Add a mux-control driver for the 24-to-1 analog multiplexer (AMUX)
embedded in the NXP MC33978/MC34978 Multiple Switch Detection
Interface (MSDI) devices.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
---
changes v9:
- rename mc33978-mux to mux-mc33978 in the Kconfig help
- fail if fwnode is NULL
changes v8:
- no chnages
changes v7:
- Simplify the return path and local variable assignment in
  mc33978_mux_set().
- Change idle_state to a signed integer to properly handle negative MUX
  subsystem constants.
- Default to MUX_IDLE_AS_IS when the "idle-state" device tree property
  is missing.
- Explicitly reject MUX_IDLE_DISCONNECT since the hardware does not
  support disconnecting the multiplexer.
changes v6:
- parse optional idle-state property
- validate idle-state against available AMUX channels
- lower-case probe error messages
changes v5:
- no changes
changes v4:
- no changes
changes v3:
- no changes
changes v2:
- Add missing <linux/err.h> include.
- Add platform_device_id table
---
 drivers/mux/Kconfig       |  14 ++++
 drivers/mux/Makefile      |   2 +
 drivers/mux/mc33978-mux.c | 141 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 157 insertions(+)
 create mode 100644 drivers/mux/mc33978-mux.c

diff --git a/drivers/mux/Kconfig b/drivers/mux/Kconfig
index c68132e38138..ffe92a714096 100644
--- a/drivers/mux/Kconfig
+++ b/drivers/mux/Kconfig
@@ -45,6 +45,20 @@ config MUX_GPIO
 	  To compile the driver as a module, choose M here: the module will
 	  be called mux-gpio.
 
+config MUX_MC33978
+	tristate "NXP MC33978/MC34978 Analog Multiplexer"
+	depends on MFD_MC33978
+	help
+	  MC33978/MC34978 24-to-1 analog multiplexer (AMUX) driver.
+
+	  This driver provides mux-control for the analog multiplexer,
+	  which can route switch voltages, temperature, and battery voltage
+	  to an external ADC. Typically used with IIO ADC drivers to measure
+	  analog values from the 22 switch inputs plus temperature and VBATP.
+
+	  To compile the driver as a module, choose M here: the module will
+	  be called mux-mc33978.
+
 config MUX_MMIO
 	tristate "MMIO/Regmap register bitfield-controlled Multiplexer"
 	depends on OF
diff --git a/drivers/mux/Makefile b/drivers/mux/Makefile
index 6e9fa47daf56..339c44b4d4f4 100644
--- a/drivers/mux/Makefile
+++ b/drivers/mux/Makefile
@@ -7,10 +7,12 @@ mux-core-objs			:= core.o
 mux-adg792a-objs		:= adg792a.o
 mux-adgs1408-objs		:= adgs1408.o
 mux-gpio-objs			:= gpio.o
+mux-mc33978-objs		:= mc33978-mux.o
 mux-mmio-objs			:= mmio.o
 
 obj-$(CONFIG_MULTIPLEXER)	+= mux-core.o
 obj-$(CONFIG_MUX_ADG792A)	+= mux-adg792a.o
 obj-$(CONFIG_MUX_ADGS1408)	+= mux-adgs1408.o
 obj-$(CONFIG_MUX_GPIO)		+= mux-gpio.o
+obj-$(CONFIG_MUX_MC33978)	+= mux-mc33978.o
 obj-$(CONFIG_MUX_MMIO)		+= mux-mmio.o
diff --git a/drivers/mux/mc33978-mux.c b/drivers/mux/mc33978-mux.c
new file mode 100644
index 000000000000..b44c862f0dbe
--- /dev/null
+++ b/drivers/mux/mc33978-mux.c
@@ -0,0 +1,141 @@
+// SPDX-License-Identifier: GPL-2.0-only
+// Copyright (c) 2026 Pengutronix, Oleksij Rempel <kernel@pengutronix.de>
+/*
+ * MC33978/MC34978 Analog Multiplexer (AMUX) Driver
+ *
+ * This driver provides mux-control for the 24-to-1 analog multiplexer.
+ * The AMUX routes one of the following signals to the external AMUX pin:
+ * - Channels 0-13: SG0-SG13 switch voltages
+ * - Channels 14-21: SP0-SP7 switch voltages
+ * - Channel 22: Internal temperature diode
+ * - Channel 23: Battery voltage (VBATP)
+ *
+ * Consumer drivers (typically IIO ADC drivers) use the mux-control
+ * subsystem to select which signal to measure.
+ *
+ * Architecture:
+ * The MC33978 does not have an internal ADC. Instead, it routes analog
+ * signals to an external AMUX pin that must be connected to an external
+ * ADC (such as the SoC's internal ADC). The IIO subsystem is responsible
+ * for coordinating the mux selection and ADC sampling.
+ */
+
+#include <linux/device.h>
+#include <linux/err.h>
+#include <linux/mod_devicetable.h>
+#include <linux/module.h>
+#include <linux/mux/driver.h>
+#include <linux/platform_device.h>
+#include <linux/property.h>
+#include <linux/regmap.h>
+
+#include <linux/mfd/mc33978.h>
+
+/* AMUX_CTRL register field definitions */
+#define MC33978_AMUX_CTRL_MASK	GENMASK(5, 0)	/* 6-bit channel select */
+
+struct mc33978_mux_priv {
+	struct device *dev;
+	struct regmap *map;
+};
+
+static int mc33978_mux_set(struct mux_control *mux, int state)
+{
+	struct mux_chip *mux_chip = mux->chip;
+	struct mc33978_mux_priv *priv = mux_chip_priv(mux_chip);
+	int ret;
+
+	if (state < 0 || state >= MC33978_NUM_AMUX_CH)
+		return -EINVAL;
+
+	ret = regmap_update_bits(priv->map, MC33978_REG_AMUX_CTRL,
+				 MC33978_AMUX_CTRL_MASK, state);
+	if (ret)
+		dev_err(priv->dev, "failed to set AMUX channel %d: %d\n",
+			state, ret);
+
+	return ret;
+}
+
+static const struct mux_control_ops mc33978_mux_ops = {
+	.set = mc33978_mux_set,
+};
+
+static int mc33978_mux_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct mc33978_mux_priv *priv;
+	struct fwnode_handle *fwnode;
+	struct mux_chip *mux_chip;
+	struct mux_control *mux;
+	s32 idle_state;
+	int ret;
+
+	mux_chip = devm_mux_chip_alloc(dev, 1, sizeof(*priv));
+	if (IS_ERR(mux_chip))
+		return dev_err_probe(dev, PTR_ERR(mux_chip), "failed to allocate mux chip\n");
+
+	fwnode = dev_fwnode(dev->parent);
+	if (!fwnode)
+		return dev_err_probe(dev, -ENODEV, "missing parent firmware node\n");
+
+	/* Borrow the parent's firmware node so consumers can find this mux chip */
+	device_set_node(&mux_chip->dev, fwnode);
+
+	priv = mux_chip_priv(mux_chip);
+	priv->dev = dev;
+
+	priv->map = dev_get_regmap(dev->parent, NULL);
+	if (!priv->map)
+		return dev_err_probe(dev, -ENODEV, "failed to get parent regmap\n");
+
+	mux_chip->ops = &mc33978_mux_ops;
+
+	mux = &mux_chip->mux[0];
+	mux->states = MC33978_NUM_AMUX_CH;
+
+	ret = device_property_read_u32(&mux_chip->dev, "idle-state",
+				       (u32 *)&idle_state);
+	if (ret < 0 && ret != -EINVAL) {
+		return dev_err_probe(dev, ret, "failed to parse idle-state\n");
+	} else if (ret == -EINVAL) {
+		mux->idle_state = MUX_IDLE_AS_IS;
+	} else {
+		if (idle_state == MUX_IDLE_DISCONNECT)
+			return dev_err_probe(dev, -EINVAL,
+					     "idle-disconnect not supported by hardware\n");
+		if (idle_state != MUX_IDLE_AS_IS &&
+		    (idle_state < 0 || idle_state >= MC33978_NUM_AMUX_CH))
+			return dev_err_probe(dev, -EINVAL, "invalid idle-state %d\n",
+					     idle_state);
+		mux->idle_state = idle_state;
+	}
+
+	ret = devm_mux_chip_register(dev, mux_chip);
+	if (ret)
+		return dev_err_probe(dev, ret, "failed to register mux chip\n");
+
+	platform_set_drvdata(pdev, mux_chip);
+
+	return 0;
+}
+
+static const struct platform_device_id mc33978_mux_id[] = {
+	{ "mc33978-mux", },
+	{ "mc34978-mux", },
+	{ }
+};
+MODULE_DEVICE_TABLE(platform, mc33978_mux_id);
+
+static struct platform_driver mc33978_mux_driver = {
+	.driver = {
+		.name = "mc33978-mux",
+	},
+	.probe = mc33978_mux_probe,
+	.id_table = mc33978_mux_id,
+};
+module_platform_driver(mc33978_mux_driver);
+
+MODULE_AUTHOR("Oleksij Rempel <kernel@pengutronix.de>");
+MODULE_DESCRIPTION("NXP MC33978/MC34978 Analog Multiplexer Driver");
+MODULE_LICENSE("GPL");
-- 
2.47.3