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

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

[Gordon Chen]

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

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

[Takashi Iwai]

On Tue, 26 May 2026 09:29:06 +0200,
Gordon Chen wrote:
> 
> 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>

Applied to for-next branch now, as it's no urgent fix like a
regression.


thanks,

Takashi

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

[Mark Shi]

Hi Gordon,

在 2026/5/26 15:29, Gordon Chen 写道:
> 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>
>
Device: Mechrevo WUJIE14Pro A-7BC4U
CPU: AMD Ryzen 7 7840HS (Codename "Phoenix")
Firmware Version: T140_PHX_V20
Firmware Release Date: 2024.1.8
Kernel version: 7.1.4

In Linux kernel 7.1.4, I found that my laptop couldn't wake up from S3 
suspend; I had to hold down the power button to force a shutdown and 
then restart it.

Using `git bisect`, I discovered that if I revert this patch, the laptop 
resumes from S3 normally.

Best Regards,
Mark Shi

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

[Takashi Iwai]

On Tue, 28 Jul 2026 16:27:35 +0200,
Mark Shi wrote:
> 
> Hi Gordon,
> 
> 在 2026/5/26 15:29, Gordon Chen 写道:
> > 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>
> > 
> Device: Mechrevo WUJIE14Pro A-7BC4U
> CPU: AMD Ryzen 7 7840HS (Codename "Phoenix")
> Firmware Version: T140_PHX_V20
> Firmware Release Date: 2024.1.8
> Kernel version: 7.1.4
> 
> In Linux kernel 7.1.4, I found that my laptop couldn't wake up from S3
> suspend; I had to hold down the power button to force a shutdown and
> then restart it.
> 
> Using `git bisect`, I discovered that if I revert this patch, the
> laptop resumes from S3 normally.

Do you have this USB-audio device (Behringer Flow 8 with USB ID
1397:050c)?  The commit must be harmless unless you have this device.
That is, if you don't have this device, it's likely a false-positive
bisection result.


thanks,

Takashi

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

[Mark Shi]

Hi Takashi,

在 2026/7/28 23:02, Takashi Iwai 写道:
> On Tue, 28 Jul 2026 16:27:35 +0200,
> Mark Shi wrote:
>> Hi Gordon,
>>
>> 在 2026/5/26 15:29, Gordon Chen 写道:
>>> 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>
>>>
>> Device: Mechrevo WUJIE14Pro A-7BC4U
>> CPU: AMD Ryzen 7 7840HS (Codename "Phoenix")
>> Firmware Version: T140_PHX_V20
>> Firmware Release Date: 2024.1.8
>> Kernel version: 7.1.4
>>
>> In Linux kernel 7.1.4, I found that my laptop couldn't wake up from S3
>> suspend; I had to hold down the power button to force a shutdown and
>> then restart it.
>>
>> Using `git bisect`, I discovered that if I revert this patch, the
>> laptop resumes from S3 normally.
> Do you have this USB-audio device (Behringer Flow 8 with USB ID
> 1397:050c)?  The commit must be harmless unless you have this device.
> That is, if you don't have this device, it's likely a false-positive
> bisection result.
>
>
> thanks,
>
> Takashi

I don't have that device, but I partially reverted the code in that 
section, and it went back to normal.


Thanks,

Mark Shi

---
  sound/usb/endpoint.c | 10 +---------
  1 file changed, 1 insertion(+), 9 deletions(-)

diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
index 24cd7692bd01..6fbcb117555c 100644
--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -1780,16 +1780,8 @@ 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 && !(ep->chip->quirk_flags & 
QUIRK_FLAG_IFB_SILENCE_ON_EMPTY))
+        if (bytes == 0)
              return;

          spin_lock_irqsave(&ep->lock, flags);
-- 
2.55.0