[PATCH 7/9] ALSA: usb-audio: Add QUIRK_FLAG_MIXER_{PLAYBACK,CAPTURE}_LINEAR_VOL

Rong Zhang
2026-03-01T21:38:14Z

Some quirky devices tune their volume by linearly tuning the voltage
level (linear volume). In other words, such devices has a linear TLV
mapping of DECLARE_TLV_DB_LINEAR(scale, TLV_DB_GAIN_MUTE, 0).

Add quirk flags MIXER_PLAYBACK_LINEAR_VOL and MIXER_CAPTURE_LINEAR_VOL
to represent this case respectively for playback and capture mixers.

No functional change intended.

Signed-off-by: Rong Zhang <i@rong.moe>
---
Documentation/sound/alsa-configuration.rst | 7 +++++
sound/usb/mixer_quirks.c | 34 ++++++++++++++++++++++
sound/usb/quirks.c | 2 ++
sound/usb/usbaudio.h | 12 ++++++++
4 files changed, 55 insertions(+)

diff --git a/Documentation/sound/alsa-configuration.rst b/Documentation/sound/alsa-configuration.rst
index 55b845d382368..f75f087639415 100644
--- a/Documentation/sound/alsa-configuration.rst
+++ b/Documentation/sound/alsa-configuration.rst
@@ -2376,6 +2376,13 @@ quirk_flags
Skip the probe-time interface setup (usb_set_interface,
init_pitch, init_sample_rate); redundant with
snd_usb_endpoint_prepare() at stream-open time
+ * bit 27: ``mixer_playback_linear_vol``
+ Set linear volume mapping for devices where the playback volume
+ control value is mapped to voltage (instead of dB) level linearly.
+ In short: ``x(raw) = (raw - raw_min) / (raw_max - raw_min)``;
+ ``V(x) = k * x``; ``dB(x) = 20 * log10(x)``. Overrides bit 24
+ * bit 28: ``mixer_capture_linear_vol``
+ Similar to bit 27 but for capture streams. Overrides bit 25

This module supports multiple devices, autoprobe and hotplugging.

diff --git a/sound/usb/mixer_quirks.c b/sound/usb/mixer_quirks.c
index 11e205da7964d..539044c0c6440 100644
--- a/sound/usb/mixer_quirks.c
+++ b/sound/usb/mixer_quirks.c
@@ -4634,6 +4634,25 @@ static void snd_fix_plt_name(struct snd_usb_audio *chip,
usb_audio_dbg(chip, "something wrong in kctl name %s\n", id->name);
}

+static void snd_usb_mixer_fu_quirk_linear_scale(struct usb_mixer_interface *mixer,
+ struct usb_mixer_elem_info *cval,
+ struct snd_kcontrol *kctl)
+{
+ static const DECLARE_TLV_DB_LINEAR(scale, TLV_DB_GAIN_MUTE, 0);
+
+ if (cval->min_mute) {
+ /*
+ * We are clearing SNDRV_CTL_ELEM_ACCESS_TLV_CALLBACK,
+ * resulting in min_mute being a no-op.
+ */
+ usb_audio_warn(mixer->chip, "LINEAR_VOL overrides MIN_MUTE\n");
+ }
+
+ kctl->tlv.p = scale;
+ kctl->vd[0].access |= SNDRV_CTL_ELEM_ACCESS_TLV_READ;
+ kctl->vd[0].access &= ~SNDRV_CTL_ELEM_ACCESS_TLV_CALLBACK;
+}
+
void snd_usb_mixer_fu_apply_quirk(struct usb_mixer_interface *mixer,
struct usb_mixer_elem_info *cval, int unitid,
struct snd_kcontrol *kctl)
@@ -4660,6 +4679,21 @@ void snd_usb_mixer_fu_apply_quirk(struct usb_mixer_interface *mixer,
"applying capture min mute quirk\n");
cval->min_mute = 1;
}
+
+ if (mixer->chip->quirk_flags & QUIRK_FLAG_MIXER_PLAYBACK_LINEAR_VOL)
+ if (cval->control == UAC_FU_VOLUME && strstr(kctl->id.name, "Playback")) {
+ usb_audio_info(mixer->chip,
+ "applying playback linear volume quirk\n");
+ snd_usb_mixer_fu_quirk_linear_scale(mixer, cval, kctl);
+ }
+
+ if (mixer->chip->quirk_flags & QUIRK_FLAG_MIXER_CAPTURE_LINEAR_VOL)
+ if (cval->control == UAC_FU_VOLUME && strstr(kctl->id.name, "Capture")) {
+ usb_audio_info(mixer->chip,
+ "applying capture linear volume quirk\n");
+ snd_usb_mixer_fu_quirk_linear_scale(mixer, cval, kctl);
+ }
+
/* ALSA-ify some Plantronics headset control names */
if (USB_ID_VENDOR(mixer->chip->usb_id) == 0x047f &&
(cval->control == UAC_FU_MUTE || cval->control == UAC_FU_VOLUME))
diff --git a/sound/usb/quirks.c b/sound/usb/quirks.c
index 5370bcd99e9d0..2327288e2b20f 100644
--- a/sound/usb/quirks.c
+++ b/sound/usb/quirks.c
@@ -2540,6 +2540,8 @@ static const char *const snd_usb_audio_quirk_flag_names[] = {
QUIRK_STRING_ENTRY(MIXER_PLAYBACK_MIN_MUTE),
QUIRK_STRING_ENTRY(MIXER_CAPTURE_MIN_MUTE),
QUIRK_STRING_ENTRY(SKIP_IFACE_SETUP),
+ QUIRK_STRING_ENTRY(MIXER_PLAYBACK_LINEAR_VOL),
+ QUIRK_STRING_ENTRY(MIXER_CAPTURE_LINEAR_VOL),
NULL
};

diff --git a/sound/usb/usbaudio.h b/sound/usb/usbaudio.h
index 085530cf62d92..58fd07f8c3c97 100644
--- a/sound/usb/usbaudio.h
+++ b/sound/usb/usbaudio.h
@@ -228,6 +228,14 @@ extern bool snd_usb_skip_validation;
* Skip the probe-time interface setup (usb_set_interface,
* init_pitch, init_sample_rate); redundant with
* snd_usb_endpoint_prepare() at stream-open time
+ * QUIRK_FLAG_MIXER_PLAYBACK_LINEAR_VOL
+ * Set linear volume mapping for devices where the playback volume control
+ * value is mapped to voltage (instead of dB) level linearly. In short:
+ * x(raw) = (raw - raw_min) / (raw_max - raw_min); V(x) = k * x;
+ * dB(x) = 20 * log10(x). Overrides QUIRK_FLAG_MIXER_PLAYBACK_MIN_MUTE
+ * QUIRK_FLAG_MIXER_CAPTURE_LINEAR_VOL
+ * Similar to QUIRK_FLAG_MIXER_PLAYBACK_LINEAR_VOL, but for capture streams.
+ * Overrides QUIRK_FLAG_MIXER_CAPTURE_MIN_MUTE
*/

enum {
@@ -258,6 +266,8 @@ enum {
QUIRK_TYPE_MIXER_PLAYBACK_MIN_MUTE = 24,
QUIRK_TYPE_MIXER_CAPTURE_MIN_MUTE = 25,
QUIRK_TYPE_SKIP_IFACE_SETUP = 26,
+ QUIRK_TYPE_MIXER_PLAYBACK_LINEAR_VOL = 27,
+ QUIRK_TYPE_MIXER_CAPTURE_LINEAR_VOL = 28,
/* Please also edit snd_usb_audio_quirk_flag_names */
};

@@ -290,5 +300,7 @@ enum {
#define QUIRK_FLAG_MIXER_PLAYBACK_MIN_MUTE QUIRK_FLAG(MIXER_PLAYBACK_MIN_MUTE)
#define QUIRK_FLAG_MIXER_CAPTURE_MIN_MUTE QUIRK_FLAG(MIXER_CAPTURE_MIN_MUTE)
#define QUIRK_FLAG_SKIP_IFACE_SETUP QUIRK_FLAG(SKIP_IFACE_SETUP)
+#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)

#endif /* __USBAUDIO_H */
--
2.51.0