Since commit 86aa1ea1f15c ("ALSA: usb-audio: Do not expose sticky
mixers"), the UAC mixer core utilizes volume SET_CUR and GET_CUR to
identify devices with sticky mixers. Unfortunately, even though most
devices with sticky GET_CUR also have corresponding sticky SET_CUR,
which I actually met more since the commit had been merged, there is
also a rare case that some devices may have volume mixers that responds
to SET_CUR properly but with its GET_CUR stubbed. This cause the sticky
check to consider the mixer to be sticky and unnecessarily disable it.

Add QUIRK_FLAG_MIXER_SKIP_GET_CUR_VOL to prevent sending GET_CUR to
mixers by returning -ENXIO early. The error effectively skips the sticky
check as it's only meaningful when the mixer has some sort of self-
awareness. Similar to QUIRK_FLAG_GET_SAMPLE_RATE, this should also help
if some unmet devices can't tolerate volume GET_CUR in other ways.

The Sennheiser MOMENTUM 3 needs the quirk flag. Though its UAC mixer
works fine and precisely corresponds to the reported dB range, the
mixer's volume GET_CUR method is somehow stubbed and returns a constant
value (15dB), resulting in it being disabled by the sticky check.

Signed-off-by: Rong Zhang <i@rong.moe>
---
Rong Zhang (3):
      ALSA: doc: usb-audio: Add doc for QUIRK_FLAG_IFB_SILENCE_ON_EMPTY
      ALSA: usb-audio: Add QUIRK_FLAG_MIXER_SKIP_GET_CUR_VOL
      ALSA: usb-audio: Add quirk flag for Sennheiser MOMENTUM 3

 Documentation/sound/alsa-configuration.rst | 10 ++++++++++
 sound/usb/mixer.c                          |  5 +++++
 sound/usb/quirks.c                         |  3 +++
 sound/usb/usbaudio.h                       |  6 ++++++
 4 files changed, 24 insertions(+)
---
base-commit: a23812004228d4b041a858b927db787a7ff80f50
change-id: 20260527-uac-quirk-get-cur-vol-d0b292c3e796

Thanks,
Rong