ALSA: usb-audio: Fix boot-time audio stuttering for USB Audio device

This USB Audio device (0x1e0b:0xd01e) exhibits audio stuttering
during boot when playing audio. Once the system is fully booted,
playback is normal.

The device reports its isochronous endpoints with the Asynchronous
sync type (bmAttributes = 0x03), which causes the driver to
calculate nurbs = min(max_urbs, ...) = 3, providing only ~16ms
of buffering. During boot, the higher system scheduling jitter
(e.g., from init scripts, device enumeration, and driver probing)
can exceed this buffer depth, causing audible stuttering.

This patch adds a device-specific quirk (QUIRK_FLAG_PLAYBACK_URB_FIXUP)
that applies two changes for this device:
1. Forces nurbs to MAX_URBS (12), providing sufficient buffering
2. Sets URB_ISO_ASAP flag for more consistent xHCI scheduling

Both changes are required together for stable boot-time playback:
- The larger buffer absorbs scheduling jitter during boot
- URB_ISO_ASAP ensures consistent URB submission timing, preventing
  the xHCI scheduler from introducing variable delays

Test methodology:
- Without patch: reboot and play audio → stuttering audible in all
  tests (reproduced consistently across multiple attempts)
- With nurbs=8 only: occasional minor stuttering observed after
  multiple tests (insufficient buffer depth)
- With full patch (nurbs=12 + URB_ISO_ASAP): reboot and play audio
  → no stuttering observed (tested in 10+ reboot cycles without
  reproducing the issue)

Signed-off-by: Zhang Heng <zhangheng@kylinos.cn>
Link: https://patch.msgid.link/20260728111309.1271834-1-zhangheng@kylinos.cn
Signed-off-by: Takashi Iwai <tiwai@suse.de>

0 files changed, 0 insertions, 0 deletions