Re: [PATCH] ALSA: usb-audio: add IFB_SILENCE_ON_EMPTY quirk for Behringer Flow 8
From: Mark Shi
Date: Tue Jul 28 2026 - 12:37:07 EST
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,Do you have this USB-audio device (Behringer Flow 8 with USB ID
在 2026/5/26 15:29, Gordon Chen 写道:
The Behringer Flow 8 (1397:050c) is an 8-channel USB mixer thatDevice: Mechrevo WUJIE14Pro A-7BC4U
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@xxxxxxxxxxxxxxx
Signed-off-by: Gordon Chen <chengordon326@xxxxxxxxx>
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.
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