Re: [PATCH] ALSA: usb-audio: add QUIRK_FLAG_ALWAYS_SET_RATE for Mackie DLZ Creator XS

On 8/8/26 23:14, Takashi Iwai wrote:
> On Sat, 08 Aug 2026 09:09:02 +0200,
> JJ Macalinao wrote:
>>
>> 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
>> initialisation is involved.
>>
>> The device is self-powered, so the resulting state survives a USB replug:
>> initialising 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 behaviours
>> 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]>
>> ---
>>  sound/usb/clock.c    | 3 ++-
>>  sound/usb/quirks.c   | 3 +++
>>  sound/usb/usbaudio.h | 6 ++++++
>>  3 files changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/sound/usb/clock.c b/sound/usb/clock.c
>> index 2e0c18e352..647bf55b1b 100644
>> --- a/sound/usb/clock.c
>> +++ b/sound/usb/clock.c
>> @@ -626,7 +626,8 @@ static int set_sample_rate_v2v3(struct snd_usb_audio *chip,
>>       }
>>
>>       prev_rate = get_sample_rate_v2v3(chip, fmt->iface, fmt->altsetting, clock);
>> -     if (prev_rate == rate)
>> +     if (prev_rate == rate &&
>> +         !(chip->quirk_flags & QUIRK_FLAG_ALWAYS_SET_RATE))
>>               goto validation;
> 
> With QUIRK_FLAG_ALWAYS_SET_RATE, calling get_sample_rate_v2v3() is
> rather useless, and we can skip it?  e.g.
> 
>         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;
>         }

Makes sense, just need to make sure prev_rate is initialized since it's
used in several places down the line. Will retest and update with v2.
Thank you.

JJ