[RFC PATCH v2 3/4] ALSA: usb: babyface-pro: add the front-panel poll + controls

["Ismaïl Bahloul" <i.bahloul01@gmail.com>]

Emulate the Babyface Pro FS front panel (the host is in the loop like
TotalMix): the 50 Hz 0x17 readback poll drives read-only button/wheel/
IN/OUT/MIX/DIM controls, and the wheel is translated into master fader,
gain, MIX-monitoring and balance edits on the corresponding mixer
controls.

The DSP EQ remains stubbed here; it lands in the final patch.

Signed-off-by: Ismaïl Bahloul <i.bahloul01@gmail.com>
Assisted-by: DeepSeek V4 Flash
---
 sound/usb/babyfacepro/babyfacepro-ctl.c | 926 +++++++++++++++++++++++-
 1 file changed, 920 insertions(+), 6 deletions(-)

diff --git a/sound/usb/babyfacepro/babyfacepro-ctl.c b/sound/usb/babyfacepro/babyfacepro-ctl.c
index 337aab92f..4904fe486 100644
--- a/sound/usb/babyfacepro/babyfacepro-ctl.c
+++ b/sound/usb/babyfacepro/babyfacepro-ctl.c
@@ -1344,27 +1344,941 @@ int babyface_create_controls(struct snd_usb_babyface *chip)
 }
 
 
-/* ── front-panel stubs ────────────────────────────────────────
- * Replaced by the panel patch.  The core driver calls these from
- * probe()/disconnect()/suspend()/resume() and the delayed-work init.
+/* Control indices in chip->panel_kctl[] (for snd_ctl_notify). */
+enum {
+	BF_PANEL_KCTL_BUTTON,
+	BF_PANEL_KCTL_WHEEL,
+	BF_PANEL_KCTL_IN,
+	BF_PANEL_KCTL_OUT,
+	BF_PANEL_KCTL_MIX,
+	BF_PANEL_KCTL_DIM,
+	BF_PANEL_KCTL_SELECT,
+	BF_PANEL_KCTL_NUM,
+};
+
+static const char *const bf_panel_in_texts[] = {
+	"Unknown", "Ch 1/2", "Ch 3/4", "Opt", NULL
+};
+
+static const char *const bf_panel_out_texts[] = {
+	"Unknown", "Ch 1/2", "Phones", "Opt", NULL
+};
+
+static const char *const bf_panel_select_texts[] = {
+	"Left", "Right", "Both", "None", NULL
+};
+
+/* byte3 button flash → event code (0 = none).  The idle byte3 is 0x40;
+ * a press flashes the value below the base for one or two poll frames.
  */
-int babyface_create_panel(struct snd_usb_babyface *chip)
+static int bf_panel_button_decode(u8 flash)
 {
-	return 0;
+	switch (flash) {
+	case BF_PANEL_FLASH_IN:		return BF_PANEL_BTN_IN;
+	case BF_PANEL_FLASH_SET:	return BF_PANEL_BTN_SET;
+	case BF_PANEL_FLASH_MIX:	return BF_PANEL_BTN_MIX;
+	case BF_PANEL_FLASH_OUT:	return BF_PANEL_BTN_OUT;
+	case BF_PANEL_FLASH_SELECT:	return BF_PANEL_BTN_SELECT;
+	case BF_PANEL_FLASH_DIM:	return BF_PANEL_BTN_DIM;
+	default:			return BF_PANEL_BTN_NONE;
+	}
+}
+
+/* (byte2 >> 4) & 7 = IN position 4/5/6 → enum index (0 = not in range). */
+static int bf_panel_in_decode(u8 nib)
+{
+	switch (nib) {
+	case BF_PANEL_IN_CH12:		return 1;
+	case BF_PANEL_IN_CH34:		return 2;
+	case BF_PANEL_IN_OPT:		return 3;
+	default:			return 0;
+	}
+}
+
+/* byte1 & 7 = OUT position.  Two encodings seen in captures: the
+ * gain-display mode 0x04/0x05/0x06 (cap_dim.pcap, cap_buttons2.pcap)
+ * and the base mode 0x01/0x02/0x00 (cap_buttons.pcap; 0x01 is also the
+ * idle byte1 of cap_padpan.pcap and the live device).  Accept both;
+ * 0x00 is ambiguous (could be Opt or no selection) so keep previous.
+ */
+static int bf_panel_out_decode(u8 v)
+{
+	switch (v) {
+	case BF_PANEL_OUT_CH12:		return 1;
+	case BF_PANEL_OUT_PHONES:	return 2;
+	case BF_PANEL_OUT_OPT:		return 3;
+	case 0x01:			return 1;	/* base-mode Ch 1/2 */
+	case 0x02:			return 2;	/* base-mode Phones */
+	default:			return 0;
+	}
+}
+
+/* ── MIX-mode monitoring level (fader curve) ────────────────
+ * Calibrated crosspoint-fader curve (AN1→AN1/2, cap_calib.pcap
+ * 2026-08-22; the same table as tuxmix-core/src/usb.rs FADER_CURVE).
+ * dB stored ×2 (half-dB grid): the MIX wheel steps ±0.5 dB per click
+ * on this curve (cap_mix.pcap).  0x0000 = −inf (digital mute),
+ * 0x0003 = −62 dB, … 0x2D41 = +6 dB.  Raw values interpolate linearly
+ * between the 1-dB points.
+ */
+#define BF_FADER_DB2_INF	(-130)	/* −65 dB = the wheel's −inf floor */
+
+static const struct bf_fader_pt {
+	s16 db2;	/* dB × 2 */
+	u16 raw;
+} bf_fader_curve[] = {
+	{ -124, 0x0003 }, { -122, 0x0004 }, { -120, 0x0005 },
+	{ -118, 0x0006 }, { -116, 0x0007 }, { -114, 0x0008 },
+	{ -112, 0x0009 }, { -110, 0x000a }, { -108, 0x000b },
+	{ -106, 0x000d }, { -104, 0x000e }, { -102, 0x0010 },
+	{ -100, 0x0012 }, {  -98, 0x0014 }, {  -96, 0x0017 },
+	{  -94, 0x0019 }, {  -92, 0x001d }, {  -90, 0x0020 },
+	{  -88, 0x0024 }, {  -86, 0x0029 }, {  -84, 0x002e },
+	{  -82, 0x0033 }, {  -80, 0x003a }, {  -78, 0x0041 },
+	{  -76, 0x0049 }, {  -74, 0x0051 }, {  -72, 0x005b },
+	{  -70, 0x0067 }, {  -68, 0x0073 }, {  -66, 0x0081 },
+	{  -64, 0x0091 }, {  -62, 0x00a3 }, {  -60, 0x00b7 },
+	{  -58, 0x00cd }, {  -56, 0x00e6 }, {  -54, 0x0102 },
+	{  -52, 0x0122 }, {  -50, 0x0145 }, {  -48, 0x016d },
+	{  -46, 0x019a }, {  -44, 0x01cc }, {  -42, 0x0204 },
+	{  -40, 0x0243 }, {  -38, 0x028a }, {  -36, 0x02d9 },
+	{  -34, 0x0332 }, {  -32, 0x0396 }, {  -30, 0x0406 },
+	{  -28, 0x0483 }, {  -26, 0x0510 }, {  -24, 0x05af },
+	{  -22, 0x0660 }, {  -20, 0x0727 }, {  -18, 0x0807 },
+	{  -16, 0x0902 }, {  -14, 0x0a1b }, {  -12, 0x0b57 },
+	{  -10, 0x0cb9 }, {   -8, 0x0e47 }, {   -6, 0x1004 },
+	{   -4, 0x11f9 }, {   -2, 0x142a }, {    0, 0x16a0 },
+	{    2, 0x1963 }, {    4, 0x1c7c }, {    6, 0x1ff6 },
+	{    8, 0x23dc }, {   10, 0x283d }, {   12, 0x2d41 },
+};
+
+/* Fader raw → dB×2 (linear interpolation; raw 0 = −inf). */
+static int bf_fader_raw_to_db2(u16 raw)
+{
+	int i;
+
+	if (raw == 0 || raw < bf_fader_curve[0].raw)
+		return BF_FADER_DB2_INF;
+	for (i = 0; i < ARRAY_SIZE(bf_fader_curve) - 1; i++) {
+		if (raw <= bf_fader_curve[i + 1].raw) {
+			u32 num = (u32)(raw - bf_fader_curve[i].raw) *
+				  (u32)(bf_fader_curve[i + 1].db2 - bf_fader_curve[i].db2);
+			u32 den = bf_fader_curve[i + 1].raw - bf_fader_curve[i].raw;
+
+			return bf_fader_curve[i].db2 + (int)((num + den / 2) / den);
+		}
+	}
+	return bf_fader_curve[ARRAY_SIZE(bf_fader_curve) - 1].db2;
+}
+
+/* dB×2 → fader raw (linear interpolation; below −62 dB = mute 0). */
+static u16 bf_fader_db2_to_raw(int db2)
+{
+	int i;
+
+	if (db2 <= bf_fader_curve[0].db2)
+		return db2 < bf_fader_curve[0].db2 ? 0 : bf_fader_curve[0].raw;
+	for (i = 0; i < ARRAY_SIZE(bf_fader_curve) - 1; i++) {
+		if (db2 <= bf_fader_curve[i + 1].db2) {
+			u32 num = (u32)(db2 - bf_fader_curve[i].db2) *
+				  (u32)(bf_fader_curve[i + 1].raw - bf_fader_curve[i].raw);
+			u32 den = bf_fader_curve[i + 1].db2 - bf_fader_curve[i].db2;
+
+			return bf_fader_curve[i].raw + (u16)((num + den / 2) / den);
+		}
+	}
+	return bf_fader_curve[ARRAY_SIZE(bf_fader_curve) - 1].raw;
+}
+
+/* MIX-mode VU display law — monitoring dB×2 → the 0x1A 0x000A display
+ * value.  Piecewise-linear through the captured (dB, display) points
+ * (cap_mix.pcap 2026-08-23: (−62,0) (−54,1) (−48,2) (−42.5,3)
+ * (−35,4) (−28.4,5); cap_panel.pcap: (−7.4,10) (−6.7,11)
+ * (−4.6,12)) — a log-ish VU scale (coarse at the bottom, ~1.4 dB/step
+ * near 0).  The −28..−8 dB middle is interpolated; the exact law is
+ * pending the cap_mixdisp.pcap full-range sweep (TODO 0g).
+ */
+static int bf_mix_display(int db2)
+{
+	static const struct {
+		s16 db2;
+		u8 disp;
+	} pts[] = {
+		{ -124, 0 }, { -108, 1 }, {  -96, 2 }, {  -85, 3 },
+		{  -70, 4 }, {  -57, 5 }, {  -15, 10 }, {  -13, 11 },
+		{   -9, 12 },
+	};
+	int i;
+
+	if (db2 <= pts[0].db2)
+		return 0;
+	for (i = 0; i < ARRAY_SIZE(pts) - 1; i++) {
+		if (db2 <= pts[i + 1].db2) {
+			u32 num = (u32)(db2 - pts[i].db2) *
+				  (u32)(pts[i + 1].disp - pts[i].disp);
+			u32 den = pts[i + 1].db2 - pts[i].db2;
+
+			return pts[i].disp + (int)((num + den / 2) / den);
+		}
+	}
+	/* Above −4.6 dB: keep the last slope (2 dB/step) up to +6 dB. */
+	return pts[ARRAY_SIZE(pts) - 1].disp +
+	       clamp((db2 - pts[ARRAY_SIZE(pts) - 1].db2) / 4, 0, 12);
+}
+
+/* The kernel driver plays the TotalMix role for the MIX button (the
+ * standalone emulator is hardware-validated in tuxmix-core/src/panel.rs
+ * + usb.rs): one wheel click in fader mode = ±0.5 dB on the SELECT-
+ * chosen channel(s) of the IN-selected pair, into the OUT-selected
+ * output's crosspoint block — the STANDARD map only (cap_mix.pcap /
+ * cap_select2.pcap, no low-map mirror).  Mirrors the change into the
+ * xpoint cache so the ALSA controls follow the wheel.  Takes the mutex
+ * (the 0x12 writes cycle the transaction flag like the mixer puts).
+ */
+static void bf_panel_mix_wheel(struct snd_usb_babyface *chip, int delta)
+{
+	/* Canonical output of the OUT selection (enum 1 = Ch1/2,
+	 * 2 = Phones, 3 = Opt): AN1/2, PH3/4, ADAT7/8 (the optical
+	 * output) respectively.
+	 */
+	int out = chip->panel_out == 3 ? 5 :
+		  chip->panel_out == 2 ? 1 : 0;
+	unsigned int blk = bf_xpoint_block[out];
+	u8 targets[2];
+	int n = 0;
+	int db2;
+	u16 raw, flag;
+	int i;
+
+	/* SELECT-chosen channel(s) of the IN pair (manual §5.1: SELECT
+	 * steps left/right/both; none = nothing selected = no-op wheel).
+	 * Source indices: AN1/AN2 = 0/1, AN3/AN4 = 2/3, AS1/2 = 4.
+	 */
+	if (chip->panel_in == 3) {
+		targets[0] = 4;		/* Opt: the AS1/2 pair */
+		n = 1;
+	} else if (chip->panel_select != 3) {
+		int base = chip->panel_in == 2 ? 2 : 0;
+
+		targets[0] = base + (chip->panel_select == 1 ? 1 : 0);
+		n = 1;
+		if (chip->panel_select == 2)
+			targets[n++] = base + 1;
+	}
+
+	mutex_lock(&chip->mutex);
+	db2 = bf_fader_raw_to_db2(chip->panel_mix_raw);
+	db2 = clamp(db2 + delta, BF_FADER_DB2_INF, 12);
+	raw = bf_fader_db2_to_raw(db2);
+	chip->panel_mix_raw = raw;
+	for (i = 0; i < n; i++) {
+		const struct bf_source *s = &bf_sources[targets[i]];
+
+		flag = bf_flag_cycle[chip->flag_cnt];
+		chip->flag_cnt = (chip->flag_cnt + 1) & 3;
+		bf_vendor_write(chip, BF_REQ_CROSSPOINT, raw,
+				(BF_REG_CROSS_BASE_L + BF_REG_CROSS_STRIDE * blk +
+				 s->idx_l) | flag);
+		bf_vendor_write(chip, BF_REQ_CROSSPOINT, raw,
+				(BF_REG_CROSS_BASE_R + BF_REG_CROSS_STRIDE * blk +
+				 s->idx_r) | flag);
+		chip->xpoint[out][targets[i]][0] = raw;
+		chip->xpoint[out][targets[i]][1] = raw;
+		/* MIX-mode VU display shadow (0x1A 0x000A+mic): TotalMix
+		 * mirrors the monitoring level into the panel display family
+		 * (cap_mix/cap_panel.pcap) — the input VU segments follow it.
+		 * Written only on change (the captures show TotalMix updating
+		 * it on segment crossings).  Law = bf_mix_display (TODO 0g
+		 * pending the exact full-range capture).
+		 */
+		if (targets[i] < 4) {
+			int disp = bf_mix_display(db2);
+
+			if (disp != chip->panel_mix_disp[targets[i]]) {
+				bf_vendor_write(chip, BF_REQ_GAIN,
+						(u16)disp,
+						BF_REG_PANEL_GAIN + targets[i]);
+				chip->panel_mix_disp[targets[i]] = disp;
+			}
+		}
+	}
+	mutex_unlock(&chip->mutex);
+}
+
+/* Write an output's L/R masters (8-bit companions + 16-bit with the
+ * transaction flag) and mirror into the cache — shared by the OUT
+ * volume wheel and the balance wheel.  Caller holds the mutex.
+ */
+static void bf_panel_write_master(struct snd_usb_babyface *chip, int out,
+				  u16 l, u16 r)
+{
+	u16 flag;
+
+	flag = bf_flag_cycle[chip->flag_cnt];
+	chip->flag_cnt = (chip->flag_cnt + 1) & 3;
+	bf_vendor_write(chip, BF_REQ_GAIN, bf_master_8bit(l),
+			BF_REG_MASTER_8 + 2 * out);
+	bf_vendor_write(chip, BF_REQ_GAIN, bf_master_8bit(r),
+			BF_REG_MASTER_8 + 2 * out + 1);
+	bf_vendor_write(chip, BF_REQ_CROSSPOINT, l,
+			(BF_REG_MASTER_16 + 2 * out) | flag);
+	bf_vendor_write(chip, BF_REQ_CROSSPOINT, r,
+			(BF_REG_MASTER_16 + 2 * out + 1) | flag);
+	chip->master[out][0] = l;
+	chip->master[out][1] = r;
+	chip->muted[out] = false;
+	/* A Phones change while DIM is engaged re-bases the restore. */
+	if (chip->dim && out == 1) {
+		chip->dim_saved[0] = l;
+		chip->dim_saved[1] = r;
+	}
+}
+
+/* OUT-mode wheel: the master fader of the OUT-selected output, ±0.5 dB
+ * per click (cap_set2/cap_dim.pcap: the wheel writes the 16-bit master
+ * 0x03E0+2·out on the master curve 0x2000·2^(dB/6); the driver keeps
+ * the 8-bit companion in sync like bf_master_put — the 8-bit is the
+ * real volume).  BOTH sides move by the same dB so an existing
+ * balance (hold-SELECT) is preserved.  Same output mapping as the MIX
+ * wheel (Phones = canon 1, Opt = ADAT7/8 = canon 5, else AN1/2).
+ */
+static void bf_panel_out_wheel(struct snd_usb_babyface *chip, int delta)
+{
+	int out = chip->panel_out == 3 ? 5 :
+		  chip->panel_out == 2 ? 1 : 0;
+	int hl, hr;
+	u16 l, r;
+
+	mutex_lock(&chip->mutex);
+	hl = bf_master_half_db(chip->master[out][0]) + delta;
+	hr = bf_master_half_db(chip->master[out][1]) + delta;
+	l = bf_master_16bit(clamp(hl, -128, 12));
+	r = bf_master_16bit(clamp(hr, -128, 12));
+	bf_panel_write_master(chip, out, l, r);
+	mutex_unlock(&chip->mutex);
+}
+
+/* IN-mode wheel: the gain of the SELECT-chosen channel(s) of the
+ * IN-selected pair, ±1 dB per click (manual §5.1: SELECT steps
+ * left/right/both, then the wheel changes the gain).  Writes the PANEL
+ * gain registers 0x1A 0x000A+mic (cap_select.pcap 2026-08-24 — the
+ * "ADC gain" family, which drives the same preamp as the GUI
+ * 0x0000+mic; the cache tracks the raw either way).  Opt has no
+ * preamp and SELECT None = no target.
+ */
+static void bf_panel_gain_wheel(struct snd_usb_babyface *chip, int delta)
+{
+	u8 mics[2];
+	int n = 0;
+	int i;
+
+	if (chip->panel_in == 3 || chip->panel_select == 3)
+		return;
+	{
+		int base = chip->panel_in == 2 ? 2 : 0;
+
+		mics[0] = base + (chip->panel_select == 1 ? 1 : 0);
+		n = 1;
+		if (chip->panel_select == 2)
+			mics[n++] = base + 1;
+	}
+
+	mutex_lock(&chip->mutex);
+	for (i = 0; i < n; i++) {
+		int mic = mics[i];
+		int db = clamp((int)chip->gain[mic] + delta,
+				0, bf_gain_max_db(mic));
+		u8 raw = bf_gain_raw(mic, db);
+
+		bf_vendor_write(chip, BF_REQ_GAIN, raw, BF_REG_PANEL_GAIN + mic);
+		chip->gain[mic] = db;
+	}
+	mutex_unlock(&chip->mutex);
+}
+
+/* OUT-balance wheel (hold SELECT + wheel — manual §5.1 "Output
+ * Balance"): moves the stereo image of the OUT-selected output by
+ * attenuating ONE side, linear in raw (cap_pan_stereo.pcap: the varied
+ * side = fixed·(1−|pan|), ~0x9C raw step per click at 0 dB — the PAN
+ * of the stereo hardware output in TotalMix).  The balance position is
+ * derived from the L/R master ratio (the louder side is the fixed
+ * one), so the gesture needs no extra state — and the OUT wheel below
+ * moves both sides by the same dB to preserve an existing balance.
+ */
+static void bf_panel_balance_wheel(struct snd_usb_babyface *chip, int delta)
+{
+	int out = chip->panel_out == 3 ? 5 :
+		  chip->panel_out == 2 ? 1 : 0;
+	u16 l, r;
+	int bal;		/* −100..+100; + = image right (left varies) */
+	u16 fixed, varied;
+
+	mutex_lock(&chip->mutex);
+	/* Read under the lock so the L/R pair is consistent with the
+	 * master/mute/dim writers (they update chip->master[] under the
+	 * same mutex).
+	 */
+	l = chip->master[out][0];
+	r = chip->master[out][1];
+	/* Balance from the L/R ratio: the louder side is the fixed one. */
+	if (l >= r) {
+		bal = r ? -(100 - (100 * r) / l) : -100;
+		fixed = l;
+	} else {
+		bal = l ? (100 - (100 * l) / r) : 100;
+		fixed = r;
+	}
+	bal = clamp(bal + delta * 2, -100, 100);
+	varied = (u16)((u32)fixed * (100 - abs(bal)) / 100);
+	l = bal >= 0 ? varied : fixed;
+	r = bal >= 0 ? fixed : varied;
+
+	bf_panel_write_master(chip, out, l, r);
+	mutex_unlock(&chip->mutex);
+}
+
+/* SET press (byte3 0x42 flash): toggle 48V phantom on the
+ * SELECT-chosen mic(s) of the IN-selected pair.  The hardware only
+ * does this in standalone mode (online, TotalMix ignores SET — no USB
+ * write in the captures), but the driver IS the host: it writes the
+ * preamp state itself and the P48 LEDs follow (the tuxmix-core
+ * emulator, hardware-verified).  Restricted to IN mode + Ch1/2 (the
+ * phantom-capable pair); Opt/Ch3/4 and SELECT None = no target.
+ */
+static void bf_panel_set_phantom(struct snd_usb_babyface *chip)
+{
+	u16 bits = 0;
+	int m;
+
+	if (chip->panel_mix || chip->panel_in != 1 ||
+	    chip->panel_select == 3)
+		return;
+	if (chip->panel_select != 1)
+		bits |= BF_PREAMP_48V_MIC1;
+	if (chip->panel_select != 0)
+		bits |= BF_PREAMP_48V_MIC2;
+
+	mutex_lock(&chip->mutex);
+	/* One channel selected: toggle it.  Both selected: ALIGN both to
+	 * the same state, so repeated SET presses cycle all-on <-> all-off
+	 * (a mixed phantom state cannot persist with both selected).
+	 */
+	if (chip->panel_select == 2) {
+		if ((chip->preamp & bits) == bits)
+			chip->preamp &= ~bits;
+		else
+			chip->preamp |= bits;
+	} else {
+		chip->preamp ^= bits;
+	}
+	bf_preamp_state_write(chip);
+	for (m = 0; m < 4; m++)
+		chip->panel_mix_disp[m] = 0;
+	mutex_unlock(&chip->mutex);
+}
+
+static void bf_panel_notify(struct snd_usb_babyface *chip, int ctl)
+{
+	if (chip->panel_kctl[ctl])
+		snd_ctl_notify(chip->card, SNDRV_CTL_EVENT_MASK_VALUE,
+			       &chip->panel_kctl[ctl]->id);
+}
+
+/* One 0x17 read + decode.  Called from the poll work; no locking needed —
+ * the worker is the only writer and the control get callbacks run under
+ * the ALSA controls lock (chip->panel_button/wheel are consumed there).
+ */
+static void bf_panel_tick(struct snd_usb_babyface *chip)
+{
+	u8 st[4];
+	int delta, in, out;
+	bool dim;
+	u8 cls, pcls;
+	int btn;
+	bool mix_flash, fader_now;
+
+	if (bf_vendor_read(chip, BF_REQ_PREAMP, BF_REG_PANEL_READ, st) < 0)
+		return;	/* device gone / busy — retry next tick */
+
+	if (!chip->panel_seen) {
+		chip->panel_seen = true;
+		memcpy(chip->panel_prev, st, sizeof(st));
+		/* Seed the state controls from the first snapshot. */
+		in = bf_panel_in_decode((st[2] >> BF_PANEL_IN_SHIFT) & 0x7);
+		if (in)
+			chip->panel_in = in;
+		out = bf_panel_out_decode(st[1] & 0x07);
+		if (out)
+			chip->panel_out = out;
+		chip->panel_mix = !!(st[0] & 0x80);
+		chip->panel_saw_fader = (st[2] >> 4) == 0x0;
+		chip->panel_dim = !!(st[1] & 0x20);
+		return;
+	}
+
+	/* The udev alsactl restore (~100 ms after probe) clobbers the host
+	 * SELECT with a stale stored value (the control is VOLATILE but
+	 * this alsactl stores/restores it anyway) — re-assert the device's
+	 * power-on state (nothing selected, cycle ARMED) for the first
+	 * ~3 s so the boot always starts in sync.
+	 */
+	if (time_is_after_jiffies(chip->panel_start + 3 * HZ))
+		chip->panel_select = 3;
+
+	/* Button flash (byte3 over the 0x40 idle base). */
+	btn = bf_panel_button_decode(st[3]);
+	if (btn)
+		chip->panel_button = btn;
+
+	/* Wheel: signed 4-bit wrap delta of the byte2 low nibble — only
+	 * while the mode class is unchanged.  A mode switch (IN 0x4x →
+	 * fader 0x0x on a MIX press, or the OUT counter carrying 0x8F →
+	 * 0x90 — the OUT counter is a full byte, cap_set2.pcap) must not
+	 * be read as a wheel jump.  Class: 0 = fader (0x0x), 1 = OUT
+	 * (0x8x/0x9x), 2 = IN (0x4x/0x5x/0x6x).
+	 */
+	cls = (st[2] >> 4) == 0x8 || (st[2] >> 4) == 0x9 ? 1 :
+	      (st[2] >> 4) == 0x0 ? 0 : 2;
+	pcls = (chip->panel_prev[2] >> 4) == 0x8 ||
+	       (chip->panel_prev[2] >> 4) == 0x9 ? 1 :
+	       (chip->panel_prev[2] >> 4) == 0x0 ? 0 : 2;
+	delta = (int)(st[2] & 0x0f) - (int)(chip->panel_prev[2] & 0x0f);
+	if (delta > 8)
+		delta -= 16;
+	else if (delta < -8)
+		delta += 16;
+	if (delta && cls == pcls) {
+		chip->panel_wheel = clamp(chip->panel_wheel + delta,
+					  SHRT_MIN, SHRT_MAX);
+		bf_panel_notify(chip, BF_PANEL_KCTL_WHEEL);
+		/* Wheel by mode (LINUX-VALIDATION §12, the TotalMix
+		 * emulator): MIX → monitoring level, OUT (0x8x/0x9x) → the
+		 * selected output master (or its balance while SELECT is
+		 * held), IN (0x4x/0x5x/0x6x) → the SELECT-chosen preamp
+		 * gain.
+		 */
+		if (chip->panel_mix)
+			bf_panel_mix_wheel(chip, delta);
+		else if (chip->panel_sel_hold >= 10 && cls == 1)
+			bf_panel_balance_wheel(chip, delta);
+		else if (cls == 1)
+			bf_panel_out_wheel(chip, delta);
+		else if (cls == 2)
+			bf_panel_gain_wheel(chip, delta);
+	}
+
+	/* Selections — keep the previous when the field is not in range
+	 * (the fader-mode readback drops the IN position bits).
+	 */
+	in = bf_panel_in_decode((st[2] >> BF_PANEL_IN_SHIFT) & 0x7);
+	if (in && in != chip->panel_in) {
+		chip->panel_in = in;
+		/* The card CLEARS its L/R/both selection on an IN pair
+		 * switch (user-verified 2026-08-27): re-sync the host-
+		 * tracked SELECT so SET / the wheel / MIX target nothing
+		 * until the user picks a channel again.  This is the main
+		 * anti-desync hook (the physical state is not readable).
+		 */
+		if (chip->panel_select != 3) {
+			chip->panel_select = 3;
+			bf_panel_notify(chip, BF_PANEL_KCTL_SELECT);
+		}
+		/* An IN-pair switch disarms the device's SELECT cycle: the
+		 * next press only re-arms it (no step), the one after that
+		 * cycles (device behavior, user-verified 2026-08-28).
+		 */
+		chip->panel_select_armed = false;
+		bf_panel_notify(chip, BF_PANEL_KCTL_IN);
+	}
+	out = bf_panel_out_decode(st[1] & 0x07);
+	if (out && out != chip->panel_out) {
+		chip->panel_out = out;
+		bf_panel_notify(chip, BF_PANEL_KCTL_OUT);
+	}
+
+	/* SELECT press cycles the channel selection L → R → both → none
+	 * → L (manual §5.1).  The state is NOT in the readback
+	 * (panelprobe 2026-08-24), so it is tracked host-side.
+	 */
+	if (st[3] == BF_PANEL_FLASH_SELECT &&
+	    chip->panel_prev[3] != BF_PANEL_FLASH_SELECT) {
+		if (!chip->panel_select_armed) {
+			/* Disarmed (IN switch since the last step): the press
+			 * only re-arms the cycle — the device steps on the
+			 * NEXT press (user-verified 2026-08-28).
+			 */
+			chip->panel_select_armed = true;
+		} else {
+			chip->panel_select = (chip->panel_select + 1) & 3;
+		}
+		bf_panel_notify(chip, BF_PANEL_KCTL_SELECT);
+	}
+	/* SELECT hold (the OUT-balance gesture, manual §5.1 "Output
+	 * Balance"): a tap flashes byte3 0x50 for ~2-3 frames at 20 Hz
+	 * (~100-150 ms — selhold_probe2), a hold keeps it sustained, and
+	 * byte0 does NOT gain the 0x80 engaged bit — so the duration is
+	 * the only discriminator: >= 10 ticks (200 ms at 50 Hz) = held.
+	 */
+	if (st[3] == BF_PANEL_FLASH_SELECT)
+		chip->panel_sel_hold++;
+	else
+		chip->panel_sel_hold = 0;
+
+	/* SET (A) press: host-side 48V phantom toggle on the
+	 * SELECT-chosen mic(s) (see bf_panel_set_phantom).
+	 */
+	if (st[3] == BF_PANEL_FLASH_SET &&
+	    chip->panel_prev[3] != BF_PANEL_FLASH_SET)
+		bf_panel_set_phantom(chip);
+
+	/* MIX (fader mode) — HOST-latched, like TotalMix (cap_mix.pcap,
+	 * cap_select2.pcap): the raw press readback is `0D 0D 41 44` —
+	 * byte3 flash 0x44, NO engaged bit, byte2 still in the current
+	 * mode.  The host acks the flash with `0x17 0x8480 0x8C80` → the
+	 * device latches fader mode (byte0/1 gain the 0x80 bit, byte2 =
+	 * 0x00+n counter) and STAYS there after the physical release; the
+	 * SECOND 0x44 flash exits it (`0x17 0x0400 0x8000` + `0x8080`).
+	 * A mode button (IN/OUT/SET) pressed during MIX makes the device
+	 * leave fader mode by itself → same exit writes (the user: IN
+	 * must return to gain control).  `panel_saw_fader` gates the
+	 * device-driven exit so a pre-ack readback (byte2 still 0x4x
+	 * while the 0x44 flash shows) never ends MIX before it started.
+	 */
+	mix_flash = st[3] == BF_PANEL_FLASH_MIX &&
+		    chip->panel_prev[3] != BF_PANEL_FLASH_MIX;
+	fader_now = (st[2] >> 4) == 0x0;
+
+	if (mix_flash) {
+		if (chip->panel_mix) {
+			bf_vendor_write(chip, BF_REQ_PREAMP, 0x0400, 0x8000);
+			bf_vendor_write(chip, BF_REQ_PREAMP, 0x0400, 0x8080);
+			chip->panel_mix = false;
+			chip->panel_saw_fader = false;
+		} else {
+			int ref, out;
+			int m;
+
+			bf_vendor_write(chip, BF_REQ_PREAMP, 0x8480, 0x8c80);
+			chip->panel_mix = true;
+			/* Seed the monitoring level at the reference
+			 * crosspoint's current value so the first wheel
+			 * click doesn't jump from −inf (the reference =
+			 * the first SELECT-chosen channel of the IN pair;
+			 * Opt = the AS1/2 pair).
+			 */
+			out = chip->panel_out == 3 ? 5 :
+			      chip->panel_out == 2 ? 1 : 0;
+			ref = chip->panel_in == 3 ? 4 :
+			      (chip->panel_in == 2 ? 2 : 0) +
+			      (chip->panel_select == 1 ? 1 : 0);
+			chip->panel_mix_raw = chip->xpoint[out][ref][0];
+			/* Seed the VU display shadow at the CURRENT level
+			 * (cap_panel.pcap: TotalMix writes the display value of
+			 * the current fader on engage — 10 in that session —
+			 * not a hard 0; cap_mix's 0 was because the fader sat
+			 * at the bottom).  Only the channels the wheel can move.
+			 */
+			for (m = 0; m < 4; m++)
+				chip->panel_mix_disp[m] = 0;
+			if (ref < 4) {
+				int db2 = bf_fader_raw_to_db2(chip->panel_mix_raw);
+				int disp = bf_mix_display(db2);
+
+				bf_vendor_write(chip, BF_REQ_GAIN, (u16)disp,
+						BF_REG_PANEL_GAIN + ref);
+				chip->panel_mix_disp[ref] = disp;
+			}
+		}
+		bf_panel_notify(chip, BF_PANEL_KCTL_MIX);
+	}
+	if (fader_now) {
+		chip->panel_saw_fader = true;
+	} else if (chip->panel_mix && chip->panel_saw_fader &&
+		   st[3] != BF_PANEL_FLASH_MIX) {
+		/* device left fader mode by itself (IN/OUT/SET press) */
+		bf_vendor_write(chip, BF_REQ_PREAMP, 0x0400, 0x8000);
+		bf_vendor_write(chip, BF_REQ_PREAMP, 0x0400, 0x8080);
+		chip->panel_mix = false;
+		chip->panel_saw_fader = false;
+		bf_panel_notify(chip, BF_PANEL_KCTL_MIX);
+	}
+
+	dim = !!(st[1] & 0x20);
+	if (dim != chip->panel_dim) {
+		chip->panel_dim = dim;
+		bf_panel_notify(chip, BF_PANEL_KCTL_DIM);
+	}
+
+	memcpy(chip->panel_prev, st, sizeof(st));
+}
+
+void babyface_panel_work(struct work_struct *work)
+{
+	struct snd_usb_babyface *chip = container_of(work,
+			struct snd_usb_babyface, panel_work.work);
+
+	if (chip->shutdown)
+		return;
+	bf_panel_tick(chip);
+	schedule_delayed_work(&chip->panel_work,
+			      msecs_to_jiffies(chip->panel_poll_ms));
 }
 
 void babyface_panel_start(struct snd_usb_babyface *chip)
 {
+	chip->panel_seen = false;
+	/* The device boots with NOTHING selected (the SELECT cycle starts
+	 * at none → AN1 → AN2 → both → none) — the unreadable selection
+	 * must start there too, or every later SET is off by one channel
+	 * (host at AN1 while the LEDs show nothing → first SELECT makes
+	 * the device blink AN1 but the host believes AN2).
+	 */
+	chip->panel_select = 3;	/* none */
+	chip->panel_select_armed = true;
+	chip->panel_start = jiffies;
+	schedule_delayed_work(&chip->panel_work, 0);
 }
 
 void babyface_panel_stop(struct snd_usb_babyface *chip)
 {
+	cancel_delayed_work_sync(&chip->panel_work);
 }
 
-void babyface_panel_work(struct work_struct *work)
+/* ── controls ────────────────────────── */
+
+/* The button/wheel controls hold the LATEST state and are NOT consumed
+ * on read: wireplumber subscribes to every notifying control and reads
+ * it, so a clear-on-get would let another reader eat the event.  Each
+ * consumer tracks its own baseline and acts on changes (the button is a
+ * last-press code, the wheel an accumulated signed delta).  VOLATILE
+ * keeps alsactl from caching them.
+ */
+static int bf_panel_button_info(struct snd_kcontrol *kctl,
+				struct snd_ctl_elem_info *uinfo)
 {
+	uinfo->type = SNDRV_CTL_ELEM_TYPE_INTEGER;
+	uinfo->count = 1;
+	uinfo->value.integer.min = 0;
+	uinfo->value.integer.max = BF_PANEL_BTN_DIM;
+	uinfo->value.integer.step = 1;
+	return 0;
+}
+
+static int bf_panel_button_get(struct snd_kcontrol *kctl,
+			       struct snd_ctl_elem_value *ucontrol)
+{
+	struct snd_usb_babyface *chip = snd_kcontrol_chip(kctl);
+
+	ucontrol->value.integer.value[0] = chip->panel_button;
+	return 0;
 }
 
+static int bf_panel_wheel_info(struct snd_kcontrol *kctl,
+			       struct snd_ctl_elem_info *uinfo)
+{
+	uinfo->type = SNDRV_CTL_ELEM_TYPE_INTEGER;
+	uinfo->count = 1;
+	uinfo->value.integer.min = SHRT_MIN;
+	uinfo->value.integer.max = SHRT_MAX;
+	uinfo->value.integer.step = 1;
+	return 0;
+}
+
+static int bf_panel_wheel_get(struct snd_kcontrol *kctl,
+			      struct snd_ctl_elem_value *ucontrol)
+{
+	struct snd_usb_babyface *chip = snd_kcontrol_chip(kctl);
+
+	ucontrol->value.integer.value[0] = chip->panel_wheel;
+	return 0;
+}
+
+static int bf_panel_in_info(struct snd_kcontrol *kctl,
+			    struct snd_ctl_elem_info *uinfo)
+{
+	return snd_ctl_enum_info(uinfo, 1, 4, bf_panel_in_texts);
+}
+
+static int bf_panel_in_get(struct snd_kcontrol *kctl,
+			   struct snd_ctl_elem_value *ucontrol)
+{
+	struct snd_usb_babyface *chip = snd_kcontrol_chip(kctl);
+
+	ucontrol->value.enumerated.item[0] = chip->panel_in;
+	return 0;
+}
+
+static int bf_panel_out_info(struct snd_kcontrol *kctl,
+			     struct snd_ctl_elem_info *uinfo)
+{
+	return snd_ctl_enum_info(uinfo, 1, 4, bf_panel_out_texts);
+}
+
+static int bf_panel_out_get(struct snd_kcontrol *kctl,
+			    struct snd_ctl_elem_value *ucontrol)
+{
+	struct snd_usb_babyface *chip = snd_kcontrol_chip(kctl);
+
+	ucontrol->value.enumerated.item[0] = chip->panel_out;
+	return 0;
+}
+
+static int bf_panel_select_info(struct snd_kcontrol *kctl,
+				struct snd_ctl_elem_info *uinfo)
+{
+	return snd_ctl_enum_info(uinfo, 1, 4, bf_panel_select_texts);
+}
+
+static int bf_panel_select_get(struct snd_kcontrol *kctl,
+			       struct snd_ctl_elem_value *ucontrol)
+{
+	struct snd_usb_babyface *chip = snd_kcontrol_chip(kctl);
+
+	ucontrol->value.enumerated.item[0] = chip->panel_select;
+	return 0;
+}
+
+/* Writable so software (or the user, after a driver reload) can
+ * re-sync the host-tracked SELECT state to the physical card — the
+ * L/R/both/none state is NOT in the 0x17 readback, so a reload starts
+ * at "Left" while the card may sit at any position; a desync makes
+ * SET / the wheel / MIX target the wrong channel.  Writing the
+ * physical state re-aligns the emulation (TotalMix parity: it also
+ * lets software select channels directly).
+ */
+static int bf_panel_select_put(struct snd_kcontrol *kctl,
+			       struct snd_ctl_elem_value *ucontrol)
+{
+	struct snd_usb_babyface *chip = snd_kcontrol_chip(kctl);
+	unsigned int v = ucontrol->value.enumerated.item[0];
+	int ret = 0;
+
+	if (v > 3)
+		return -EINVAL;
+	if (v != chip->panel_select) {
+		chip->panel_select = v;
+		bf_panel_notify(chip, BF_PANEL_KCTL_SELECT);
+		ret = 1;
+	}
+	return ret;
+}
+
+/* Shared boolean get — private_value selects mix (0) / dim (1). */
+static int bf_panel_bool_get(struct snd_kcontrol *kctl,
+			     struct snd_ctl_elem_value *ucontrol)
+{
+	struct snd_usb_babyface *chip = snd_kcontrol_chip(kctl);
+
+	ucontrol->value.integer.value[0] =
+		kctl->private_value ? chip->panel_dim : chip->panel_mix;
+	return 0;
+}
+
+int babyface_create_panel(struct snd_usb_babyface *chip)
+{
+	struct snd_kcontrol *kctl;
+	int err;
+
+	memset(chip->panel_kctl, 0, sizeof(chip->panel_kctl));
+
+	kctl = snd_ctl_new1(&(struct snd_kcontrol_new){
+		.iface = SNDRV_CTL_ELEM_IFACE_MIXER,
+		.name = "Front Panel Button",
+		.access = SNDRV_CTL_ELEM_ACCESS_READ |
+			  SNDRV_CTL_ELEM_ACCESS_VOLATILE,
+		.info = bf_panel_button_info,
+		.get = bf_panel_button_get,
+	}, chip);
+	err = snd_ctl_add(chip->card, kctl);
+	if (err < 0)
+		return err;
+	chip->panel_kctl[BF_PANEL_KCTL_BUTTON] = kctl;
+
+	kctl = snd_ctl_new1(&(struct snd_kcontrol_new){
+		.iface = SNDRV_CTL_ELEM_IFACE_MIXER,
+		.name = "Front Panel Wheel",
+		.access = SNDRV_CTL_ELEM_ACCESS_READ |
+			  SNDRV_CTL_ELEM_ACCESS_VOLATILE,
+		.info = bf_panel_wheel_info,
+		.get = bf_panel_wheel_get,
+	}, chip);
+	err = snd_ctl_add(chip->card, kctl);
+	if (err < 0)
+		return err;
+	chip->panel_kctl[BF_PANEL_KCTL_WHEEL] = kctl;
+
+	kctl = snd_ctl_new1(&(struct snd_kcontrol_new){
+		.iface = SNDRV_CTL_ELEM_IFACE_MIXER,
+		.name = "Front Panel In",
+		.access = SNDRV_CTL_ELEM_ACCESS_READ |
+			  SNDRV_CTL_ELEM_ACCESS_VOLATILE,
+		.info = bf_panel_in_info,
+		.get = bf_panel_in_get,
+	}, chip);
+	err = snd_ctl_add(chip->card, kctl);
+	if (err < 0)
+		return err;
+	chip->panel_kctl[BF_PANEL_KCTL_IN] = kctl;
+
+	kctl = snd_ctl_new1(&(struct snd_kcontrol_new){
+		.iface = SNDRV_CTL_ELEM_IFACE_MIXER,
+		.name = "Front Panel Out",
+		.access = SNDRV_CTL_ELEM_ACCESS_READ |
+			  SNDRV_CTL_ELEM_ACCESS_VOLATILE,
+		.info = bf_panel_out_info,
+		.get = bf_panel_out_get,
+	}, chip);
+	err = snd_ctl_add(chip->card, kctl);
+	if (err < 0)
+		return err;
+	chip->panel_kctl[BF_PANEL_KCTL_OUT] = kctl;
+
+	kctl = snd_ctl_new1(&(struct snd_kcontrol_new){
+		.iface = SNDRV_CTL_ELEM_IFACE_MIXER,
+		.name = "Front Panel Mix",
+		.access = SNDRV_CTL_ELEM_ACCESS_READ |
+			  SNDRV_CTL_ELEM_ACCESS_VOLATILE,
+		.info = snd_ctl_boolean_mono_info,
+		.get = bf_panel_bool_get,
+	}, chip);
+	err = snd_ctl_add(chip->card, kctl);
+	if (err < 0)
+		return err;
+	chip->panel_kctl[BF_PANEL_KCTL_MIX] = kctl;
+
+	kctl = snd_ctl_new1(&(struct snd_kcontrol_new){
+		.iface = SNDRV_CTL_ELEM_IFACE_MIXER,
+		.name = "Front Panel Dim",
+		.access = SNDRV_CTL_ELEM_ACCESS_READ |
+			  SNDRV_CTL_ELEM_ACCESS_VOLATILE,
+		.info = snd_ctl_boolean_mono_info,
+		.get = bf_panel_bool_get,
+		.private_value = 1,
+	}, chip);
+	err = snd_ctl_add(chip->card, kctl);
+	if (err < 0)
+		return err;
+	chip->panel_kctl[BF_PANEL_KCTL_DIM] = kctl;
+
+	kctl = snd_ctl_new1(&(struct snd_kcontrol_new){
+		.iface = SNDRV_CTL_ELEM_IFACE_MIXER,
+		.name = "Front Panel Select",
+		.access = SNDRV_CTL_ELEM_ACCESS_READ |
+			  SNDRV_CTL_ELEM_ACCESS_WRITE |
+			  SNDRV_CTL_ELEM_ACCESS_VOLATILE,
+		.info = bf_panel_select_info,
+		.get = bf_panel_select_get,
+		.put = bf_panel_select_put,
+	}, chip);
+	err = snd_ctl_add(chip->card, kctl);
+	if (err < 0)
+		return err;
+	chip->panel_kctl[BF_PANEL_KCTL_SELECT] = kctl;
+
+	return 0;
+}
+
+
 /* ── DSP-EQ stubs ─────────────────────────────────────────────
  * Replaced by the eq patch.  The core driver calls these from
  * probe() and hw_params() (fs-dependent re-upload).
-- 
2.55.0