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

[Takashi Iwai <tiwai@suse.de>]

On Tue, 28 Jul 2026 17:46:05 +0200,
Mark Shi wrote:
> 
> 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.

Hmm, that's odd.

> 
> 
> 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);

Could you put a debug print what value does ep->chip->quirk_flags
have on your device?


thanks,

Takashi