ALSA: usb-audio: Add QUIRK_FLAG_MIXER_{PLAYBACK,CAPTURE}_LINEAR_VOL

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>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20260303194805.266158-7-i@rong.moe

Author: Rongronggg9

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.
 

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))

sound/usb/quirks.c

@@ -2543,6 +2543,8 @@ 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
 };
 

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 */