ALSA: usb-audio: add QUIRK_FLAG_ALWAYS_SET_RATE for Mackie DLZ Creator XS

set_sample_rate_v2v3() returns early when the clock already reports the
requested rate:

	prev_rate = get_sample_rate_v2v3(chip, fmt->iface,
					 fmt->altsetting, clock);
	if (prev_rate == rate)
		goto validation;

A device advertising exactly one sample rate always takes this branch, so
it never receives a SET_CUR for CS_SAM_FREQ_CONTROL at all.

The Mackie DLZ Creator XS (0a73:003a, 14 in / 4 out, 48 kHz only) requires
that write.  Without it the device drops off the USB bus roughly 0.2-1.8 s
into any stream, clearing its port CONNECTION bit; captured audio is
byte-correct until the instant it vanishes.

USBPcap traces of a cold-booted device on Windows show SET_CUR 48000 issued
unconditionally on every stream start, followed by clean streaming.  The
device is otherwise driven with plain class-compliant UAC2 - it also works
on iOS, which cannot load a vendor driver - so no vendor-specific
initialization is involved.

The device is self-powered, so the resulting state survives a USB replug:
initializing it on any host that issues the write leaves it working on
Linux until it is power-cycled, which made the failure look intermittent.

Add a quirk flag rather than dropping the early exit, since the opposite
requirement also exists in-tree: QUIRK_FLAG_FIXED_RATE suppresses rate
setting for single-rate devices (JBL Quantum610/810).  The two behaviors
are device-dependent and cannot both be the default.

A/B on identically cold-booted hardware, same kernel, same port, repeated
twice:

  without the flag  device dropped after 5-6 s, then again after 3-4 s
  with the flag     20 s playback followed by 20 s of 14-channel capture,
                    960000 frames, zero re-enumerations

This change was developed with an AI coding assistant.  The assistant did
the trace analysis that located the bug and wrote the patch and this
changelog; the hardware testing, the cold-boot cycles and the decision to
submit were the author's.  Several earlier hypotheses it proposed - URB
queue depth, isochronous packet under-allocation, endpoint start ordering -
were disproven by measurement before this one.

The bug was located with usbmon on Linux and USBPcap on Windows, by
diffing an enumeration capture of a cold-booted device on each host.
Verified on physical hardware by the A/B above.

Assisted-by: Claude-Code:claude-opus-5
Signed-off-by: JJ Macalinao <jj@macalinao.org>
Link: https://patch.msgid.link/20260808172726.1107550-1-jj@macalinao.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>

0 files changed, 0 insertions, 0 deletions