Re: [PATCH] ALSA: usb-audio: Check sticky mixers precisely

[Rong Zhang]

Hi Alexander,

On Tue, 2026-08-18 at 17:24 +0200, Alexander Niemeyer wrote:
> Hi Rong,
> 
> one important follow-up to my previous message:
> 
> I realized that the detachable microphone boom was not physically connected
> to the headset during the direct libusb tests I reported earlier.
> 
> I therefore repeated the relevant tests with the boom microphone attached,
> and the result changes in an important way.
> 
> With the boom microphone attached, on a fresh device:
> 
> Mic GET_RES = 256
> no SET_RES
> 
> Playback:
>    GET_CUR = -3840 (-15 dB)
>    SET_CUR = -2048 (-8 dB)
>    GET_CUR changes to -2048 after 65.3 ms
> 
> Result: PASS
> 
> After another power cycle, with the boom still attached:
> 
> Mic GET_RES before = 256
> Mic SET_RES(128)    = success
> Mic GET_RES after  = 256
> 
> Playback:
>    GET_CUR = -3840 (-15 dB)
>    SET_CUR = -2048 (-8 dB)
>    GET_CUR changes to -2048 after 66.3 ms
> 
> Result: PASS
> 
> I also repeated the complete Mic SET_RES sequence:
> 
> SET_RES 128
> SET_RES 64
> SET_RES 32
> SET_RES 16
> SET_RES 8
> SET_RES 4
> SET_RES 2
> SET_RES 1
> 
> All requests return success, GET_RES remains 256, and Playback 
> SET_CUR(-8 dB)
> still works afterwards, with GET_CUR changing after 76.0 ms.
> 
> However, with the boom microphone attached, the stock Fedora 7.1.8
> snd-usb-audio driver still reproduces the original failure:
> 
>    sticky mixer values (-16384/0/256 => -3840), disabling
> 
> and no PCM Playback Volume control is exposed.

Could you test my patch with Mic attached?

> 
> So I need to correct the conclusion from my previous message:
> 
> The Mic SET_RES behavior is real, but it depends on whether the detachable
> microphone boom is physically connected.
> 
> With the boom disconnected:
>    a single Mic SET_RES(128) is sufficient to make subsequent Playback
>    SET_CUR ineffective for more than 1200 ms.
> 
> With the boom connected:
>    both a single Mic SET_RES(128) and the full SET_RES sequence leave
>    Playback SET_CUR working normally.
> 

Thanks for your tests.

Let me conclude:

- Mic detached: SET_RES on Mic breaks Playback
- Mic attached: SET_RES on Mic breaks nothing

Still, I doubt if Playback SET_CUR is really broken in the first case.
There is a chance that Playback SET_CUR is effective while GET_CUR
becomes broken.

Could you test:

- Leave the Mic detached
- SET_RES on Mic
- GET_CUR on Playback
- Play some audio, listen to it
- SET_CUR on Playback
- GET_CUR on Playback
- Play some audio again, listen to it, and tell if the physical volume
has changed

The procedure can be achieved by hacking into the snd-usb-audio driver
and nullifying its cache mechanism, so that you can fetch GET_CUR values
from standard ALSA mixer interface.

If you can notice physical volume changes, it implies SET_RES on Mic
break Playback GET_CUR without breaking SET_CUR. Otherwise, it break
Playback SET_CUR.

If it only breaks GET_CUR, it implies the methodology of sticky check is
problematic. In this case I would probably consider demoting the sticky
check's severity.

I am also thinking about how we should demote the severity. Could you
also test if applying QUIRK_FLAG_MIXER_GET_CUR_BROKEN (without this
patch) breaks mixer change notifications? I.e., tune the volume with the
buttons/knobs on the device and see if the audio stack reflects the
change. I guess it will break the notifications, but I would still like
to wait for your confirmation. 

Thanks,
Rong

> Therefore Mic SET_RES alone does not explain the complete snd-usb-audio
> failure in all device configurations. There must be another difference in
> the full snd-usb-audio initialization path when the boom is attached.
> 
> The separate Playback -64 dB minimum issue I reported remains unchanged:
> -64 dB is ineffective, -63 dB works, and attempting -64 dB does not prevent
> subsequent valid Playback SET_CUR requests from working.
> 
> Sorry for not noticing the detachable-microphone state earlier. I wanted to
> send this correction as soon as I confirmed it.
> 
> I still have the usbmon/pcapng captures and can run further targeted tests
> if useful.
> 
> Thanks,
> Alexander
> 
> Am 18.08.2026 um 16:41 schrieb Alexander Niemeyer:
> > Hi Rong,
> > 
> > I think we found the reason for the different behavior.
> > 
> > I reproduced the snd-usb-audio initialization sequence step by step 
> > with direct libusb UAC1 control transfers and isolated the problem to 
> > SET_RES on the *Mic Capture Volume control (Feature Unit 3)*.
> > 
> > A fresh-device control test looks like this:
> > 
> > Mic GET_RES = 256
> > no SET_RES
> > Playback:
> > GET_CUR = -3840 (-15 dB)
> > SET_CUR = -2048 (-8 dB)
> > GET_CUR changes to -2048 after 68.0 ms
> > Result: PASS
> > 
> > After another power cycle, I repeated the same test but issued just 
> > *one* SET_RES request to the Mic Feature Unit first:
> > 
> > Mic GET_RES before = 256
> > Mic SET_RES(128) = success
> > Mic GET_RES after = 256
> > Playback:
> > GET_CUR = -3840 (-15 dB)
> > SET_CUR = -2048 (-8 dB)
> > GET_CUR remains -3840 for more than 1200 ms
> > Result: FAIL
> > 
> > So a single successful |SET_RES(128)| on Feature Unit 3 is sufficient 
> > to make subsequent |SET_CUR| requests to the Playback Volume control 
> > on Feature Unit 2 ineffective.
> > 
> > I also tested the complete Mic SET_RES sequence used by snd-usb-audio:
> > 
> > SET_RES 128
> > SET_RES 64
> > SET_RES 32
> > SET_RES 16
> > SET_RES 8
> > SET_RES 4
> > SET_RES 2
> > SET_RES 1
> > 
> > All requests return success, while GET_RES remains 256. After that 
> > sequence, Playback SET_CUR also remains ineffective for more than 1200 
> > ms.
> > 
> > Interestingly, the Mic control itself still works after this. In an 
> > ALSA-like Mic probe I could successfully change Mic Volume from 0 dB 
> > to -64 dB and then +1 dB, with GET_CUR reflecting those changes 
> > essentially immediately (~0.3 ms). Playback remained broken afterwards.
> > 
> > I also checked whether SET_RES on the Playback Feature Unit itself 
> > causes the problem. It does not:
> > 
> > Playback GET_RES = 256
> > SET_RES 128 -> 64 -> 32 -> 16 -> 8 -> 4 -> 2 -> 1
> > GET_RES still = 256
> > Playback SET_CUR(-8 dB)
> > GET_CUR changes successfully after 87.7 ms
> > 
> > So the problematic operation appears specifically to be *SET_RES on 
> > the Mic Feature Unit affecting the Playback Feature Unit*.
> > 
> > I also clarified the separate advertised-minimum issue:
> > 
> > Playback SET_CUR(-64 dB):
> > no change after >1200 ms
> > followed by SET_CUR(-8 dB):
> > works normally after 54.9 ms
> > Playback SET_CUR(-63 dB):
> > works after 75.8 ms
> > followed by SET_CUR(-8 dB):
> > works after 43.6 ms
> > 
> > Therefore the broken -64 dB endpoint does not leave the device in the 
> > broken state; it is a separate issue. -63 dB works normally.
> > 
> > I also captured usbmon/pcapng traces for both a working direct-libusb 
> > SET_CUR sequence and the failing snd-usb-audio initialization, so I 
> > can send those as well if they are useful.
> > 
> > This also seems to explain why the sticky-check changes did not help: 
> > by the time snd-usb-audio reaches the Playback Volume sticky check, 
> > the earlier Mic SET_RES sanity test has already put the device into 
> > the state where Playback SET_CUR no longer takes effect.
> > 
> > Let me know if you would like me to test a patch or capture any 
> > additional traces.
> > 
> > Thanks,
> > Alexander
> > 
> > 
> > Am 16.08.2026 um 17:08 schrieb Rong Zhang:
> > > Hi Alexander,
> > > 
> > > On Sun, 2026-08-16 at 16:09 +0200, Alexander Niemeyer wrote:
> > > > Hi Rong,
> > > > 
> > > > Sure. The libusb tests were direct USB Audio Class 1 control transfers
> > > > to the headset using libusb/PyUSB, not ALSA mixer operations.
> > > > 
> > > > I accessed Feature Unit 2 on AudioControl interface 0, master 
> > > > channel 0,
> > > > with the UAC1 Volume control selector:
> > > > 
> > > > wValue = 0x0200 /* Volume control, master channel */
> > > > wIndex = 0x0200 /* Feature Unit 2, interface 0 */
> > > > 
> > > > I used the standard class-specific requests directly, including 
> > > > GET_CUR,
> > > > GET_MIN, GET_MAX, GET_RES and SET_CUR, with signed 16-bit little-endian
> > > > volume values in 1/256 dB units.
> > > > 
> > > > The device reported:
> > > > 
> > > > GET_CUR: 0 ( 0 dB in that test)
> > > > GET_MIN: -16384 (-64 dB)
> > > > GET_MAX: 0 ( 0 dB)
> > > > GET_RES: 256 ( 1 dB)
> > > > 
> > > > For the timing tests I issued SET_CUR for a target value and then
> > > > repeatedly queried GET_CUR until the value changed or the timeout 
> > > > expired.
> > > > 
> > > > Valid values became visible after roughly:
> > > > 
> > > > -1 dB ~81 ms
> > > > -2 dB ~52 ms
> > > > -4 dB ~47 ms
> > > > -8 dB ~47 ms
> > > > -16 dB ~52 ms
> > > > -32 dB ~47 ms
> > > > 
> > > > The advertised -64 dB minimum behaved differently: SET_CUR returned
> > > > successfully, but GET_CUR did not change even after 1000 ms.
> > > > 
> > > > To access the AudioControl interface with libusb, I unbound the
> > > > AudioControl interface from snd-usb-audio for the duration of the test.
> > > > 
> > > > I did not intentionally open a playback stream during those libusb
> > > > tests. Because the AudioControl interface had been unbound from
> > > > snd-usb-audio, I also do not believe there was an active ALSA playback
> > > > stream at that point.
> > > Thanks for the information.
> > > 
> > > Unfortunately, I still don't exactly see why the device behaved
> > > differently when GET_CUR/SET_CUR requests were sent from snd-usb-audio
> > > compared to your libusb tests.
> > > 
> > > snd-usb-audio also tries SET_RES to test the sanity of GET_RES. Could 
> > > you
> > > test if it breaks your device's GET_CUR?
> > > 
> > > Maybe comparing them with usbmon can show some clues. You can use
> > > Wireshark to sniff /dev/usbmon*.
> > > 
> > > Hint: a Thunderbolt port usually corresponds to a dedicated USB root 
> > > hub.
> > > If you have one, plug the device to it to get pure usbmon trace results
> > > with no noisy URBs from other devices.
> > > 
> > > Thanks,
> > > Rong
> > > 
> > > > If the open-stream state is important, I can repeat the experiment
> > > > specifically controlling for playback-stream-open versus
> > > > playback-stream-closed.
> > > > 
> > > > Thanks,
> > > > Alexander
> > > > 
> > > > Am 16.08.2026 um 15:50 schrieb Rong Zhang:
> > > > > Hi Alexander,
> > > > > 
> > > > > On Sun, 2026-08-16 at 07:14 +0200, Alexander Niemeyer wrote:
> > > > > > Hi Rong,
> > > > > > 
> > > > > > I tested the sticky-check part of your patch on the Logitech PRO X
> > > > > > Wireless (046d:0aba) on Fedora 44, kernel 7.1.8-200.fc44.x86_64.
> > > > > > 
> > > > > > Since your patch is based on a newer tree, I used a minimal 
> > > > > > backport of
> > > > > > the new ~16-value / 10 ms sticky-check logic to the 7.1.8 code. The
> > > > > > GET_CUR-broken handling from the newer tree was not included; GET_CUR
> > > > > > itself succeeds on this device.
> > > > > > 
> > > > > > Unfortunately, the playback control is still classified as sticky:
> > > > > > 
> > > > > > 2:0: sticky mixer values (-16384/0/256 => -3840), disabling
> > > > > > 
> > > > > > I then instrumented the check and tried an additional diagnostic: 
> > > > > > after
> > > > > > every successful SET_CUR, wait 100 ms and perform another GET_CUR 
> > > > > > before
> > > > > > issuing the next SET_CUR.
> > > > > > 
> > > > > > For the playback volume, the saved value was -3840 and GET_CUR 
> > > > > > remained
> > > > > > at -3840 for every tested value, even after 100 ms, for example:
> > > > > > 
> > > > > > test=-15104 immediate=-3840 after100ms=-3840
> > > > > > test=-13824 immediate=-3840 after100ms=-3840
> > > > > > test=-3584 immediate=-3840 after100ms=-3840
> > > > > > test=-2304 immediate=-3840 after100ms=-3840
> > > > > > test=-1024 immediate=-3840 after100ms=-3840
> > > > > > test=0 immediate=-3840 after100ms=-3840
> > > > > > 
> > > > > > So in this case the issue does not appear to be simply that the
> > > > > > accumulated 10 ms sleeps are too short. During the probe-time sticky
> > > > > > check, SET_CUR succeeds but GET_CUR for the playback control remains
> > > > > > unchanged even when each SET_CUR is given 100 ms before the next one.
> > > > > > 
> > > > > > This differs from my previous direct libusb tests with the 
> > > > > > AudioControl
> > > > > > interface unbound, where valid SET_CUR values became visible through
> > > > > > GET_CUR after roughly 47–81 ms.
> > > > > Really interesting. Maybe the mixer changes its value only when 
> > > > > there is
> > > > > an opened playback stream.
> > > > > 
> > > > > Could you clarify your "libusb tests"?
> > > > > 
> > > > > Thanks,
> > > > > Rong
> > > > > 
> > > > > > The first debug line I saw with |saved=0| was from the Mic Capture
> > > > > > Volume control; that control changed immediately and returned as
> > > > > > non-sticky. The sequence above with |saved=-3840| is the 
> > > > > > problematic PCM
> > > > > > Playback Volume control.
> > > > > > 
> > > > > > I'd be happy to test another version or run additional diagnostics if
> > > > > > useful.
> > > > > > 
> > > > > > Best regards,
> > > > > > Alexander
> > > > > > 
> > > > > > 
> > > > > > Am 15.08.2026 um 23:47 schrieb Rong Zhang:
> > > > > > > Some mixers are asynchronous, and some have broken min/max. They are
> > > > > > > mistakenly considered sticky due to how the check is implemented.
> > > > > > > 
> > > > > > > Check sticky mixers more precisely by checking approximately 16 
> > > > > > > values
> > > > > > > and adding a msleep(10) between each check, so that asynchronous 
> > > > > > > mixers
> > > > > > > have enough time to change the value and mixers with broken 
> > > > > > > min/max are
> > > > > > > checked properly. Additionally, mark GET_CUR as broken when
> > > > > > > get_cur_mix_raw() fails, instead of returning successfully.
> > > > > > > 
> > > > > > > Reported-by: Alexander Niemeyer<adventureFAN@xxxxxx>
> > > > > > > Closes:https://lore.kernel.org/r/6262cbbd-d1f2-4c9d-a1c7-9c5d12636f4b@xxxxxx 
> > > > > > > 
> > > > > > > Signed-off-by: Rong Zhang<i@xxxxxxxx>
> > > > > > > ---
> > > > > > >     sound/usb/mixer.c | 51 
> > > > > > > ++++++++++++++++++++++++++++++++++++++++++++-------
> > > > > > >     1 file changed, 44 insertions(+), 7 deletions(-)
> > > > > > > 
> > > > > > > diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
> > > > > > > index 703c118f9d4e..3d0f97730a06 100644
> > > > > > > --- a/sound/usb/mixer.c
> > > > > > > +++ b/sound/usb/mixer.c
> > > > > > > @@ -1256,22 +1256,59 @@ static void init_cur_mix_raw(struct 
> > > > > > > usb_mixer_elem_info *cval, int ch, int idx)
> > > > > > >     static int check_sticky_volume_control(struct 
> > > > > > > usb_mixer_elem_info *cval,
> > > > > > >                            int channel, int saved)
> > > > > > >     {
> > > > > > > -    int sticky_test_values[] = { cval->min, cval->max };
> > > > > > > -    int test, check, i;
> > > > > > > +    int test, check, res;
> > > > > > > +
> > > > > > > +    /*
> > > > > > > +     * Check approximately 16 values (15 intervals).
> > > > > > > +     * If the resolution is not fine enough, check fewer values.
> > > > > > > +     */
> > > > > > > +    res = DIV_ROUND_UP(cval->max - cval->min, 15);
> > > > > > > +    res = res ? roundup(res, cval->res) : cval->res;
> > > > > > > +
> > > > > > > +    /*
> > > > > > > +     * If (cval->max - cval->min) is not a multiple of 
> > > > > > > cval->res, we still
> > > > > > > +     * want to test cval->max anyway.
> > > > > > > +     */
> > > > > > > +    for (test = cval->min; test < cval->max + res; test += res) {
> > > > > > > +        if (test > cval->max)
> > > > > > > +            test = cval->max;
> > > > > > >     -    for (i = 0; i < ARRAY_SIZE(sticky_test_values); i++) {
> > > > > > > -        test = sticky_test_values[i];
> > > > > > >             if (test == saved)
> > > > > > >                 continue;
> > > > > > >                 /* Assume non-sticky on failure. */
> > > > > > > -        if (snd_usb_set_cur_mix_value(cval, channel, 0, test) ||
> > > > > > > -            get_cur_mix_raw(cval, channel, &check) ||
> > > > > > > -            check != saved) /* SET_CUR effective, non-sticky. */
> > > > > > > +        if (snd_usb_set_cur_mix_value(cval, channel, 0, test))
> > > > > > > +            return 0;
> > > > > > > +
> > > > > > > +        if (get_cur_mix_raw(cval, channel, &check))
> > > > > > > +            goto get_cur_broken;
> > > > > > > +        if (check != saved) /* SET_CUR effective, non-sticky. */
> > > > > > >                 return 0;
> > > > > > > +
> > > > > > > +        /*
> > > > > > > +         * Leave some time for asynchronous mixers to change the 
> > > > > > > value.
> > > > > > > +         *
> > > > > > > +         * Note that there is no need to wait between SET_CUR and
> > > > > > > +         * GET_CUR, as we don't care whether the GET_CUR value 
> > > > > > > matches
> > > > > > > +         * the SET_CUR one. IOW, what we expect is just a 
> > > > > > > GET_CUR value
> > > > > > > +         * differing from the saved one.
> > > > > > > +         *
> > > > > > > +         * Mixers of most devices are synchronous. The should have
> > > > > > > +         * returned early without extra sleep. Asynchronous 
> > > > > > > mixers will
> > > > > > > +         * return once the accumulated time is enough for them 
> > > > > > > to change
> > > > > > > +         * the value.
> > > > > > > +         */
> > > > > > > +        msleep(10);
> > > > > > >         }
> > > > > > >     +    /* Check again after the last msleep(). */
> > > > > > > +    if (get_cur_mix_raw(cval, channel, &check))
> > > > > > > +        goto get_cur_broken;
> > > > > > > +    if (check != saved)
> > > > > > > +        return 0;
> > > > > > > +
> > > > > > >         if (cval->head.mixer->chip->quirk_flags & 
> > > > > > > QUIRK_FLAG_MIXER_GET_CUR_BROKEN) {
> > > > > > > +get_cur_broken:
> > > > > > >             usb_audio_info(cval->head.mixer->chip,
> > > > > > >                        "%d:%d: broken mixer GET_CUR (%d/%d/%d => 
> > > > > > > %d)\n",
> > > > > > >                        cval->head.id, 
> > > > > > > mixer_ctrl_intf(cval->head.mixer),
> > > > > > > 
> > > > > > > ---
> > > > > > > base-commit: 3eb40771c00a8488fa6ed2cc1fe203477908bf38
> > > > > > > change-id: 74676fce-uac-precise-sticky-check-94474a22b57d
> > > > > > > 
> > > > > > > Thanks,
> > > > > > > Rong
> > > > > > >