ALSA: usb-audio: lowlatency playback runs 25% slow, no xrun (Scarlett 2i2)

[Alexander Toth <alexandertoth96@gmail.com>]

Hi,

On a Focusrite Scarlett 2i2 3rd Gen (1235:8210), playback with
lowlatency=1 (the default) and a two-period ring runs at about 75% of
the nominal rate when the application does some work between
snd_pcm_wait() and snd_pcm_writei(). No xrun is reported; hw_ptr just
advances slower than real time. With lowlatency=0 every case runs at
the full rate.

Setup: S32_LE, 2 ch, 48 kHz on hw:CARD=USB,DEV=0 (EP 0x01 OUT, sync
type SYNC, 125 us packets). Kernel 6.14.0-24-generic (Ubuntu 24.04),
alsa-lib 1.2.9.

The test loop is: snd_pcm_wait() for a free period, busy-wait N us,
then write one period of silence. The rate is frames written minus
snd_pcm_delay(), over 5 s. Frames consumed per second:

  period x n    work    lowlatency=1   lowlatency=0
  128 x 2       0 us       47,999         47,999
  128 x 2     300 us       36,159         47,997
  128 x 2    1500 us       36,569         47,999
  128 x 3     300 us       48,002         47,999
  256 x 2     300 us       41,090         47,999
  512 x 2     300 us       48,010         47,999

Other observations:
- SCHED_RR priority 20 doesn't help, so it isn't late wakeups.
- Writing the previous period first and computing afterwards holds 48k.
- The loss doesn't scale with the work: a run is either at 48k or at
  about 36k (one period in four lost). Near 50 us, runs flip between
  the two.
- At 128x2, delay is 510 frames with lowlatency=1 and 512 with
  lowlatency=0, so low-latency mode gains nothing on this card.

From reading sound/usb in v6.14 (not traced): snd_usb_pcm_playback_ack()
only queues pending output URBs when !ep->active_mask. If the write
lands after one URB has completed short of data but while the other is
still in flight, the new data waits for a whole URB. With two URBs per
ring the stream loses time, but submitted_urbs never reaches zero, so
notify_xrun() never fires. Is it intended that ack only queues URBs
when none are in flight?

I'm happy to test a patch or collect traces. The reproducer is a short
alsa-lib program, and I can send it if it helps.

This was investigated with Claude Code (Anthropic's Claude Opus 5.5
model). It ran the measurements, wrote the test program, read the
sound/usb source and drafted this mail. I steered and approved each
step, ran the lowlatency=0 tests myself and reviewed this mail before
sending it. No fix has been written, no trace taken, and only this one
device has been tested.

Thanks,
Alexander Toth