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

[Takashi Iwai <tiwai@suse.de>]

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