[alsa-devel] Fixed sampling freq UAC2 doubt

Jassi Brar jassisinghbrar at gmail.com
Fri Jan 21 09:35:52 CET 2011


 While testing my under-development generic USB Audio Class 2.0 _gadget_ driver
with ALSA's USB Audio Class driver, I came across an apparent discrepency.
Or so do I think.

Page-98 Section of 'Audio 20 final.pdf' specifies :-
[In many cases, the Clock Source Entity represents a crystal
oscillator based generator
 with a single fixed frequency. In that case, the Set request is not supported.]

Here 'not supported' means setup requests for CUR/RANGE CS_SAM_FREQ_CONTROL
should stall. Right ?

The snd_usb_hw_params in sound/usb/pcm.c apparently attempts to do that with
disregard to Clock-Frequency-Control bitmask in bmControls field of the
Clock Source Descriptor.

The confusion is, subs->cur_rate is initialized only if snd_usb_init_sample_rate
succeeds. Which does SET control tranfer, albeit with the supported frequency.
Couldn't a fixed sampling freq UAC2 compliant device refuse to work in
that case,
thereby failing any attempt to use the USB card?

Could someone please point where I got it wrong and/or if such a card
works with ALSA


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