[alsa-devel] USB Audio initialization race

Clemens Ladisch clemens at ladisch.de
Sun Sep 15 10:49:40 CEST 2013

Alan Stern wrote:
> On Sat, 14 Sep 2013, Clemens Ladisch wrote:
>> Alan Stern wrote:
>>> So here's my question: If the sound driver recognizes that interface B
>>> is connected with interface A when B is probed, why can't it recognize
>>> this fact when A is probed?  It could claim B while A's probe is
>>> running.  Then the sound card would be registered with the PCM
>>> component already in place.
>> The sound driver already does this.
> It does?  Why does the comment preceding snd_usb_audio_probe() say:
>  * thus we check the usb device pointer and creates the card instance
>  * only at the first time.  the successive calls of this function will
>  * append the pcm interface to the corresponding card.

For UAC devices with correct descriptors, and for devices with multi-
interface quirks, the driver claims all interfaces at once.  But it is
still possible to have devices with zero or two audio control interfaces.


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