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From: Jassi Brar <jassisinghbrar@gmail.com>
To: alsa-devel@alsa-project.org
Cc: Takashi Iwai <tiwai@suse.de>
Subject: Fixed sampling freq UAC2 doubt
Date: Fri, 21 Jan 2011 17:35:52 +0900	[thread overview]
Message-ID: <AANLkTinCZyrS6GDKfs3WerNrQ7DaF6_nMHLCaWqERmxC@mail.gmail.com> (raw)

Hi,

 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 5.2.5.1.1 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
USB?

Thanks.

             reply	other threads:[~2011-01-21  8:35 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-01-21  8:35 Jassi Brar [this message]
2011-01-24 13:21 ` Fixed sampling freq UAC2 doubt Takashi Iwai
2011-01-27  8:14 ` Clemens Ladisch
2011-01-29 11:29   ` Jassi Brar

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