[PATCH] ALSA: usb-audio: Add implicit feedback quirk for Zoom UAC-2

[Takashi Iwai]

On Tue, 27 Oct 2020 08:18:41 +0100,
Keith Winstein wrote:
> 
> On Mon, Oct 26, 2020 at 02:52:28PM +0100, Takashi Iwai wrote:
> > On Mon, 26 Oct 2020 09:01:43 +0100,
> > Keith Winstein wrote:
> > > 
> > > Here is the patch again without line-wrapping.
> > 
> > Could you put the patch description in the patch itself?
> > Basically I'm fine to merge this as is, just missing information for
> > git.
> 
> Sure, here you go:
> 
> >From 51fac91fa3a5da656ca6a49ca6cc98be7664efd7 Mon Sep 17 00:00:00 2001
> From: Keith Winstein <keithw at cs.stanford.edu>
> Date: Sun, 25 Oct 2020 22:05:47 -0700
> Subject: [PATCH] ALSA: usb-audio: Add implicit feedback quirk for Zoom UAC-2
> 
> The Zoom UAC-2 USB audio interface provides an async playback endpoint ("1 OUT (ASYNC)") and
> capture endpoint ("2 IN (ASYNC)"), both with 2-channel S32_LE in 44.1, 48, 88.2, 96, 176.4,
> or 192 kilosamples/s. The device provides explicit feedback to adjust the host's playback
> rate, but the feedback appears unstable and biased relative to the device's capture rate.
> "alsaloop -t 1000" experiences playback underruns and tries to resample the captured audio
> to match the varying playback rate. Forcing the kernel to use implicit feedback appears to
> produce more stable results. This causes the host to transmit one playback sample for each
> capture sample received. (Zoom North America has been notified of this change.)
> 
> Signed-off-by: Keith Winstein <keithw at cs.stanford.edu>
> Tested-by: Keith Winstein <keithw at cs.stanford.edu>

Applied now.  Thanks.


Takashi