[PATCH] ALSA: usb-audio: add IFB_SILENCE_ON_EMPTY quirk for Behringer Flow 8

[Gordon Chen <chengordon326@gmail.com>]

The Behringer Flow 8 (1397:050c) is an 8-channel USB mixer that
declares OUT EP 0x01 with implicit feedback from capture EP 0x81 via
its UAC2 endpoint companion descriptor. After 5-35 minutes of
continuous playback, the device occasionally returns a capture URB in
which every iso_frame_desc has a non-zero status (-EXDEV bursts,
visible as rate-limited "frame N active: -18" lines in dmesg from
pcm.c).

In that case snd_usb_handle_sync_urb() at endpoint.c counts bytes==0
and falls into the early "skip empty packets" return originally added
for M-Audio Fast Track Ultra. As a result the playback EP loses its
sole IFB-driven feeder and the OUT ring starves permanently: hw_ptr
stops advancing while substream state remains RUNNING. Only USB
re-enumeration recovers.

Three independent ftrace captures (taken at the moment of stall via a
userspace watchdog) consistently show:

  - 60-70 capture URB completions in the 70ms window before the marker
  - 0 retire_playback_urb / queue_pending_output_urbs /
    snd_usb_endpoint_implicit_feedback_sink calls
  - every usb_submit_urb in the window comes from
    snd_complete_urb+0x64e (capture self-resubmit), none from the
    queue_pending_output_urbs path

Add a new opt-in quirk QUIRK_FLAG_IFB_SILENCE_ON_EMPTY: when set, the
early return is skipped and we fall through to enqueue a packet_info
whose packet_size[i] are all 0 (the existing loop already maps
status!=0 packets to size 0). prepare_outbound_urb then emits a
silence packet, the OUT ring keeps moving, and the device rides
through the glitch.

The default behaviour (early return) is preserved for all existing
devices including M-Audio Fast Track Ultra. Only Flow 8 opts in here.

Cc: stable@vger.kernel.org
Signed-off-by: Gordon Chen <chengordon326@gmail.com>
---
 sound/usb/endpoint.c | 10 +++++++++-
 sound/usb/quirks.c   |  3 +++
 sound/usb/usbaudio.h |  8 ++++++++
 3 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
index 6fbcb1175..24cd7692b 100644
--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -1780,8 +1780,16 @@ static void snd_usb_handle_sync_urb(struct snd_usb_endpoint *ep,
 		/*
 		 * skip empty packets. At least M-Audio's Fast Track Ultra stops
 		 * streaming once it received a 0-byte OUT URB
+		 *
+		 * However, on devices where bytes==0 means every sync-source
+		 * packet errored (e.g. Behringer Flow 8 returning -EXDEV bursts
+		 * for entire capture URBs), an unconditional return starves the
+		 * IFB-fed OUT ring permanently. Such devices set
+		 * QUIRK_FLAG_IFB_SILENCE_ON_EMPTY to fall through and enqueue a
+		 * packet_info with size 0 packets, so playback emits silence
+		 * and the OUT ring keeps moving.
 		 */
-		if (bytes == 0)
+		if (bytes == 0 && !(ep->chip->quirk_flags & QUIRK_FLAG_IFB_SILENCE_ON_EMPTY))
 			return;
 
 		spin_lock_irqsave(&ep->lock, flags);
diff --git a/sound/usb/quirks.c b/sound/usb/quirks.c
index 31cbe383a..e2c95be38 100644
--- a/sound/usb/quirks.c
+++ b/sound/usb/quirks.c
@@ -2365,6 +2365,8 @@ static const struct usb_audio_quirk_flags_table quirk_flags_table[] = {
 		   QUIRK_FLAG_PLAYBACK_FIRST | QUIRK_FLAG_GENERIC_IMPLICIT_FB),
 	DEVICE_FLG(0x1397, 0x0509, /* Behringer UMC404HD */
 		   QUIRK_FLAG_PLAYBACK_FIRST | QUIRK_FLAG_GENERIC_IMPLICIT_FB),
+	DEVICE_FLG(0x1397, 0x050c, /* Behringer Flow 8 */
+		   QUIRK_FLAG_IFB_SILENCE_ON_EMPTY),
 	DEVICE_FLG(0x13e5, 0x0001, /* Serato Phono */
 		   QUIRK_FLAG_IGNORE_CTL_ERROR),
 	DEVICE_FLG(0x152a, 0x880a, /* NeuralDSP Quad Cortex */
@@ -2602,6 +2604,7 @@ static const char *const snd_usb_audio_quirk_flag_names[] = {
 	QUIRK_STRING_ENTRY(SKIP_IFACE_SETUP),
 	QUIRK_STRING_ENTRY(MIXER_PLAYBACK_LINEAR_VOL),
 	QUIRK_STRING_ENTRY(MIXER_CAPTURE_LINEAR_VOL),
+	QUIRK_STRING_ENTRY(IFB_SILENCE_ON_EMPTY),
 	NULL
 };
 
diff --git a/sound/usb/usbaudio.h b/sound/usb/usbaudio.h
index 58fd07f8c..9afcad8f1 100644
--- a/sound/usb/usbaudio.h
+++ b/sound/usb/usbaudio.h
@@ -236,6 +236,12 @@ extern bool snd_usb_skip_validation;
  * QUIRK_FLAG_MIXER_CAPTURE_LINEAR_VOL
  *  Similar to QUIRK_FLAG_MIXER_PLAYBACK_LINEAR_VOL, but for capture streams.
  *  Overrides QUIRK_FLAG_MIXER_CAPTURE_MIN_MUTE
+ * QUIRK_FLAG_IFB_SILENCE_ON_EMPTY
+ *  In implicit feedback mode, when an entire capture URB returns with
+ *  all iso_frame_desc[i].status != 0 (bytes==0), do not silently return
+ *  from snd_usb_handle_sync_urb. Instead fall through and enqueue a
+ *  packet_info containing only size-0 packets, so the OUT ring keeps
+ *  moving (emits silence). Needed by Behringer Flow 8 (1397:050c).
  */
 
 enum {
@@ -268,6 +274,7 @@ enum {
 	QUIRK_TYPE_SKIP_IFACE_SETUP		= 26,
 	QUIRK_TYPE_MIXER_PLAYBACK_LINEAR_VOL	= 27,
 	QUIRK_TYPE_MIXER_CAPTURE_LINEAR_VOL	= 28,
+	QUIRK_TYPE_IFB_SILENCE_ON_EMPTY		= 29,
 /* Please also edit snd_usb_audio_quirk_flag_names */
 };
 
@@ -302,5 +309,6 @@ enum {
 #define QUIRK_FLAG_SKIP_IFACE_SETUP		QUIRK_FLAG(SKIP_IFACE_SETUP)
 #define QUIRK_FLAG_MIXER_PLAYBACK_LINEAR_VOL	QUIRK_FLAG(MIXER_PLAYBACK_LINEAR_VOL)
 #define QUIRK_FLAG_MIXER_CAPTURE_LINEAR_VOL	QUIRK_FLAG(MIXER_CAPTURE_LINEAR_VOL)
+#define QUIRK_FLAG_IFB_SILENCE_ON_EMPTY		QUIRK_FLAG(IFB_SILENCE_ON_EMPTY)
 
 #endif /* __USBAUDIO_H */
-- 
2.54.0