[PATCH v3] ALSA: usb-audio: Skip broken playback feature unit on XiiSound chips

[Cristiano Souza <cristianocps@gmail.com>]

Weltrend XiiSound-based devices (040b:0897, e.g. Redragon H878
wireless headset) expose a UAC1 playback feature unit whose volume
and mute controls are unusable in practice: the reported per-channel
dB range is only 0..+0.39dB, yet setting any volume below the
maximum mutes one or even both channels, and the mapping between
logical and physical channels is unreliable.  Any attempt by
userspace to drive the hardware volume therefore results in muted
or one-sided audio.

Ignore the playback feature unit via a usbmix_name_map so that
userspace falls back to software volume, and pin the unit's master
and per-channel volumes to the reported maximum with mutes cleared
when the mixer is created, so the signal path always stays fully
open.  The capture path (microphone feature unit) is not affected
and keeps working.

Tested with a Redragon H878 wireless headset on PipeWire/PulseAudio:
stereo output works on both channels at any volume, software volume
applies smoothly, and mute/unmute works.

Cc: stable@vger.kernel.org
Signed-off-by: Cristiano Souza <cristianocps@gmail.com>
---
Changes in v3:
- Re-pin the playback feature unit on resume via
  snd_usb_mixer_resume_quirk(), as the device may lose its state
  over suspend (Sashiko AI review).
- Validate the UAC_GET_MAX read length instead of only checking for
  negative errors, to avoid pinning with a short read (Sashiko AI
  review).

Changes in v2:
- Skip the broken playback feature unit via a usbmix_name_map in
  mixer_maps.c instead of open-coding the skip in the FU parser, as
  suggested by Takashi Iwai.
 sound/usb/mixer_maps.c   | 19 +++++++++++++
 sound/usb/mixer_quirks.c | 59 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 78 insertions(+)

diff --git a/sound/usb/mixer_maps.c b/sound/usb/mixer_maps.c
index 98b4146..9218ca6 100644
--- a/sound/usb/mixer_maps.c
+++ b/sound/usb/mixer_maps.c
@@ -374,6 +374,20 @@ static const struct usbmix_name_map dell_alc4020_map[] = {
 	{ 0 }
 };
 
+/*
+ * Weltrend XiiSound chips (040b:0897, e.g. Redragon H878):
+ * the playback feature unit is broken; its volume controls act like
+ * mute switches below the maximum value and the channel mapping is
+ * unreliable, so any hardware volume change mutes one or both
+ * channels.  Skip the unit entirely and let userspace fall back to
+ * software volume; the unit is pinned to maximum/unmuted by the
+ * mixer quirk in mixer_quirks.c.
+ */
+static const struct usbmix_name_map xiisound_map[] = {
+	{ 5, NULL },	/* playback feature unit */
+	{ 0 }
+};
+
 /*
  * Corsair Virtuoso calls everything "Headset" without this, leading to
  * applications moving the sidetone control instead of the main one.
@@ -790,6 +804,11 @@ static const struct usbmix_ctl_map usbmix_ctl_maps[] = {
 		.id = USB_ID(0x1395, 0x0025),
 		.map = sennheiser_pc8_map,
 	},
+	{
+		/* Weltrend XiiSound (Redragon H878) */
+		.id = USB_ID(0x040b, 0x0897),
+		.map = xiisound_map,
+	},
 	{
 		/* Microsoft USB Link headset */
 		.id = USB_ID(0x045e, 0x083c),
diff --git a/sound/usb/mixer_quirks.c b/sound/usb/mixer_quirks.c
index 5839719..4c7d683 100644
--- a/sound/usb/mixer_quirks.c
+++ b/sound/usb/mixer_quirks.c
@@ -4603,6 +4603,59 @@ static int snd_audiofuse_controls_create(struct usb_mixer_interface *mixer)
 	return err;
 }
 
+/*
+ * Weltrend XiiSound chips (040b:0897, e.g. Redragon H878):
+ * the playback feature unit (id 5) is broken; its volume controls act like
+ * mute switches below the maximum value and the channel mapping is
+ * unreliable, so the unit is hidden from userspace entirely (see the
+ * quirk in mixer_maps.c) and userspace falls back to software volume.
+ * Pin the unit's master and channel volumes to the reported maximum
+ * and unmute them, so the audio path stays fully open.
+ */
+static int snd_xiisound_init_fu_pins(struct usb_mixer_interface *mixer)
+{
+	struct snd_usb_audio *chip = mixer->chip;
+	unsigned char buf[2];
+	int ifnum, ch, err;
+
+	if (mixer->protocol != UAC_VERSION_1)
+		return 0;
+
+	ifnum = snd_usb_ctrl_intf(chip->ctrl_intf);
+
+	/* unit 5 is the playback feature unit on these chips */
+	for (ch = 0; ch <= 2; ch++) {
+		err = snd_usb_ctl_msg(chip->dev, usb_rcvctrlpipe(chip->dev, 0),
+				      UAC_GET_MAX,
+				      USB_DIR_IN | USB_TYPE_CLASS |
+				      USB_RECIP_INTERFACE,
+				      (UAC_FU_VOLUME << 8) | ch,
+				      (5 << 8) | ifnum, buf, 2);
+		if (err < 2)
+			return 0; /* no such unit/channels, nothing to pin */
+
+		err = snd_usb_ctl_msg(chip->dev, usb_sndctrlpipe(chip->dev, 0),
+				      UAC_SET_CUR,
+				      USB_DIR_OUT | USB_TYPE_CLASS |
+				      USB_RECIP_INTERFACE,
+				      (UAC_FU_VOLUME << 8) | ch,
+				      (5 << 8) | ifnum, buf, 2);
+		if (err < 0)
+			return err;
+
+		buf[0] = 0; /* unmute */
+		err = snd_usb_ctl_msg(chip->dev, usb_sndctrlpipe(chip->dev, 0),
+				      UAC_SET_CUR,
+				      USB_DIR_OUT | USB_TYPE_CLASS |
+				      USB_RECIP_INTERFACE,
+				      (UAC_FU_MUTE << 8) | ch,
+				      (5 << 8) | ifnum, buf, 1);
+		if (err < 0)
+			return err;
+	}
+	return 0;
+}
+
 int snd_usb_mixer_apply_create_quirk(struct usb_mixer_interface *mixer)
 {
 	int err = 0;
@@ -4616,6 +4669,9 @@ int snd_usb_mixer_apply_create_quirk(struct usb_mixer_interface *mixer)
 	case USB_ID(0x0644, 0x8047):
 		err = snd_us16x08_controls_create(mixer);
 		break;
+	case USB_ID(0x040b, 0x0897):
+		err = snd_xiisound_init_fu_pins(mixer);
+		break;
 	case USB_ID(0x041e, 0x3020):
 	case USB_ID(0x041e, 0x3040):
 	case USB_ID(0x041e, 0x3042):
@@ -4807,6 +4863,9 @@ void snd_usb_mixer_resume_quirk(struct usb_mixer_interface *mixer)
 	case USB_ID(0x0bda, 0x4014): /* Dell WD15 dock */
 		dell_dock_mixer_init(mixer);
 		break;
+	case USB_ID(0x040b, 0x0897): /* Weltrend XiiSound (Redragon H878) */
+		snd_xiisound_init_fu_pins(mixer);
+		break;
 	}
 }
 
-- 
2.43.0