From f15cfca818d756dd1c9492530091dfd583359db3 Mon Sep 17 00:00:00 2001
From: Keith Winstein <keithw@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
Git-commit: f15cfca818d756dd1c9492530091dfd583359db3
Patch-mainline: v5.10-rc3
References: git-fixes

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@cs.stanford.edu>
Tested-by: Keith Winstein <keithw@cs.stanford.edu>
Cc: <stable@vger.kernel.org>
Buglink: https://lore.kernel.org/r/20201027071841.GA164525@trolley.csail.mit.edu
Signed-off-by: Takashi Iwai <tiwai@suse.de>

---
 sound/usb/pcm.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index b401ee894e1b..31b1edf53c41 100644
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -352,6 +352,10 @@ static int set_sync_ep_implicit_fb_quirk(struct snd_usb_substream *subs,
 		ep = 0x81;
 		ifnum = 2;
 		goto add_sync_ep_from_ifnum;
+	case USB_ID(0x1686, 0xf029): /* Zoom UAC-2 */
+		ep = 0x82;
+		ifnum = 2;
+		goto add_sync_ep_from_ifnum;
 	case USB_ID(0x1397, 0x0001): /* Behringer UFX1604 */
 	case USB_ID(0x1397, 0x0002): /* Behringer UFX1204 */
 		ep = 0x81;
-- 
2.26.2