'm sometimes seeing that USB audio devices are not initialized 
> correctly by PulseAudio, but I can't reproduce this myself.
> I have, however, spotted what I think is a race condition in the driver 
> code, if a device has more than one interface. snd_card_register is 
> called once for every interface, which means that it will be shown to 
> userspace after the first interface has been probed. So the race could 
> look like:
> 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.

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

Alan Stern

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