[PATCH v2] 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 <[email protected]>
---
v2: skip the GET_CUR entirely when the flag is set, per Takashi's review,
    rather than issuing it and discarding the result.

    prev_rate is initialized to 0 because it is still read below - the
    !cur_rate fallback and the TEAC 0x0644 block. That is not a new state:
    get_sample_rate_v2v3() already returns 0 when the GET fails, so both
    paths handle it today.

    Retested on cold-booted hardware, same unit and port: 20 s playback
    followed by 20 s of 14-channel capture, 960000 frames, no
    re-enumeration.

 sound/usb/clock.c    | 11 +++++++----
 sound/usb/quirks.c   |  3 +++
 sound/usb/usbaudio.h |  6 ++++++
 3 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/sound/usb/clock.c b/sound/usb/clock.c
index 2e0c18e352..34832183a1 100644
--- a/sound/usb/clock.c
+++ b/sound/usb/clock.c
@@ -600,7 +600,7 @@ int snd_usb_set_sample_rate_v2v3(struct snd_usb_audio *chip,
 static int set_sample_rate_v2v3(struct snd_usb_audio *chip,
 				const struct audioformat *fmt, int rate)
 {
-	int cur_rate, prev_rate;
+	int cur_rate, prev_rate = 0;
 	int clock;
 
 	/* First, try to find a valid clock. This may trigger
@@ -625,9 +625,12 @@ static int set_sample_rate_v2v3(struct snd_usb_audio *chip,
 			return clock;
 	}
 
-	prev_rate = get_sample_rate_v2v3(chip, fmt->iface, fmt->altsetting, clock);
-	if (prev_rate == rate)
-		goto validation;
+	if (!(chip->quirk_flags & QUIRK_FLAG_ALWAYS_SET_RATE)) {
+		prev_rate = get_sample_rate_v2v3(chip, fmt->iface,
+						 fmt->altsetting, clock);
+		if (prev_rate == rate)
+			goto validation;
+	}
 
 	cur_rate = snd_usb_set_sample_rate_v2v3(chip, fmt, clock, rate);
 	if (cur_rate < 0) {
diff --git a/sound/usb/quirks.c b/sound/usb/quirks.c
index 3330eb6049..d00d6a543a 100644
--- a/sound/usb/quirks.c
+++ b/sound/usb/quirks.c
@@ -2333,6 +2333,8 @@ static const struct usb_audio_quirk_flags_table quirk_flags_table[] = {
 		   QUIRK_FLAG_IGNORE_CTL_ERROR),
 	DEVICE_FLG(0x0951, 0x16ad, /* Kingston HyperX */
 		   QUIRK_FLAG_CTL_MSG_DELAY_1M),
+	DEVICE_FLG(0x0a73, 0x003a, /* Mackie DLZ Creator XS */
+		   QUIRK_FLAG_ALWAYS_SET_RATE),
 	DEVICE_FLG(0x0b05, 0x18a6, /* ASUSTek Computer, Inc. */
 		   QUIRK_FLAG_MIXER_CAPTURE_MIN_MUTE),
 	DEVICE_FLG(0x0b0e, 0x0349, /* Jabra 550a */
@@ -2638,6 +2640,7 @@ static const char *const snd_usb_audio_quirk_flag_names[] = {
 	QUIRK_STRING_ENTRY(IFB_SILENCE_ON_EMPTY),
 	QUIRK_STRING_ENTRY(MIXER_GET_CUR_BROKEN),
 	QUIRK_STRING_ENTRY(PLAYBACK_URB_FIXUP),
+	QUIRK_STRING_ENTRY(ALWAYS_SET_RATE),
 	NULL
 };
 
diff --git a/sound/usb/usbaudio.h b/sound/usb/usbaudio.h
index 31e6125000..c49709d7ad 100644
--- a/sound/usb/usbaudio.h
+++ b/sound/usb/usbaudio.h
@@ -260,6 +260,10 @@ extern bool snd_usb_skip_validation;
  *  to insufficient buffer depth combined with xHCI scheduling variability.
  *  The larger buffer (MAX_URBS = 12, ~64ms) absorbs system scheduling
  *  jitter during boot, while URB_ISO_ASAP ensures consistent xHCI scheduling.
+ * QUIRK_FLAG_ALWAYS_SET_RATE:
+ *  Issue SET_CUR for the sample rate even when the clock already reports the
+ *  requested rate.  A device advertising a single rate is otherwise never sent
+ *  the request at all, and some require it before streaming will start.
  */
 
 enum {
@@ -295,6 +299,7 @@ enum {
 	QUIRK_TYPE_IFB_SILENCE_ON_EMPTY		= 29,
 	QUIRK_TYPE_MIXER_GET_CUR_BROKEN		= 30,
 	QUIRK_TYPE_PLAYBACK_URB_FIXUP		= 31,
+	QUIRK_TYPE_ALWAYS_SET_RATE		= 32,
 /* Please also edit snd_usb_audio_quirk_flag_names */
 };
 
@@ -332,5 +337,6 @@ enum {
 #define QUIRK_FLAG_IFB_SILENCE_ON_EMPTY		QUIRK_FLAG(IFB_SILENCE_ON_EMPTY)
 #define QUIRK_FLAG_MIXER_GET_CUR_BROKEN		QUIRK_FLAG(MIXER_GET_CUR_BROKEN)
 #define QUIRK_FLAG_PLAYBACK_URB_FIXUP		QUIRK_FLAG(PLAYBACK_URB_FIXUP)
+#define QUIRK_FLAG_ALWAYS_SET_RATE		QUIRK_FLAG(ALWAYS_SET_RATE)
 
 #endif /* __USBAUDIO_H */

base-commit: 140fe610af607fd70da95c447943fc2690855519
-- 
2.55.0