[PATCH v3 1/2] ALSA: usb-audio: Add QUIRK_FLAG_MIXER_GET_CUR_BROKEN
From: Rong Zhang
Date: Thu May 28 2026 - 14:45:48 EST
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.
As the sticky check can't distinguish between sticky mixers and working
SET_CUR but broken GET_CUR, add QUIRK_FLAG_MIXER_GET_CUR_BROKEN to tell
that the device should fall into the second category when GET_CUR
returns a constant value. In this case, the sticky check becomes
non-fatal and only disables GET_CUR instead of the whole mixer. The
current volume will then be provided by the internal cache that stores
the last set volume.
Signed-off-by: Rong Zhang <i@xxxxxxxx>
---
Documentation/sound/alsa-configuration.rst | 12 +++++++
sound/usb/mixer.c | 58 ++++++++++++++++++++++++------
sound/usb/mixer.h | 1 +
sound/usb/quirks.c | 1 +
sound/usb/usbaudio.h | 13 +++++++
5 files changed, 75 insertions(+), 10 deletions(-)
diff --git a/Documentation/sound/alsa-configuration.rst b/Documentation/sound/alsa-configuration.rst
index 4b30cd63c5a5..78fb484e8b04 100644
--- a/Documentation/sound/alsa-configuration.rst
+++ b/Documentation/sound/alsa-configuration.rst
@@ -2389,6 +2389,18 @@ quirk_flags
from snd_usb_handle_sync_urb. Instead fall through and enqueue a
packet_info containing only size-0 packets, so the OUT ring keeps
moving (emits silence). Needed by Behringer Flow 8 (1397:050c).
+ * bit 30: ``mixer_get_cur_broken``
+ Some mixers are sticky, which means that setting their current volume
+ is a no-op, and reading the current volume returns a constant value.
+ The sticky check disables these mixers to prevent confusing userspace.
+ However, some devices do have a tunable volume despite the reported
+ current volume being constant. As the sticky check can't distinguish
+ between the two categories, setting this flag tells that the device
+ should fall into the second category when GET_CUR returns a constant
+ value, resulting in the sticky check being non-fatal and only
+ disabling GET_CUR instead of the whole mixer. The current volume will
+ then be provided by the internal cache that stores the last set
+ volume
This module supports multiple devices, autoprobe and hotplugging.
diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index d61bde654219..b98222e5f697 100644
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -434,6 +434,11 @@ int snd_usb_get_cur_mix_value(struct usb_mixer_elem_info *cval,
*value = cval->cache_val[index];
return 0;
}
+
+ /* The current value is always provided by the cache after initialization. */
+ if (cval->get_cur_broken)
+ return -ENXIO;
+
err = get_cur_mix_raw(cval, channel, value);
if (err < 0) {
if (!cval->head.mixer->ignore_ctl_error)
@@ -1223,7 +1228,7 @@ static void init_cur_mix_raw(struct usb_mixer_elem_info *cval, int ch, int idx)
err = snd_usb_get_cur_mix_value(cval, ch, idx, &val);
if (!err)
return;
- if (!cval->head.mixer->ignore_ctl_error)
+ if (!cval->head.mixer->ignore_ctl_error && !cval->get_cur_broken)
usb_audio_warn(cval->head.mixer->chip,
"%d:%d: failed to get current value for ch %d (%d)\n",
cval->head.id, mixer_ctrl_intf(cval->head.mixer),
@@ -1237,8 +1242,16 @@ static void init_cur_mix_raw(struct usb_mixer_elem_info *cval, int ch, int idx)
* Some devices' volume control mixers are sticky, which accept SET_CUR but
* do absolutely nothing.
*
- * Prevent sticky mixers from being registered, otherwise they confuses
- * userspace and results in ineffective volume control.
+ * Check the return values of GET_CUR with different SET_CUR values. Consider
+ * the mixer as sticky if GET_CUR always returns a constant value.
+ *
+ * Some devices have effective SET_CUR despite GET_CUR being constant. Do not
+ * consider the mixer as sticky if a quirk flag indicates that.
+ *
+ * Gate the registration of sticky mixers to prevent confusing userspace, so
+ * that they won't cause ineffective volume control. However, for mixers with
+ * effective SET_CUR but broken GET_CUR, the registration can continue normally
+ * but further GET_CUR requests will be gated.
*/
static int check_sticky_volume_control(struct usb_mixer_elem_info *cval,
int channel, int saved)
@@ -1258,6 +1271,16 @@ static int check_sticky_volume_control(struct usb_mixer_elem_info *cval,
return 0;
}
+ if (cval->head.mixer->chip->quirk_flags & QUIRK_FLAG_MIXER_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),
+ cval->min, cval->max, cval->res, saved);
+
+ cval->get_cur_broken = 1;
+ return -ENXIO;
+ }
+
usb_audio_err(cval->head.mixer->chip,
"%d:%d: sticky mixer values (%d/%d/%d => %d), disabling\n",
cval->head.id, mixer_ctrl_intf(cval->head.mixer),
@@ -1304,7 +1327,7 @@ static void check_volume_control_res(struct usb_mixer_elem_info *cval,
static int get_min_max_with_quirks(struct usb_mixer_elem_info *cval,
int default_min, struct snd_kcontrol *kctl)
{
- int i, idx, ret;
+ int i, idx, ret = 0;
/* for failsafe */
cval->min = default_min;
@@ -1360,10 +1383,12 @@ static int get_min_max_with_quirks(struct usb_mixer_elem_info *cval,
goto no_checks;
ret = check_sticky_volume_control(cval, minchn, saved);
- if (ret < 0) {
+ if (ret == -ENODEV) {
snd_usb_set_cur_mix_value(cval, minchn, 0, saved);
return ret;
}
+ if (ret)
+ goto no_checks;
if (cval->min + cval->res < cval->max)
check_volume_control_res(cval, minchn, saved);
@@ -1372,6 +1397,16 @@ static int get_min_max_with_quirks(struct usb_mixer_elem_info *cval,
}
no_checks:
+ /*
+ * Got a non-fatal failure during sanity checks.
+ *
+ * Do not propagate mixer values written by sanity checks.
+ * Instead, rely on init_cur_mix_raw() to initialize the mixer
+ * properly.
+ */
+ if (ret)
+ cval->cached = 0;
+
cval->initialized = 1;
}
@@ -3513,7 +3548,8 @@ void snd_usb_mixer_notify_id(struct usb_mixer_interface *mixer, int unitid)
continue;
info = mixer_elem_list_to_info(list);
/* invalidate cache, so the value is read from the device */
- info->cached = 0;
+ if (!info->get_cur_broken)
+ info->cached = 0;
snd_ctl_notify(mixer->chip->card, SNDRV_CTL_EVENT_MASK_VALUE,
&list->kctl->id);
}
@@ -3610,10 +3646,12 @@ static void snd_usb_mixer_interrupt_v2(struct usb_mixer_interface *mixer,
switch (attribute) {
case UAC2_CS_CUR:
/* invalidate cache, so the value is read from the device */
- if (channel)
- info->cached &= ~BIT(channel);
- else /* master channel */
- info->cached = 0;
+ if (!info->get_cur_broken) {
+ if (channel)
+ info->cached &= ~BIT(channel);
+ else /* master channel */
+ info->cached = 0;
+ }
snd_ctl_notify(mixer->chip->card, SNDRV_CTL_EVENT_MASK_VALUE,
&info->head.kctl->id);
diff --git a/sound/usb/mixer.h b/sound/usb/mixer.h
index afbb3dd9f177..3fa1bd96f858 100644
--- a/sound/usb/mixer.h
+++ b/sound/usb/mixer.h
@@ -94,6 +94,7 @@ struct usb_mixer_elem_info {
int cache_val[MAX_CHANNELS];
u8 initialized;
u8 min_mute;
+ u8 get_cur_broken;
void *private_data;
};
diff --git a/sound/usb/quirks.c b/sound/usb/quirks.c
index e2c95be38aca..ac2f0f6039be 100644
--- a/sound/usb/quirks.c
+++ b/sound/usb/quirks.c
@@ -2605,6 +2605,7 @@ static const char *const snd_usb_audio_quirk_flag_names[] = {
QUIRK_STRING_ENTRY(MIXER_PLAYBACK_LINEAR_VOL),
QUIRK_STRING_ENTRY(MIXER_CAPTURE_LINEAR_VOL),
QUIRK_STRING_ENTRY(IFB_SILENCE_ON_EMPTY),
+ QUIRK_STRING_ENTRY(MIXER_GET_CUR_BROKEN),
NULL
};
diff --git a/sound/usb/usbaudio.h b/sound/usb/usbaudio.h
index 9afcad8f143a..e472aef6eb87 100644
--- a/sound/usb/usbaudio.h
+++ b/sound/usb/usbaudio.h
@@ -242,6 +242,17 @@ extern bool snd_usb_skip_validation;
* from snd_usb_handle_sync_urb. Instead fall through and enqueue a
* packet_info containing only size-0 packets, so the OUT ring keeps
* moving (emits silence). Needed by Behringer Flow 8 (1397:050c).
+ * QUIRK_FLAG_MIXER_GET_CUR_BROKEN
+ * Some mixers are sticky, which means that setting their current volume is a
+ * no-op, and reading the current volume returns a constant value. The sticky
+ * check disables these mixers to prevent confusing userspace. However, some
+ * devices do have a tunable volume despite the reported current volume being
+ * constant. As the sticky check can't distinguish between the two categories,
+ * setting this flag tells that the device should fall into the second
+ * category when GET_CUR returns a constant value, resulting in the sticky
+ * check being non-fatal and only disabling GET_CUR instead of the whole mixer.
+ * The current volume will then be provided by the internal cache that stores
+ * the last set volume
*/
enum {
@@ -275,6 +286,7 @@ enum {
QUIRK_TYPE_MIXER_PLAYBACK_LINEAR_VOL = 27,
QUIRK_TYPE_MIXER_CAPTURE_LINEAR_VOL = 28,
QUIRK_TYPE_IFB_SILENCE_ON_EMPTY = 29,
+ QUIRK_TYPE_MIXER_GET_CUR_BROKEN = 30,
/* Please also edit snd_usb_audio_quirk_flag_names */
};
@@ -310,5 +322,6 @@ enum {
#define QUIRK_FLAG_MIXER_PLAYBACK_LINEAR_VOL QUIRK_FLAG(MIXER_PLAYBACK_LINEAR_VOL)
#define QUIRK_FLAG_MIXER_CAPTURE_LINEAR_VOL QUIRK_FLAG(MIXER_CAPTURE_LINEAR_VOL)
#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)
#endif /* __USBAUDIO_H */
--
2.53.0