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

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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>
Link: https://patch.msgid.link/20260526072906.90106-1-chengordon326@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
This commit is contained in:
Gordon Chen 2026-05-26 15:29:06 +08:00 • committed by Takashi Iwai
commit a238120042
3 changed files with 20 additions and 1 deletions
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