iio: adc: Add support for LTC2378 and similar ADCs

Support for LTC2378-20 and similar analog-to-digital converters.

Signed-off-by Marcelo Schmitt <[email protected]>

---
Change log v3 -> v4:
- Reworked the driver to make IIO channels static according to v3 feedback.
- Updated to use default 8 bits_per_word for non-offloaded transfers.
- Handled refin voltage reference supply for LTC2338.
- Used spi_bpw_to_bytes() where applicable.

 MAINTAINERS               |   1 +
 drivers/iio/adc/Kconfig   |  12 ++
 drivers/iio/adc/Makefile  |   1 +
 drivers/iio/adc/ltc2378.c | 395 ++++++++++++++++++++++++++++++++++++++
 4 files changed, 409 insertions(+)
 create mode 100644 drivers/iio/adc/ltc2378.c

drivers/iio/adc/ltc2378.c new file +15 -0

new file mode 100644
index 000000000000..9a9f32e4989b
@@ -0,0 +1,395 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Analog Devices LTC2378 ADC series driver
+ *
+ * Copyright (C) 2026 Analog Devices Inc.
+ * Author: Marcelo Schmitt <[email protected]>
+ */
+
+#include <linux/bitops.h>
+#include <linux/bits.h>
+#include <linux/cleanup.h>
+#include <linux/delay.h>
+#include <linux/err.h>
+#include <linux/gpio/consumer.h>
+#include <linux/mod_devicetable.h>

drivers/iio/adc/ltc2378.c new file +12 -0

+{										\
+	.type = IIO_VOLTAGE,							\
+	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |				\
+			      BIT(IIO_CHAN_INFO_SCALE) |			\
+			      (_offl ? BIT(IIO_CHAN_INFO_SAMP_FREQ) : 0),	\
+	.info_mask_separate_available = _offl ? BIT(IIO_CHAN_INFO_SAMP_FREQ) : 0,\
+	.scan_index = 0,							\
+	.scan_type = {								\
+		.format = _sign ? IIO_SCAN_FORMAT_SIGNED_INT :			\
+				  IIO_SCAN_FORMAT_UNSIGNED_INT,			\
+		.realbits = _real_bits,						\
+		.storagebits = _storage_bits,					\

drivers/iio/adc/ltc2378.c new file +3 -0

+	struct iio_chan_spec chan;
+};
+

drivers/iio/adc/ltc2378.c new file +10 -0

+
+	ret = devm_regulator_get_enable_read_voltage(dev, "ref");
+	if (ret < 0)
+		return dev_err_probe(dev, ret, "failed to read ref regulator\n");
+
+	st->ref_uV = ret;
+
+	return 0;
+}
+

Quoted 17 lines

drivers/iio/adc/ltc2378.c new file +17 -0

+{										\
+	.type = IIO_VOLTAGE,							\
+	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |				\
+			      BIT(IIO_CHAN_INFO_SCALE) |			\
+			      (_offl ? BIT(IIO_CHAN_INFO_SAMP_FREQ) : 0),	\
+	.info_mask_separate_available = _offl ? BIT(IIO_CHAN_INFO_SAMP_FREQ) : 0,\
+	.scan_index = 0,							\
+	.scan_type = {								\
+		.format = _sign ? IIO_SCAN_FORMAT_SIGNED_INT :			\
+				  IIO_SCAN_FORMAT_UNSIGNED_INT,			\
+		.realbits = _real_bits,						\
+		.storagebits = _storage_bits,					\
+		.shift = (_offl ? 0 : _storage_bits - _real_bits),		\
+		.endianness = _offl ? IIO_CPU : IIO_BE				\
+	},									\
+}
+

drivers/iio/adc/ltc2378.c line 47

Quoted 5 lines

drivers/iio/adc/ltc2378.c new file +5 -0

+#define LTC2378_BIPOLAR_DIFF_CHANNEL(_real_bits)				\
+	__LTC2378_DIFF_CHANNEL(1, _real_bits, (((_real_bits) > 16) ? 32 : 16), 0)
+
+#define LTC2378_UNIPOLAR_DIFF_CHANNEL(_real_bits)				\
+	__LTC2378_DIFF_CHANNEL(0, _real_bits, (((_real_bits) > 16) ? 32 : 16), 0)

Why not move the (((_real_bits) > 16) ? 32 : 16) into the __LTC2378_DIFF_CHANNEL() macro to avoid repeating it?

Quoted 4 lines

drivers/iio/adc/ltc2378.c new file +4 -0

+
+struct ltc2378_chip_info {
+	const char *name;
+	unsigned int internal_ref_uv;

drivers/iio/adc/ltc2378.c line 274

Quoted 9 lines

drivers/iio/adc/ltc2378.c new file +8 -0

+static int ltc2378_regulator_setup(struct device *dev, struct ltc2378_state *st)
+{
+	int ret;
+
+	ret = devm_regulator_get_enable_read_voltage(dev, "refin");
+	if (ret < 0 && ret != -ENODEV) {
+		return dev_err_probe(dev, ret, "failed to read refin regulator\n");
+	} else if (ret > 0) {

Else is not needed here.

drivers/iio/adc/ltc2378.c line 282

Quoted 9 lines

drivers/iio/adc/ltc2378.c new file +8 -0

+		st->ref_uV = ret;
+		return 0;
+	}
+
+	if (st->info->internal_ref_uv) {
+		st->ref_uV = st->info->internal_ref_uv;
+		return 0;
+	}

I would be tempted to have two separate functions here and only call one depending on the chip. Otherwise, it allows incorrect devicetree.

drivers/iio/adc/ltc2378.c line 292

Quoted 11 lines

drivers/iio/adc/ltc2378.c new file +10 -0

+
+	ret = devm_regulator_get_enable_read_voltage(dev, "ref");
+	if (ret < 0)
+		return dev_err_probe(dev, ret, "failed to read ref regulator\n");
+
+	st->ref_uV = ret;
+
+	return 0;
+}
+

Quoted 17 lines

Quoted 17 lines

drivers/iio/adc/ltc2378.c new file +17 -0

+{										\
+	.type = IIO_VOLTAGE,							\
+	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |				\
+			      BIT(IIO_CHAN_INFO_SCALE) |			\
+			      (_offl ? BIT(IIO_CHAN_INFO_SAMP_FREQ) : 0),	\
+	.info_mask_separate_available = _offl ? BIT(IIO_CHAN_INFO_SAMP_FREQ) : 0,\
+	.scan_index = 0,							\
+	.scan_type = {								\
+		.format = _sign ? IIO_SCAN_FORMAT_SIGNED_INT :			\
+				  IIO_SCAN_FORMAT_UNSIGNED_INT,			\
+		.realbits = _real_bits,						\
+		.storagebits = _storage_bits,					\
+		.shift = (_offl ? 0 : _storage_bits - _real_bits),		\
+		.endianness = _offl ? IIO_CPU : IIO_BE				\
+	},									\
+}
+

Quoted 9 lines

drivers/iio/adc/ltc2378.c line 47

Quoted 5 lines

drivers/iio/adc/ltc2378.c new file +5 -0

+#define LTC2378_BIPOLAR_DIFF_CHANNEL(_real_bits)				\
+	__LTC2378_DIFF_CHANNEL(1, _real_bits, (((_real_bits) > 16) ? 32 : 16), 0)
+
+#define LTC2378_UNIPOLAR_DIFF_CHANNEL(_real_bits)				\
+	__LTC2378_DIFF_CHANNEL(0, _real_bits, (((_real_bits) > 16) ? 32 : 16), 0)

Why not move the (((_real_bits) > 16) ? 32 : 16) into the __LTC2378_DIFF_CHANNEL() macro to avoid repeating it?

Because that would go wrong for LTC2378_OFFLOAD_BIPOLAR_DIFF_CHANNEL() in patch 3.

Quoted 4 lines

Quoted 4 lines

drivers/iio/adc/ltc2378.c new file +4 -0

+
+struct ltc2378_chip_info {
+	const char *name;
+	unsigned int internal_ref_uv;

...

Quoted 11 lines

drivers/iio/adc/ltc2378.c line 274

Quoted 9 lines

drivers/iio/adc/ltc2378.c new file +8 -0

+static int ltc2378_regulator_setup(struct device *dev, struct ltc2378_state *st)
+{
+	int ret;
+
+	ret = devm_regulator_get_enable_read_voltage(dev, "refin");
+	if (ret < 0 && ret != -ENODEV) {
+		return dev_err_probe(dev, ret, "failed to read refin regulator\n");
+	} else if (ret > 0) {

Else is not needed here.

Why not?

On 6/29/26 9:57 AM, Marcelo Schmitt wrote:

It could use _offl ? ... in the macro for that. Not a big deal to me though.

Quoted 10 lines

Quoted 10 lines

drivers/iio/adc/ltc2378.c line 274

Quoted 8 lines

drivers/iio/adc/ltc2378.c new file +8 -0

+static int ltc2378_regulator_setup(struct device *dev, struct ltc2378_state *st)
+{
+	int ret;
+
+	ret = devm_regulator_get_enable_read_voltage(dev, "refin");
+	if (ret < 0 && ret != -ENODEV) {
+		return dev_err_probe(dev, ret, "failed to read refin regulator\n");
+	} else if (ret > 0) {

Else is not needed here.

Quoted 1 line

Why not?

The if returns unconditionally, so else is not needed. This is just a general principal.

Quoted 10 lines

Quoted 10 lines

Quoted 10 lines

drivers/iio/adc/ltc2378.c line 274

Quoted 8 lines

drivers/iio/adc/ltc2378.c new file +8 -0

+static int ltc2378_regulator_setup(struct device *dev, struct ltc2378_state *st)
+{
+	int ret;
+
+	ret = devm_regulator_get_enable_read_voltage(dev, "refin");
+	if (ret < 0 && ret != -ENODEV) {
+		return dev_err_probe(dev, ret, "failed to read refin regulator\n");
+	} else if (ret > 0) {

Else is not needed here.

Quoted 4 lines

Quoted 1 line

Why not?

The if returns unconditionally, so else is not needed. This is just a general principal.

Unfortunately in this case ENODEV will go further, I don't see full context, but it may lead to something unexpected.

Personally I prefer the ladder of

	if (ret == -ENODEV)
		...
	else if (ret < 0)
		...
	else
		...

Make use of SPI transfer offloading to speed up data capture, enabling data acquisition at faster sample rates (up to 2 MSPS).

drivers/iio/adc/ltc2378.c +1 -0

+	unsigned int tconv_ns;
 };

drivers/iio/adc/ltc2378.c +1 -0

+		return -ERANGE;

drivers/iio/adc/ltc2378.c +10 -0

+	/*
+	 * Configure SPI offload PWM trigger.
+	 * The trigger should fire after tBUSYLH + tCONV + tDSDOBUSYL.
+	 * Minimum time needed: TBUSYLH (13ns) + TCONV (part-specific) + TDSDOBUSYL (5ns)
+	 *
+	 * Use the same period as CNV PWM to avoid timing issues.
+	 * Convert back from period to frequency for the SPI offload API.
+	 */
+	offload_period_ns = cnv_wf.period_length_ns;
+	config->periodic.frequency_hz = DIV_ROUND_UP(HZ_PER_GHZ, offload_period_ns);

drivers/iio/adc/ltc2378.c static int ltc2378_read_raw(struct iio_dev *indio_dev, · line 426

Quoted 3 lines

drivers/iio/adc/ltc2378.c +3 -0

+	/* Double check the minimum CNV high time is met */
+	if (cnv_wf.duty_length_ns < LTC2378_TCNV_HIGH_NS)
+		return -EIO;

Hmm... How is this IO issue?

Quoted 5 lines

drivers/iio/adc/ltc2378.c static int ltc2378_read_raw(struct iio_dev *indio_dev, · line 426

Quoted 3 lines

drivers/iio/adc/ltc2378.c +3 -0

+	/* Double check the minimum CNV high time is met */
+	if (cnv_wf.duty_length_ns < LTC2378_TCNV_HIGH_NS)
+		return -EIO;

Hmm... How is this IO issue?

Okay, what about EDOM for PWM setup failures? Any error code suggestion otherwise? Will follow the other suggestions.

Enable users to run triggered data captures with LTC2378 and similar ADCs.

This patch series adds support for LTC2378 and similar low noise, low power, high speed, successive approximation register (SAR) ADCs. These ADCs are similar among each other, varying mainly on the amount of precision bits, maximum sample rate, and input configuration (either fully differential or pseudo-differential).

Quoted 5 lines · Marcelo Schmitt

This patch series adds support for LTC2378 and similar low noise, low power, high speed, successive approximation register (SAR) ADCs. These ADCs are similar among each other, varying mainly on the amount of precision bits, maximum sample rate, and input configuration (either fully differential or pseudo-differential).

I had a few minor comments and I suspect Sashiko found a few legitimate things. But this is looking really nice now.

Reviewed-by David Lechner <[email protected]>