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

On Tue, 28 Jul 2026 16:27:35 +0200,
Mark Shi wrote:
> 
> Hi Gordon,
> 
>  2026/5/26 15:29, 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: [email protected]
> > Signed-off-by: Gordon Chen <[email protected]>
> > 
> 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