[PATCH 2/3] iio: adc: mt6397-auxadc: add mt6397 PMIC AUXADC driver

[Ryan Brue]

The mt6397 AUXADC is a 10-bit ADC behind the SoC's PMIC wrapper. On boards
built around this PMIC it is the only way to read the battery: the SoC's
AUXADC is wired to board thermistors and the charger ICs these boards use
have no ADC of their own.

Add a driver exposing the battery voltage and battery temperature
channels. Only those two are described, so a channel ID in the device tree
is an index into the driver's channel array rather than the PMIC's channel
number, as mt6323-auxadc does. The ready bit lives in a channel's raw
result register, but the value comes from the chip's trimmed copy of it,
which is what the vendor driver reads for a measurement.

Both channels need more than that, as the vendor programs them. The
battery voltage is measured through ISENSE, because a board with a
switching charger in the power path leaves BATSNS on the charger's system
rail instead of on the pack. The thermistor only reads correctly with the
PMIC's battery-detect bias and input buffer enabled, which take 20 ms to
settle. Both are switched back off afterwards.

Reads average sixteen conversions in software; the chip's sample
accumulator makes no measurable difference at any setting, so it is left
at one sample per conversion.

Assisted-by: LLM
Signed-off-by: Ryan Brue <ryanbrue.dev@xxxxxxxxx>
---
 MAINTAINERS                     |   7 +
 drivers/iio/adc/Kconfig         |  11 ++
 drivers/iio/adc/Makefile        |   1 +
 drivers/iio/adc/mt6397-auxadc.c | 340 ++++++++++++++++++++++++++++++++++++++++
 4 files changed, 359 insertions(+)

diff --git a/MAINTAINERS b/MAINTAINERS
index c2414447892c..9b254f98b5d7 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -16713,6 +16713,13 @@ S:	Maintained
 F:	drivers/iio/adc/mt6323-auxadc.c
 F:	include/dt-bindings/iio/adc/mediatek,mt6323-auxadc.h
 
+MEDIATEK MT6397 PMIC AUXADC DRIVER
+M:	Ryan Brue <ryanbrue.dev@xxxxxxxxx>
+L:	linux-iio@xxxxxxxxxxxxxxx
+S:	Maintained
+F:	drivers/iio/adc/mt6397-auxadc.c
+F:	include/dt-bindings/iio/adc/mediatek,mt6397-auxadc.h
+
 MEDIATEK MT6735 CLOCK & RESET DRIVERS
 M:	Yassine Oudjana <y.oudjana@xxxxxxxxxxxxxx>
 L:	linux-clk@xxxxxxxxxxxxxxx
diff --git a/drivers/iio/adc/Kconfig b/drivers/iio/adc/Kconfig
index 415e519ad4eb..9a1d36ed53cb 100644
--- a/drivers/iio/adc/Kconfig
+++ b/drivers/iio/adc/Kconfig
@@ -1233,6 +1233,17 @@ config MEDIATEK_MT6370_ADC
 	  This driver can also be built as a module. If so, the module
 	  will be called "mt6370-adc".
 
+config MEDIATEK_MT6397_AUXADC
+	tristate "MediaTek MT6397 PMIC AUXADC driver"
+	depends on MFD_MT6397
+	help
+	  Say yes here to enable support for MediaTek MT6397 PMIC Auxiliary ADC.
+	  This driver provides the battery voltage and battery temperature
+	  channels used by the fuel gauge on boards built around this PMIC.
+
+	  This driver can also be built as a module. If so, the module will be
+	  called mt6397-auxadc.
+
 config MEDIATEK_MT6577_AUXADC
 	tristate "MediaTek AUXADC driver"
 	depends on ARCH_MEDIATEK || COMPILE_TEST
diff --git a/drivers/iio/adc/Makefile b/drivers/iio/adc/Makefile
index dcec0abb03b7..13cb4206bdb7 100644
--- a/drivers/iio/adc/Makefile
+++ b/drivers/iio/adc/Makefile
@@ -105,6 +105,7 @@ obj-$(CONFIG_MEDIATEK_MT6323_AUXADC) += mt6323-auxadc.o
 obj-$(CONFIG_MEDIATEK_MT6359_AUXADC) += mt6359-auxadc.o
 obj-$(CONFIG_MEDIATEK_MT6360_ADC) += mt6360-adc.o
 obj-$(CONFIG_MEDIATEK_MT6370_ADC) += mt6370-adc.o
+obj-$(CONFIG_MEDIATEK_MT6397_AUXADC) += mt6397-auxadc.o
 obj-$(CONFIG_MEDIATEK_MT6577_AUXADC) += mt6577_auxadc.o
 obj-$(CONFIG_MEN_Z188_ADC) += men_z188_adc.o
 obj-$(CONFIG_MESON_SARADC) += meson_saradc.o
diff --git a/drivers/iio/adc/mt6397-auxadc.c b/drivers/iio/adc/mt6397-auxadc.c
new file mode 100644
index 000000000000..dbfa82341d6c
--- /dev/null
+++ b/drivers/iio/adc/mt6397-auxadc.c
@@ -0,0 +1,340 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * MediaTek MT6397 PMIC AUXADC IIO driver
+ *
+ * Copyright (c) 2026 Ryan Brue <ryanbrue.dev@xxxxxxxxx>
+ *
+ * Based on drivers/iio/adc/mt6323-auxadc.c
+ */
+
+#include <linux/array_size.h>
+#include <linux/bitfield.h>
+#include <linux/bits.h>
+#include <linux/cleanup.h>
+#include <linux/delay.h>
+#include <linux/iio/iio.h>
+#include <linux/math.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/platform_device.h>
+#include <linux/regmap.h>
+#include <linux/stringify.h>
+#include <linux/time.h>
+#include <linux/types.h>
+
+#include <linux/mfd/mt6397/core.h>
+
+#include <dt-bindings/iio/adc/mediatek,mt6397-auxadc.h>
+
+/* The ready bit is in the raw result register, the value in the trimmed one. */
+#define MT6397_AUXADC_ADC(n)		(0x0514 + 2 * (n))
+#define MT6397_AUXADC_ADC_RDY		BIT(15)
+#define MT6397_AUXADC_ADC_VAL		GENMASK(9, 0)
+#define MT6397_AUXADC_ADC_TRIM(n)	(0x052a + 2 * (n))	/* ADC11 + n */
+
+/* The thermistor needs the battery-detect bias and the input buffer on. */
+#define MT6397_AUXADC_CON0		0x0542
+#define MT6397_AUXADC_CON0_BUF_PWD_B	BIT(1)
+#define MT6397_AUXADC_CON0_BUF_PWD_ON	BIT(3)
+#define MT6397_CHR_CON7			0x000e
+#define MT6397_CHR_CON7_BATON_TDET_EN	BIT(2)
+#define MT6397_AUXADC_BATTEMP_SETTLE_US	(20 * USEC_PER_MSEC)
+
+#define MT6397_AUXADC_CON1		0x0544
+#define MT6397_AUXADC_CON1_CHSEL	GENMASK(10, 7)
+#define MT6397_AUXADC_CON1_START	BIT(0)
+
+/*
+ * CHR_CON16 and the two SOURCE_CH0 selects route either BATSNS or ISENSE onto
+ * the battery channel. Only ISENSE is used: with a switching charger in the
+ * power path BATSNS sits on the system rail rather than on the pack.
+ */
+#define MT6397_CHR_CON16		0x0020
+#define MT6397_CHR_CON16_ADCIN_VSEN_EN	BIT(11)
+#define MT6397_CHR_CON16_ADCIN_VBAT_EN	BIT(10)
+#define MT6397_CHR_CON16_ADCIN_SEL	(MT6397_CHR_CON16_ADCIN_VSEN_EN | \
+					 MT6397_CHR_CON16_ADCIN_VBAT_EN)
+#define MT6397_AUXADC_CON14		0x055e
+#define MT6397_AUXADC_CON14_CH0_NORM_SEL BIT(2)
+#define MT6397_AUXADC_CON14_CH0_LBAT_SEL BIT(0)
+#define MT6397_AUXADC_ISENSE_SETTLE_US	USEC_PER_MSEC
+
+/* Samples accumulated per conversion; the chip's accumulator is left off. */
+#define MT6397_AUXADC_CON0_SPL_NUM	GENMASK(11, 7)
+
+/* Conversions averaged in software per read. */
+#define MT6397_AUXADC_SAMPLES		16
+
+/* Hardware limitation: the result is not ready for this long after START. */
+#define MT6397_AUXADC_START_US		30
+
+/* The battery channel is divided down by 4; the thermistor is not divided. */
+#define MT6397_AUXADC_ISENSE_DIVIDER	4
+
+/* The PMIC's own channel numbers, as the CON1 select field takes them. */
+#define MT6397_AUXADC_HWCHAN_BATSNS	0
+#define MT6397_AUXADC_HWCHAN_BAT_TEMP	3
+
+struct mt6397_auxadc {
+	struct regmap *regmap;
+	/* AUXADC doesn't support reading multiple channels simultaneously. */
+	struct mutex lock;
+};
+
+#define MTK_PMIC_IIO_CHAN(_name, _chan, _hwchan)                \
+{                                                               \
+	.type = IIO_VOLTAGE,                                    \
+	.indexed = 1,                                           \
+	.channel = _chan,                                       \
+	.address = _hwchan,                                     \
+	.datasheet_name = __stringify(_name),                   \
+	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |          \
+			      BIT(IIO_CHAN_INFO_SCALE),         \
+}
+
+/*
+ * A device tree channel ID indexes this array; .address holds the PMIC channel
+ * number. The thermistor reports as a voltage because that is all the PMIC
+ * measures: the divider across an NTC whose curve belongs to the board.
+ */
+static const struct iio_chan_spec mt6397_auxadc_channels[] = {
+	MTK_PMIC_IIO_CHAN(isense,   MT6397_AUXADC_ISENSE,
+			  MT6397_AUXADC_HWCHAN_BATSNS),
+	MTK_PMIC_IIO_CHAN(bat_temp, MT6397_AUXADC_BAT_TEMP,
+			  MT6397_AUXADC_HWCHAN_BAT_TEMP),
+};
+
+static int mt6397_auxadc_read_once(struct mt6397_auxadc *adc,
+				   const struct iio_chan_spec *chan, int *val)
+{
+	struct regmap *map = adc->regmap;
+	unsigned int reg;
+	int ret;
+
+	ret = regmap_update_bits(map, MT6397_AUXADC_CON1,
+				 MT6397_AUXADC_CON1_CHSEL,
+				 FIELD_PREP(MT6397_AUXADC_CON1_CHSEL,
+					    chan->address));
+	if (ret)
+		return ret;
+
+	/* START is edge triggered: it has to be lowered before being raised. */
+	ret = regmap_clear_bits(map, MT6397_AUXADC_CON1,
+				MT6397_AUXADC_CON1_START);
+	if (ret)
+		return ret;
+
+	ret = regmap_set_bits(map, MT6397_AUXADC_CON1,
+			      MT6397_AUXADC_CON1_START);
+	if (ret)
+		return ret;
+
+	fsleep(MT6397_AUXADC_START_US);
+
+	ret = regmap_read_poll_timeout(map, MT6397_AUXADC_ADC(chan->address),
+				       reg, reg & MT6397_AUXADC_ADC_RDY,
+				       100, 100 * USEC_PER_MSEC);
+	if (ret)
+		return ret;
+
+	ret = regmap_read(map, MT6397_AUXADC_ADC_TRIM(chan->address), &reg);
+	if (ret)
+		return ret;
+
+	*val = FIELD_GET(MT6397_AUXADC_ADC_VAL, reg);
+
+	return 0;
+}
+
+static int mt6397_auxadc_battemp_bias(struct mt6397_auxadc *adc, bool on)
+{
+	struct regmap *map = adc->regmap;
+	int ret;
+
+	if (on) {
+		ret = regmap_set_bits(map, MT6397_AUXADC_CON0,
+				      MT6397_AUXADC_CON0_BUF_PWD_ON);
+		if (ret)
+			return ret;
+		ret = regmap_set_bits(map, MT6397_AUXADC_CON0,
+				      MT6397_AUXADC_CON0_BUF_PWD_B);
+		if (ret)
+			return ret;
+		return regmap_set_bits(map, MT6397_CHR_CON7,
+				       MT6397_CHR_CON7_BATON_TDET_EN);
+	}
+
+	ret = regmap_clear_bits(map, MT6397_CHR_CON7,
+				MT6397_CHR_CON7_BATON_TDET_EN);
+	ret = ret ?: regmap_clear_bits(map, MT6397_AUXADC_CON0,
+				       MT6397_AUXADC_CON0_BUF_PWD_B);
+	return ret ?: regmap_clear_bits(map, MT6397_AUXADC_CON0,
+					MT6397_AUXADC_CON0_BUF_PWD_ON);
+}
+
+static int mt6397_auxadc_isense_enable(struct mt6397_auxadc *adc)
+{
+	struct regmap *map = adc->regmap;
+	int ret;
+
+	ret = regmap_update_bits(map, MT6397_CHR_CON16,
+				 MT6397_CHR_CON16_ADCIN_SEL,
+				 MT6397_CHR_CON16_ADCIN_VSEN_EN);
+	if (ret)
+		return ret;
+
+	return regmap_set_bits(map, MT6397_AUXADC_CON14,
+			       MT6397_AUXADC_CON14_CH0_NORM_SEL |
+			       MT6397_AUXADC_CON14_CH0_LBAT_SEL);
+}
+
+static int mt6397_auxadc_isense_disable(struct mt6397_auxadc *adc)
+{
+	struct regmap *map = adc->regmap;
+	int ret;
+
+	ret = regmap_clear_bits(map, MT6397_AUXADC_CON14,
+				MT6397_AUXADC_CON14_CH0_NORM_SEL |
+				MT6397_AUXADC_CON14_CH0_LBAT_SEL);
+
+	return ret ?: regmap_clear_bits(map, MT6397_CHR_CON16,
+					MT6397_CHR_CON16_ADCIN_SEL);
+}
+
+static int mt6397_auxadc_read_channel(struct mt6397_auxadc *adc,
+				      const struct iio_chan_spec *chan,
+				      int *val)
+{
+	unsigned int i, sum = 0;
+	int ret, sample;
+
+	/* Held across the whole burst: the channel select is shared state. */
+	guard(mutex)(&adc->lock);
+
+	if (chan->channel == MT6397_AUXADC_ISENSE) {
+		ret = mt6397_auxadc_isense_enable(adc);
+		if (ret)
+			return ret;
+		fsleep(MT6397_AUXADC_ISENSE_SETTLE_US);
+	} else {
+		ret = mt6397_auxadc_battemp_bias(adc, true);
+		if (ret)
+			return ret;
+		fsleep(MT6397_AUXADC_BATTEMP_SETTLE_US);
+	}
+
+	for (i = 0; i < MT6397_AUXADC_SAMPLES; i++) {
+		ret = mt6397_auxadc_read_once(adc, chan, &sample);
+		if (ret)
+			break;
+
+		sum += sample;
+	}
+
+	/* Lower START so the converter is not left armed between reads. */
+	regmap_clear_bits(adc->regmap, MT6397_AUXADC_CON1,
+			  MT6397_AUXADC_CON1_START);
+
+	if (chan->channel == MT6397_AUXADC_ISENSE)
+		mt6397_auxadc_isense_disable(adc);
+	else
+		mt6397_auxadc_battemp_bias(adc, false);
+
+	if (ret)
+		return ret;
+
+	*val = DIV_ROUND_CLOSEST(sum, MT6397_AUXADC_SAMPLES);
+
+	return 0;
+}
+
+static int mt6397_auxadc_read_raw(struct iio_dev *indio_dev,
+				  const struct iio_chan_spec *chan,
+				  int *val, int *val2, long mask)
+{
+	struct mt6397_auxadc *adc = iio_priv(indio_dev);
+	int ret;
+
+	switch (mask) {
+	case IIO_CHAN_INFO_RAW:
+		ret = mt6397_auxadc_read_channel(adc, chan, val);
+		if (ret)
+			return ret;
+
+		return IIO_VAL_INT;
+
+	case IIO_CHAN_INFO_SCALE:
+		/* 1200 mV full range with 10-bit resolution. */
+		*val = 1200;
+		if (chan->channel == MT6397_AUXADC_ISENSE)
+			*val *= MT6397_AUXADC_ISENSE_DIVIDER;
+		*val2 = 10;
+
+		return IIO_VAL_FRACTIONAL_LOG2;
+
+	default:
+		return -EINVAL;
+	}
+}
+
+static int mt6397_auxadc_init(struct mt6397_auxadc *adc)
+{
+	return regmap_update_bits(adc->regmap, MT6397_AUXADC_CON0,
+				  MT6397_AUXADC_CON0_SPL_NUM,
+				  FIELD_PREP(MT6397_AUXADC_CON0_SPL_NUM, 1));
+}
+
+static const struct iio_info mt6397_auxadc_info = {
+	.read_raw = mt6397_auxadc_read_raw,
+};
+
+static int mt6397_auxadc_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct mt6397_chip *chip = dev_get_drvdata(dev->parent);
+	struct mt6397_auxadc *adc;
+	struct iio_dev *indio_dev;
+	int ret;
+
+	indio_dev = devm_iio_device_alloc(dev, sizeof(*adc));
+	if (!indio_dev)
+		return -ENOMEM;
+
+	adc = iio_priv(indio_dev);
+	adc->regmap = chip->regmap;
+
+	ret = devm_mutex_init(dev, &adc->lock);
+	if (ret)
+		return ret;
+
+	ret = mt6397_auxadc_init(adc);
+	if (ret)
+		return dev_err_probe(dev, ret, "failed to initialize auxadc\n");
+
+	indio_dev->name = "mt6397-auxadc";
+	indio_dev->info = &mt6397_auxadc_info;
+	indio_dev->modes = INDIO_DIRECT_MODE;
+	indio_dev->channels = mt6397_auxadc_channels;
+	indio_dev->num_channels = ARRAY_SIZE(mt6397_auxadc_channels);
+
+	return devm_iio_device_register(dev, indio_dev);
+}
+
+static const struct of_device_id mt6397_auxadc_of_match[] = {
+	{ .compatible = "mediatek,mt6397-auxadc" },
+	{ /* sentinel */ }
+};
+MODULE_DEVICE_TABLE(of, mt6397_auxadc_of_match);
+
+static struct platform_driver mt6397_auxadc_driver = {
+	.driver = {
+		.name = "mt6397-auxadc",
+		.of_match_table = mt6397_auxadc_of_match,
+	},
+	.probe = mt6397_auxadc_probe,
+};
+module_platform_driver(mt6397_auxadc_driver);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Ryan Brue <ryanbrue.dev@xxxxxxxxx>");
+MODULE_DESCRIPTION("MediaTek MT6397 PMIC AUXADC Driver");

-- 
2.55.0