[RFC 1/2] ALSA: usb-audio: expose the Topping M62's analogue gains as mixer controls

[Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>]

The M62 (152a:875c) keeps its analogue input gains and its output
volumes behind a vendor protocol on a HID-class interface, and exposes
none of them through UAC. What UAC does offer on the capture side is a
digital trim after the converter, which cannot buy signal-to-noise:
raising it lifts the converter's own floor along with the signal. A
noise-floor ladder against the card shows exactly that, so on Linux
today the one knob worth setting is the one that cannot be reached, and
a measurement has to begin by asking a human to touch the front panel.

The protocol was read off the vendor application's traffic, the way
mixer_scarlett2.c describes reading Focusrite's. Frames are fifteen
bytes -- start magic, a constant, a target, a property, a signed 32-bit
big-endian value, CRC-16/MODBUS over the middle stored big-endian, end
magic -- and rebuilding all 2619 captured frames from that description
reproduces them byte for byte. The device says nothing until it is
subscribed; one write starts the stream, a second makes it announce its
whole state, after which every change arrives unsolicited, including a
front panel press. So the controls are populated by asking rather than
by caching what was written, which matters here because the vendor
application on another host pushes its own cached state onto the card
on connect.

The control pipe cannot carry this: GET_REPORT and SET_REPORT stall
with EPIPE for every report type, so the interrupt endpoints on the HID
interface are the only route and this driver has to own that interface.
hid_ignore_list keeps usbhid away. Nothing is lost by that: the report
descriptor the device offers is a fig leaf -- a Generic Desktop
application collection, eight unnamed usages, sixteen bytes in and out,
no report ID -- so hid-generic can only make a nonexistent mouse of it.

The controls are a table: a name, the target and property that carry
the knob, the second target that must be written in step with it, the
range and the scale. Adding a knob is adding a row. Six rows here --
the two microphone preamps in whole decibels, AUX and Bluetooth on the
input side, headphone and OTG on the output side -- and the outputs
come in pairs because the device answers on only one of each pair and
the other would drift away unheard.

The two volume tapers are measured, not guessed: index 0 is mute, index
99 the maximum, the step is 0.5 dB above -10 dB and 1 dB below it, and
the family that must cover 97 dB in 98 steps takes 2 dB below -52 dB as
well. Both express as DB_RANGE. The microphone preamps are ordinary
1 dB steps from 0 to 88.

One thing a mixer quirk cannot do for itself: usb_audio_driver is
private to card.c, so claiming an interface the audio class knows
nothing about needs a helper there. snd_usb_claim_iface() is that
helper, and it is the only change outside the new file and its
dispatch.

Not included: OTG IN, which has no front panel control and therefore
never announced itself in any capture, so its property is unknown. It
is one row when it is known.

Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
---
 MAINTAINERS               |   6 +
 drivers/hid/hid-ids.h     |   3 +
 drivers/hid/hid-quirks.c  |   2 +
 sound/usb/Makefile        |   1 +
 sound/usb/card.c          |  14 ++
 sound/usb/mixer_quirks.c  |   5 +
 sound/usb/mixer_topping.c | 495 ++++++++++++++++++++++++++++++++++++++
 sound/usb/mixer_topping.h |   7 +
 sound/usb/usbaudio.h      |   3 +
 9 files changed, 536 insertions(+)
 create mode 100644 sound/usb/mixer_topping.c
 create mode 100644 sound/usb/mixer_topping.h

diff --git a/MAINTAINERS b/MAINTAINERS
index d2b7ed2..41ec6b7 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -27349,6 +27349,12 @@ S:	Maintained
 W:	https://tomoyo.sourceforge.net/
 F:	security/tomoyo/
 
+TOPPING M62 MIXER DRIVER
+M:	Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
+L:	linux-sound@vger.kernel.org
+S:	Maintained
+F:	sound/usb/mixer_topping.*
+
 TOPSTAR LAPTOP EXTRAS DRIVER
 M:	Herton Ronaldo Krzesinski <herton@canonical.com>
 L:	platform-driver-x86@vger.kernel.org
diff --git a/drivers/hid/hid-ids.h b/drivers/hid/hid-ids.h
index 341bf58..092b2a9 100644
--- a/drivers/hid/hid-ids.h
+++ b/drivers/hid/hid-ids.h
@@ -1470,6 +1470,9 @@
 #define USB_DEVICE_ID_TIVO_SLIDE	0x1201
 #define USB_DEVICE_ID_TIVO_SLIDE_PRO	0x1203
 
+#define USB_VENDOR_ID_TOPPING		0x152a
+#define USB_DEVICE_ID_TOPPING_M62	0x875c
+
 #define USB_VENDOR_ID_TOPRE			0x0853
 #define USB_DEVICE_ID_TOPRE_REALFORCE_R2_108			0x0148
 #define USB_DEVICE_ID_TOPRE_REALFORCE_R2_87			0x0146
diff --git a/drivers/hid/hid-quirks.c b/drivers/hid/hid-quirks.c
index 8a0b51d..3c156d1 100644
--- a/drivers/hid/hid-quirks.c
+++ b/drivers/hid/hid-quirks.c
@@ -981,6 +981,8 @@ static const struct hid_device_id hid_ignore_list[] = {
 	{ HID_USB_DEVICE(USB_VENDOR_ID_SYNAPTICS, USB_DEVICE_ID_SYNAPTICS_WTP) },
 	{ HID_USB_DEVICE(USB_VENDOR_ID_SYNAPTICS, USB_DEVICE_ID_SYNAPTICS_DPAD) },
 #endif
+	/* the M62's vendor control channel, driven by snd-usb-audio */
+	{ HID_USB_DEVICE(USB_VENDOR_ID_TOPPING, USB_DEVICE_ID_TOPPING_M62) },
 	{ HID_USB_DEVICE(USB_VENDOR_ID_YEALINK, USB_DEVICE_ID_YEALINK_P1K_P4K_B2K) },
 	{ HID_USB_DEVICE(USB_VENDOR_ID_QUANTA, USB_DEVICE_ID_QUANTA_HP_5MP_CAMERA_5473) },
 	{ }
diff --git a/sound/usb/Makefile b/sound/usb/Makefile
index e62794a..151b481 100644
--- a/sound/usb/Makefile
+++ b/sound/usb/Makefile
@@ -14,6 +14,7 @@ snd-usb-audio-y := 	card.o \
 			mixer_quirks.o \
 			mixer_scarlett.o \
 			mixer_scarlett2.o \
+			mixer_topping.o \
 			mixer_us16x08.o \
 			mixer_s1810c.o \
 			pcm.o \
diff --git a/sound/usb/card.c b/sound/usb/card.c
index 24112e4..e99b934 100644
--- a/sound/usb/card.c
+++ b/sound/usb/card.c
@@ -325,6 +325,20 @@ static int snd_usb_create_stream(struct snd_usb_audio *chip, int ctrlif, int int
 	return 0;
 }
 
+/*
+ * Claim an interface of this device for snd-usb-audio.
+ *
+ * A mixer quirk may need an interface the audio class knows nothing
+ * about -- a vendor control channel that happens to wear the HID class,
+ * for instance -- and cannot claim it itself, because usb_audio_driver
+ * is private to this file.
+ */
+int snd_usb_claim_iface(struct snd_usb_audio *chip, struct usb_interface *iface)
+{
+	return usb_driver_claim_interface(&usb_audio_driver, iface,
+					  USB_AUDIO_IFACE_UNUSED);
+}
+
 /*
  * parse audio control descriptor and create pcm/midi streams
  */
diff --git a/sound/usb/mixer_quirks.c b/sound/usb/mixer_quirks.c
index a1f5592..10f3302 100644
--- a/sound/usb/mixer_quirks.c
+++ b/sound/usb/mixer_quirks.c
@@ -36,6 +36,7 @@
 #include "mixer_quirks.h"
 #include "mixer_scarlett.h"
 #include "mixer_scarlett2.h"
+#include "mixer_topping.h"
 #include "mixer_us16x08.h"
 #include "mixer_s1810c.h"
 #include "helper.h"
@@ -4531,6 +4532,10 @@ int snd_usb_mixer_apply_create_quirk(struct usb_mixer_interface *mixer)
 		err = snd_fcp_init(mixer);
 		break;
 
+	case USB_ID(0x152a, 0x875c): /* Topping M62 */
+		err = snd_topping_init(mixer);
+		break;
+
 	case USB_ID(0x041e, 0x323b): /* Creative Sound Blaster E1 */
 		err = snd_soundblaster_e1_switch_create(mixer);
 		break;
diff --git a/sound/usb/mixer_topping.c b/sound/usb/mixer_topping.c
new file mode 100644
index 0000000..0c89a51
--- /dev/null
+++ b/sound/usb/mixer_topping.c
@@ -0,0 +1,495 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Mixer controls for Topping interfaces behind a vendor HID channel
+ *
+ * Copyright (c) 2026 Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
+ *
+ * The M62 (152a:875c) puts its analogue input gains and its output
+ * volumes behind a vendor protocol on a HID-class interface, and
+ * exposes nothing of them through UAC.  What UAC does expose on the
+ * capture side is a digital trim AFTER the converter, which cannot buy
+ * signal-to-noise: raising it lifts the converter's own floor with the
+ * signal.  So the only knob worth automating is unreachable, and a
+ * measurement application on Linux has to ask a human to set it by
+ * hand on the front panel.
+ *
+ * The protocol was read off the vendor application's traffic.  Frames
+ * are fifteen bytes:
+ *
+ *	22 33 | 20 01 01 | TT | PP | s32 value BE | CRC16 BE | 66 77
+ *
+ * with TT a target (an input, an output, or the device itself), PP a
+ * property of that target, and the checksum CRC-16/MODBUS over bytes
+ * 2..10 stored most significant byte first.  Reports arriving from the
+ * device are the same frame plus one trailing pad byte; an idle poll
+ * returns sixteen zeroes.  The vendor application sends 00 00 in place
+ * of the checksum and the device accepts it, so the device evidently
+ * does not verify what it receives -- this driver signs its writes
+ * anyway, and validates what it reads.
+ *
+ * The device says nothing until it is subscribed: one write of
+ * 0x11/0x24 starts the notification stream, after which every change,
+ * including a front panel button, arrives unsolicited.  A second
+ * write, 0x11/0x26, makes the device announce its whole state, which
+ * is how the controls are populated without caching what we wrote.
+ *
+ * Note that the control pipe is not an option here: GET_REPORT and
+ * SET_REPORT both stall with EPIPE for every report type, so the
+ * interrupt endpoints on the HID interface are the only route and this
+ * driver has to own that interface.  hid_ignore_list keeps usbhid off
+ * it; the report descriptor it would bind to describes nothing anyway
+ * (a Generic Desktop application collection with eight unnamed usages
+ * and no report ID), so no HID functionality is lost.
+ */
+
+#include <linux/crc16.h>
+#include <linux/unaligned.h>
+#include <linux/init.h>
+#include <linux/slab.h>
+#include <linux/usb.h>
+
+#include <sound/control.h>
+#include <sound/core.h>
+#include <sound/tlv.h>
+
+#include "usbaudio.h"
+#include "mixer.h"
+#include "mixer_topping.h"
+
+#define TOPPING_FRAME_LEN	15	/* what we send */
+#define TOPPING_REPORT_LEN	16	/* what arrives, one pad byte more */
+#define TOPPING_EP_BUF		64	/* the endpoints' packet size */
+
+/* device-scope properties */
+#define TOPPING_TT_DEVICE	0x11
+#define TOPPING_PP_SUBSCRIBE	0x24
+#define TOPPING_PP_ANNOUNCE	0x26
+
+/*
+ * The two volume tapers, measured against the vendor application's own
+ * readout: index 0 is always mute, index 99 always the maximum, the
+ * step is 0.5 dB above -10 dB and 1 dB below it, and the family that
+ * has to cover 97 dB in 98 steps takes 2 dB below -52 dB as well.
+ */
+static const DECLARE_TLV_DB_SCALE(topping_tlv_gain, 0, 100, 0);
+
+static const unsigned int topping_tlv_out_9[] = {
+	TLV_DB_RANGE_HEAD(4),
+	0, 0, SNDRV_CTL_TLVD_DB_SCALE_ITEM(SNDRV_CTL_TLVD_DB_GAIN_MUTE, 0, 1),
+	1, 19, SNDRV_CTL_TLVD_DB_SCALE_ITEM(-8800, 200, 0),
+	20, 61, SNDRV_CTL_TLVD_DB_SCALE_ITEM(-5100, 100, 0),
+	62, 99, SNDRV_CTL_TLVD_DB_SCALE_ITEM(-950, 50, 0),
+};
+
+static const unsigned int topping_tlv_out_0[] = {
+	TLV_DB_RANGE_HEAD(3),
+	0, 0, SNDRV_CTL_TLVD_DB_SCALE_ITEM(SNDRV_CTL_TLVD_DB_GAIN_MUTE, 0, 1),
+	1, 79, SNDRV_CTL_TLVD_DB_SCALE_ITEM(-8800, 100, 0),
+	80, 99, SNDRV_CTL_TLVD_DB_SCALE_ITEM(-950, 50, 0),
+};
+
+/*
+ * One row per knob.  A row is the whole description of a control: what
+ * to call it, which target and property carry it, the second target
+ * that has to be written in step with the first, the range, and the
+ * scale.  Adding a knob is adding a row.
+ *
+ * The outputs come in pairs and the device announces only the second
+ * of each pair, so both are written and the second is the one listened
+ * for.  OTG IN is missing on purpose: it has no front panel control,
+ * so it never announced itself in any capture, and its property is not
+ * known.  It is one row when it is.
+ */
+struct topping_ctl_desc {
+	const char *name;
+	u8 target;		/* the target that reports */
+	u8 target_pair;		/* written too, or 0 */
+	u8 prop;
+	int min, max;
+	const unsigned int *tlv;
+};
+
+static const struct topping_ctl_desc topping_m62_ctls[] = {
+	{ "Mic-1 Analog Capture Volume", 0x21, 0, 0x04, 0, 88,
+	  topping_tlv_gain },
+	{ "Mic-2 Analog Capture Volume", 0x22, 0, 0x04, 0, 88,
+	  topping_tlv_gain },
+	{ "Aux Capture Volume", 0x23, 0, 0x04, 0, 99,
+	  topping_tlv_out_9 },
+	{ "Bluetooth Capture Volume", 0x25, 0, 0x04, 0, 99,
+	  topping_tlv_out_0 },
+	{ "Headphone Playback Volume", 0x64, 0x63, 0x03, 0, 99,
+	  topping_tlv_out_9 },
+	{ "OTG Playback Volume", 0x62, 0x61, 0x03, 0, 99,
+	  topping_tlv_out_0 },
+};
+
+struct topping_mixer {
+	struct usb_mixer_interface *mixer;
+	struct usb_interface *iface;
+	const struct topping_ctl_desc *ctls;
+	int num_ctls;
+	struct urb *urb;
+	u8 *inbuf;
+	dma_addr_t inbuf_dma;
+	unsigned int pipe_in, pipe_out;
+	int interval;
+	spinlock_t lock;	/* guards val[] against the URB */
+	int *val;
+	struct snd_kcontrol **kctl;
+};
+
+static void topping_build(u8 *f, u8 target, u8 prop, s32 value)
+{
+	u16 crc;
+
+	f[0] = 0x22;
+	f[1] = 0x33;
+	f[2] = 0x20;
+	f[3] = 0x01;
+	f[4] = 0x01;
+	f[5] = target;
+	f[6] = prop;
+	put_unaligned_be32(value, f + 7);
+	crc = crc16(0xffff, f + 2, 9);
+	put_unaligned_be16(crc, f + 11);
+	f[13] = 0x66;
+	f[14] = 0x77;
+}
+
+static int topping_send(struct topping_mixer *tm, u8 target, u8 prop,
+			s32 value)
+{
+	u8 *buf;
+	int err, actual;
+
+	buf = kzalloc(TOPPING_EP_BUF, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+	topping_build(buf, target, prop, value);
+	err = usb_interrupt_msg(tm->mixer->chip->dev, tm->pipe_out,
+				buf, TOPPING_FRAME_LEN, &actual, 1000);
+	kfree(buf);
+	if (err < 0)
+		usb_audio_err(tm->mixer->chip,
+			      "Topping: write %02x/%02x failed: %d\n",
+			      target, prop, err);
+	return err;
+}
+
+/* -1 when this frame is not one of ours */
+static int topping_index_of(struct topping_mixer *tm, u8 target, u8 prop)
+{
+	int i;
+
+	for (i = 0; i < tm->num_ctls; i++)
+		if (tm->ctls[i].target == target && tm->ctls[i].prop == prop)
+			return i;
+	return -1;
+}
+
+static void topping_urb_complete(struct urb *urb)
+{
+	struct topping_mixer *tm = urb->context;
+	const u8 *f = urb->transfer_buffer;
+	unsigned long flags;
+	int idx, value, err;
+
+	if (urb->status)
+		return;		/* resubmitted below only when running */
+	if (urb->actual_length < TOPPING_FRAME_LEN)
+		goto resubmit;
+	if (f[0] != 0x22 || f[1] != 0x33 || f[13] != 0x66 || f[14] != 0x77)
+		goto resubmit;
+	if (get_unaligned_be16(f + 11) != crc16(0xffff, f + 2, 9))
+		goto resubmit;
+
+	idx = topping_index_of(tm, f[5], f[6]);
+	if (idx < 0)
+		goto resubmit;		/* a meter, or something unnamed */
+
+	value = get_unaligned_be32(f + 7);
+	if (value < tm->ctls[idx].min || value > tm->ctls[idx].max)
+		goto resubmit;
+
+	spin_lock_irqsave(&tm->lock, flags);
+	if (tm->val[idx] == value) {
+		spin_unlock_irqrestore(&tm->lock, flags);
+		goto resubmit;
+	}
+	tm->val[idx] = value;
+	spin_unlock_irqrestore(&tm->lock, flags);
+
+	if (tm->kctl[idx])
+		snd_ctl_notify(tm->mixer->chip->card,
+			       SNDRV_CTL_EVENT_MASK_VALUE,
+			       &tm->kctl[idx]->id);
+
+resubmit:
+	err = usb_submit_urb(urb, GFP_ATOMIC);
+	if (err < 0 && err != -ENODEV && err != -ESHUTDOWN)
+		usb_audio_err(tm->mixer->chip,
+			      "Topping: cannot resubmit: %d\n", err);
+}
+
+static int topping_ctl_info(struct snd_kcontrol *kctl,
+			    struct snd_ctl_elem_info *uinfo)
+{
+	struct usb_mixer_elem_info *elem = kctl->private_data;
+	struct topping_mixer *tm = elem->head.mixer->private_data;
+	int idx = elem->control;
+
+	uinfo->type = SNDRV_CTL_ELEM_TYPE_INTEGER;
+	uinfo->count = 1;
+	uinfo->value.integer.min = tm->ctls[idx].min;
+	uinfo->value.integer.max = tm->ctls[idx].max;
+	uinfo->value.integer.step = 1;
+	return 0;
+}
+
+static int topping_ctl_get(struct snd_kcontrol *kctl,
+			   struct snd_ctl_elem_value *ucontrol)
+{
+	struct usb_mixer_elem_info *elem = kctl->private_data;
+	struct topping_mixer *tm = elem->head.mixer->private_data;
+	unsigned long flags;
+
+	spin_lock_irqsave(&tm->lock, flags);
+	ucontrol->value.integer.value[0] = tm->val[elem->control];
+	spin_unlock_irqrestore(&tm->lock, flags);
+	return 0;
+}
+
+static int topping_ctl_put(struct snd_kcontrol *kctl,
+			   struct snd_ctl_elem_value *ucontrol)
+{
+	struct usb_mixer_elem_info *elem = kctl->private_data;
+	struct usb_mixer_interface *mixer = elem->head.mixer;
+	struct topping_mixer *tm = mixer->private_data;
+	const struct topping_ctl_desc *d = &tm->ctls[elem->control];
+	unsigned long flags;
+	int value, err;
+
+	value = ucontrol->value.integer.value[0];
+	if (value < d->min || value > d->max)
+		return -EINVAL;
+
+	spin_lock_irqsave(&tm->lock, flags);
+	if (tm->val[elem->control] == value) {
+		spin_unlock_irqrestore(&tm->lock, flags);
+		return 0;
+	}
+	spin_unlock_irqrestore(&tm->lock, flags);
+
+	err = topping_send(tm, d->target, d->prop, value);
+	if (err < 0)
+		return err;
+	if (d->target_pair) {
+		/*
+		 * The device announces only one of a pair, so the other
+		 * would drift away unheard.
+		 */
+		err = topping_send(tm, d->target_pair, d->prop, value);
+		if (err < 0)
+			return err;
+	}
+
+	spin_lock_irqsave(&tm->lock, flags);
+	tm->val[elem->control] = value;
+	spin_unlock_irqrestore(&tm->lock, flags);
+	return 1;
+}
+
+static const struct snd_kcontrol_new topping_ctl = {
+	.iface = SNDRV_CTL_ELEM_IFACE_MIXER,
+	.access = SNDRV_CTL_ELEM_ACCESS_READWRITE |
+		  SNDRV_CTL_ELEM_ACCESS_TLV_READ,
+	.info = topping_ctl_info,
+	.get = topping_ctl_get,
+	.put = topping_ctl_put,
+};
+
+static int topping_add_ctl(struct topping_mixer *tm, int idx)
+{
+	struct usb_mixer_elem_info *elem;
+	struct snd_kcontrol *kctl;
+	int err;
+
+	elem = kzalloc_obj(*elem);
+	if (!elem)
+		return -ENOMEM;
+
+	elem->head.mixer = tm->mixer;
+	elem->head.id = 0;
+	elem->control = idx;
+	elem->channels = 1;
+	elem->val_type = USB_MIXER_BESPOKEN;
+
+	kctl = snd_ctl_new1(&topping_ctl, elem);
+	if (!kctl) {
+		kfree(elem);
+		return -ENOMEM;
+	}
+	kctl->private_free = snd_usb_mixer_elem_free;
+	kctl->tlv.p = tm->ctls[idx].tlv;
+	strscpy(kctl->id.name, tm->ctls[idx].name, sizeof(kctl->id.name));
+
+	err = snd_usb_mixer_add_control(&elem->head, kctl);
+	if (err < 0)
+		return err;
+
+	tm->kctl[idx] = kctl;
+	return 0;
+}
+
+static void topping_private_free(struct usb_mixer_interface *mixer)
+{
+	struct topping_mixer *tm = mixer->private_data;
+
+	if (!tm)
+		return;
+	if (tm->urb) {
+		usb_kill_urb(tm->urb);
+		usb_free_coherent(mixer->chip->dev, TOPPING_EP_BUF,
+				  tm->inbuf, tm->inbuf_dma);
+		usb_free_urb(tm->urb);
+	}
+	kfree(tm->val);
+	kfree(tm->kctl);
+	kfree(tm);
+	mixer->private_data = NULL;
+}
+
+/* the HID interface, by class rather than by a number in a comment */
+static struct usb_interface *topping_find_iface(struct snd_usb_audio *chip,
+						int *ep_in, int *ep_out,
+						int *interval)
+{
+	struct usb_device *dev = chip->dev;
+	struct usb_host_interface *alts;
+	struct usb_interface *iface;
+	int i, e;
+
+	for (i = 0; i < 256; i++) {
+		iface = usb_ifnum_to_if(dev, i);
+		if (!iface)
+			continue;
+		alts = &iface->altsetting[0];
+		if (alts->desc.bInterfaceClass != USB_CLASS_HID)
+			continue;
+		*ep_in = *ep_out = 0;
+		for (e = 0; e < alts->desc.bNumEndpoints; e++) {
+			struct usb_endpoint_descriptor *ep;
+
+			ep = &alts->endpoint[e].desc;
+			if (!usb_endpoint_xfer_int(ep))
+				continue;
+			if (usb_endpoint_dir_in(ep)) {
+				*ep_in = usb_endpoint_num(ep);
+				*interval = ep->bInterval;
+			} else {
+				*ep_out = usb_endpoint_num(ep);
+			}
+		}
+		if (*ep_in && *ep_out)
+			return iface;
+	}
+	return NULL;
+}
+
+int snd_topping_init(struct usb_mixer_interface *mixer)
+{
+	struct snd_usb_audio *chip = mixer->chip;
+	struct usb_interface *iface;
+	struct topping_mixer *tm;
+	int ep_in = 0, ep_out = 0, interval = 5;
+	int i, err;
+
+	iface = topping_find_iface(chip, &ep_in, &ep_out, &interval);
+	if (!iface) {
+		usb_audio_err(chip, "Topping: no vendor HID interface\n");
+		return 0;	/* not fatal: the card still plays */
+	}
+	if (usb_interface_claimed(iface)) {
+		usb_audio_err(chip,
+			      "Topping: the HID interface is already claimed\n");
+		return 0;
+	}
+
+	tm = kzalloc_obj(*tm);
+	if (!tm)
+		return -ENOMEM;
+
+	tm->mixer = mixer;
+	tm->iface = iface;
+	tm->ctls = topping_m62_ctls;
+	tm->num_ctls = ARRAY_SIZE(topping_m62_ctls);
+	tm->pipe_in = usb_rcvintpipe(chip->dev, ep_in);
+	tm->pipe_out = usb_sndintpipe(chip->dev, ep_out);
+	tm->interval = interval;
+	spin_lock_init(&tm->lock);
+
+	tm->val = kcalloc(tm->num_ctls, sizeof(*tm->val), GFP_KERNEL);
+	tm->kctl = kcalloc(tm->num_ctls, sizeof(*tm->kctl), GFP_KERNEL);
+	if (!tm->val || !tm->kctl) {
+		err = -ENOMEM;
+		goto fail;
+	}
+
+	err = snd_usb_claim_iface(chip, iface);
+	if (err < 0)
+		goto fail;
+
+	tm->urb = usb_alloc_urb(0, GFP_KERNEL);
+	if (!tm->urb) {
+		err = -ENOMEM;
+		goto fail;
+	}
+	tm->inbuf = usb_alloc_coherent(chip->dev, TOPPING_EP_BUF, GFP_KERNEL,
+				       &tm->inbuf_dma);
+	if (!tm->inbuf) {
+		err = -ENOMEM;
+		goto fail;
+	}
+	usb_fill_int_urb(tm->urb, chip->dev, tm->pipe_in,
+			 tm->inbuf, TOPPING_EP_BUF,
+			 topping_urb_complete, tm, tm->interval);
+	tm->urb->transfer_dma = tm->inbuf_dma;
+	tm->urb->transfer_flags |= URB_NO_TRANSFER_DMA_MAP;
+
+	mixer->private_data = tm;
+	mixer->private_free = topping_private_free;
+
+	for (i = 0; i < tm->num_ctls; i++) {
+		err = topping_add_ctl(tm, i);
+		if (err < 0)
+			return err;	/* private_free cleans up */
+	}
+
+	err = usb_submit_urb(tm->urb, GFP_KERNEL);
+	if (err < 0) {
+		usb_audio_err(chip, "Topping: cannot listen: %d\n", err);
+		return err;
+	}
+
+	/*
+	 * Subscribe, then ask for the state.  The device answers in two
+	 * waves -- identification at once, the gains about 3.7 s later,
+	 * which is the same delay a phantom rail takes to settle -- so
+	 * nothing here waits for them: each value lands through the URB
+	 * and notifies its own control.
+	 */
+	topping_send(tm, TOPPING_TT_DEVICE, TOPPING_PP_SUBSCRIBE, 1);
+	topping_send(tm, TOPPING_TT_DEVICE, TOPPING_PP_ANNOUNCE, 1);
+	return 0;
+
+fail:
+	if (tm->inbuf)
+		usb_free_coherent(chip->dev, TOPPING_EP_BUF, tm->inbuf,
+				  tm->inbuf_dma);
+	usb_free_urb(tm->urb);
+	kfree(tm->val);
+	kfree(tm->kctl);
+	kfree(tm);
+	return err;
+}
diff --git a/sound/usb/mixer_topping.h b/sound/usb/mixer_topping.h
new file mode 100644
index 0000000..15e16b5
--- /dev/null
+++ b/sound/usb/mixer_topping.h
@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+#ifndef __USB_MIXER_TOPPING_H
+#define __USB_MIXER_TOPPING_H
+
+int snd_topping_init(struct usb_mixer_interface *mixer);
+
+#endif /* __USB_MIXER_TOPPING_H */
diff --git a/sound/usb/usbaudio.h b/sound/usb/usbaudio.h
index c49709d..4f9770a 100644
--- a/sound/usb/usbaudio.h
+++ b/sound/usb/usbaudio.h
@@ -80,6 +80,9 @@ struct snd_usb_audio {
 
 #define USB_AUDIO_IFACE_UNUSED	((void *)-1L)
 
+int snd_usb_claim_iface(struct snd_usb_audio *chip,
+			struct usb_interface *iface);
+
 #define usb_audio_err(chip, fmt, args...) \
 	dev_err(&(chip)->dev->dev, fmt, ##args)
 #define usb_audio_err_ratelimited(chip, fmt, args...) \
-- 
2.43.0