[PATCH v9 3/6] iio: adc: ltc2378: Enable high-speed data capture

[Marcelo Schmitt <marcelo.schmitt@analog.com>]

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

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
---
Change log v8 -> v9:
No changes in v9.

 drivers/iio/adc/Kconfig   |   6 +
 drivers/iio/adc/ltc2378.c | 404 +++++++++++++++++++++++++++++++++++++-
 2 files changed, 407 insertions(+), 3 deletions(-)

diff --git a/drivers/iio/adc/Kconfig b/drivers/iio/adc/Kconfig
index 220c6739a8f2..96df8453b826 100644
--- a/drivers/iio/adc/Kconfig
+++ b/drivers/iio/adc/Kconfig
@@ -948,6 +948,12 @@ config LTC2378
 	depends on SPI
 	depends on REGULATOR || COMPILE_TEST
 	depends on GPIOLIB
+	depends on PWM
+	select IIO_BUFFER
+	select IIO_BUFFER_DMA
+	select IIO_BUFFER_DMAENGINE
+	select SPI_OFFLOAD
+	select SPI_OFFLOAD_TRIGGER_PWM
 	help
 	  Say yes here to build support for Analog Devices LTC2378-20 and
 	  similar analog to digital converters.
diff --git a/drivers/iio/adc/ltc2378.c b/drivers/iio/adc/ltc2378.c
index d161435fc3a3..48380a2d7dc6 100644
--- a/drivers/iio/adc/ltc2378.c
+++ b/drivers/iio/adc/ltc2378.c
@@ -16,16 +16,27 @@
 #include <linux/device-id/of.h>
 #include <linux/err.h>
 #include <linux/gpio/consumer.h>
+#include <linux/math64.h>
+#include <linux/minmax.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
 #include <linux/regulator/consumer.h>
+#include <linux/pwm.h>
 #include <linux/spi/spi.h>
+#include <linux/spi/offload/consumer.h>
+#include <linux/spi/offload/types.h>
+#include <linux/time64.h>
 #include <linux/types.h>
 #include <linux/units.h>
 
+#include <linux/iio/buffer.h>
+#include <linux/iio/buffer-dmaengine.h>
 #include <linux/iio/iio.h>
 #include <linux/iio/types.h>
 
+#define LTC2378_TDSDOBUSYL_NS		5
+#define LTC2378_TBUSYLH_NS		13
+#define LTC2378_TCNV_HIGH_NS		20
 #define LTC2378_MAX_DATA_WAIT_US	4 /* max(TBUSYLH + TCONV + TDSDOBUSYL) */
 
 #define LTC2378_CHANNEL(_sign, _real_bits, _storage_bits)			\
@@ -54,9 +65,49 @@
 #define LTC2378_PSEUDO_DIFF_CHANNEL(_real_bits)					\
 	LTC2378_CHANNEL(0, _real_bits, (((_real_bits) > 16) ? 32 : 16))
 
+#define LTC2378_OFFLOAD_CHANNEL(_sign, _real_bits, _storage_bits)		\
+{										\
+	.type = IIO_VOLTAGE,							\
+	.indexed = 1,								\
+	.differential = _sign,							\
+	.channel = 0,								\
+	.channel2 = _sign ? 1 : 0,						\
+	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |				\
+			      BIT(IIO_CHAN_INFO_SCALE) |			\
+			      BIT(IIO_CHAN_INFO_SAMP_FREQ),			\
+	.info_mask_separate_available = BIT(IIO_CHAN_INFO_SAMP_FREQ),		\
+	.scan_index = 0,							\
+	.scan_type = {								\
+		.format = _sign ? IIO_SCAN_FORMAT_SIGNED_INT :			\
+				  IIO_SCAN_FORMAT_UNSIGNED_INT,			\
+		.realbits = _real_bits,						\
+		.storagebits = _storage_bits,					\
+		.shift = 0,							\
+		.endianness = IIO_CPU,						\
+	},									\
+}
+
+/*
+ * Currently, the available offload hardware + DMA configuration only supports
+ * pushing 32-bit data elements to DMA IIO buffers in CPU endianness. For 16-bit
+ * precision parts, those 32-bit elements (in CPU endianness) contain 2 bytes
+ * with data and 2 bytes always zeroed out. Nevertheless, for the offload use
+ * case, the IIO buffer is configured for 32 storage bits in CPU endianness so
+ * data is correctly aligned in user space despite 2 out of the 4 bytes being
+ * zeros.
+ */
+#define LTC2378_OFFLOAD_DIFF_CHANNEL(_real_bits)			\
+	LTC2378_OFFLOAD_CHANNEL(1, (_real_bits), 32)
+
+#define LTC2378_OFFLOAD_PSEUDO_DIFF_CHANNEL(_real_bits)				\
+	LTC2378_OFFLOAD_CHANNEL(0, (_real_bits), 32)
+
 struct ltc2378_chip_info {
 	const char *name;
 	struct iio_chan_spec chan;
+	struct iio_chan_spec offload_chan;
+	unsigned int max_sample_rate_Hz;
+	unsigned int tconv_ns;
 };
 
 struct ltc2378_state {
@@ -66,6 +117,15 @@ struct ltc2378_state {
 	struct mutex lock; /* Protect data acquisition cycle */
 	int ref_uV;
 	struct spi_transfer xfer;
+	struct spi_transfer offload_xfer;
+	struct spi_offload *offload;
+	struct spi_offload_trigger *offload_trigger;
+	struct pwm_waveform cnv_wf;
+	struct spi_message offload_msg;
+	struct spi_offload_trigger_config offload_trigger_config;
+	struct pwm_device *cnv_trigger;
+	unsigned int cnv_Hz;
+	unsigned int sample_freq_range[3];
 
 	/*
 	 * DMA (thus cache coherency maintenance) requires the transfer buffers
@@ -85,96 +145,153 @@ struct ltc2378_state {
 static const struct ltc2378_chip_info ltc2364_16_chip_info = {
 	.name = "ltc2364-16",
 	.chan = LTC2378_PSEUDO_DIFF_CHANNEL(16),
+	.offload_chan = LTC2378_OFFLOAD_PSEUDO_DIFF_CHANNEL(16),
+	.max_sample_rate_Hz = 250 * HZ_PER_KHZ,
+	.tconv_ns = 3000,
 };
 
 static const struct ltc2378_chip_info ltc2364_18_chip_info = {
 	.name = "ltc2364-18",
 	.chan = LTC2378_PSEUDO_DIFF_CHANNEL(18),
+	.offload_chan = LTC2378_OFFLOAD_PSEUDO_DIFF_CHANNEL(18),
+	.max_sample_rate_Hz = 250 * HZ_PER_KHZ,
+	.tconv_ns = 3000,
 };
 
 static const struct ltc2378_chip_info ltc2367_16_chip_info = {
 	.name = "ltc2367-16",
 	.chan = LTC2378_PSEUDO_DIFF_CHANNEL(16),
+	.offload_chan = LTC2378_OFFLOAD_PSEUDO_DIFF_CHANNEL(16),
+	.max_sample_rate_Hz = 500 * HZ_PER_KHZ,
+	.tconv_ns = 1500,
 };
 
 static const struct ltc2378_chip_info ltc2367_18_chip_info = {
 	.name = "ltc2367-18",
 	.chan = LTC2378_PSEUDO_DIFF_CHANNEL(18),
+	.offload_chan = LTC2378_OFFLOAD_PSEUDO_DIFF_CHANNEL(18),
+	.max_sample_rate_Hz = 500 * HZ_PER_KHZ,
+	.tconv_ns = 1500,
 };
 
 static const struct ltc2378_chip_info ltc2368_16_chip_info = {
 	.name = "ltc2368-16",
 	.chan = LTC2378_PSEUDO_DIFF_CHANNEL(16),
+	.offload_chan = LTC2378_OFFLOAD_PSEUDO_DIFF_CHANNEL(16),
+	.max_sample_rate_Hz = 1 * HZ_PER_MHZ,
+	.tconv_ns = 527,
 };
 
 static const struct ltc2378_chip_info ltc2368_18_chip_info = {
 	.name = "ltc2368-18",
 	.chan = LTC2378_PSEUDO_DIFF_CHANNEL(18),
+	.offload_chan = LTC2378_OFFLOAD_PSEUDO_DIFF_CHANNEL(18),
+	.max_sample_rate_Hz = 1 * HZ_PER_MHZ,
+	.tconv_ns = 527,
 };
 
 static const struct ltc2378_chip_info ltc2369_18_chip_info = {
 	.name = "ltc2369-18",
 	.chan = LTC2378_PSEUDO_DIFF_CHANNEL(18),
+	.offload_chan = LTC2378_OFFLOAD_PSEUDO_DIFF_CHANNEL(18),
+	.max_sample_rate_Hz = 1600 * HZ_PER_KHZ,
+	.tconv_ns = 412,
 };
 
 static const struct ltc2378_chip_info ltc2370_16_chip_info = {
 	.name = "ltc2370-16",
 	.chan = LTC2378_PSEUDO_DIFF_CHANNEL(16),
+	.offload_chan = LTC2378_OFFLOAD_PSEUDO_DIFF_CHANNEL(16),
+	.max_sample_rate_Hz = 2 * HZ_PER_MHZ,
+	.tconv_ns = 322,
 };
 
 static const struct ltc2378_chip_info ltc2376_16_chip_info = {
 	.name = "ltc2376-16",
 	.chan = LTC2378_DIFF_CHANNEL(16),
+	.offload_chan = LTC2378_OFFLOAD_DIFF_CHANNEL(16),
+	.max_sample_rate_Hz = 250 * HZ_PER_KHZ,
+	.tconv_ns = 3000,
 };
 
 static const struct ltc2378_chip_info ltc2376_18_chip_info = {
 	.name = "ltc2376-18",
 	.chan = LTC2378_DIFF_CHANNEL(18),
+	.offload_chan = LTC2378_OFFLOAD_DIFF_CHANNEL(18),
+	.max_sample_rate_Hz = 250 * HZ_PER_KHZ,
+	.tconv_ns = 3000,
 };
 
 static const struct ltc2378_chip_info ltc2376_20_chip_info = {
 	.name = "ltc2376-20",
 	.chan = LTC2378_DIFF_CHANNEL(20),
+	.offload_chan = LTC2378_OFFLOAD_DIFF_CHANNEL(20),
+	.max_sample_rate_Hz = 250 * HZ_PER_KHZ,
+	.tconv_ns = 3000,
 };
 
 static const struct ltc2378_chip_info ltc2377_16_chip_info = {
 	.name = "ltc2377-16",
 	.chan = LTC2378_DIFF_CHANNEL(16),
+	.offload_chan = LTC2378_OFFLOAD_DIFF_CHANNEL(16),
+	.max_sample_rate_Hz = 500 * HZ_PER_KHZ,
+	.tconv_ns = 1500,
 };
 
 static const struct ltc2378_chip_info ltc2377_18_chip_info = {
 	.name = "ltc2377-18",
 	.chan = LTC2378_DIFF_CHANNEL(18),
+	.offload_chan = LTC2378_OFFLOAD_DIFF_CHANNEL(18),
+	.max_sample_rate_Hz = 500 * HZ_PER_KHZ,
+	.tconv_ns = 1500,
 };
 
 static const struct ltc2378_chip_info ltc2377_20_chip_info = {
 	.name = "ltc2377-20",
 	.chan = LTC2378_DIFF_CHANNEL(20),
+	.offload_chan = LTC2378_OFFLOAD_DIFF_CHANNEL(20),
+	.max_sample_rate_Hz = 500 * HZ_PER_KHZ,
+	.tconv_ns = 1500,
 };
 
 static const struct ltc2378_chip_info ltc2378_16_chip_info = {
 	.name = "ltc2378-16",
 	.chan = LTC2378_DIFF_CHANNEL(16),
+	.offload_chan = LTC2378_OFFLOAD_DIFF_CHANNEL(16),
+	.max_sample_rate_Hz = 1 * HZ_PER_MHZ,
+	.tconv_ns = 527,
 };
 
 static const struct ltc2378_chip_info ltc2378_18_chip_info = {
 	.name = "ltc2378-18",
 	.chan = LTC2378_DIFF_CHANNEL(18),
+	.offload_chan = LTC2378_OFFLOAD_DIFF_CHANNEL(18),
+	.max_sample_rate_Hz = 1 * HZ_PER_MHZ,
+	.tconv_ns = 527,
 };
 
 static const struct ltc2378_chip_info ltc2378_20_chip_info = {
 	.name = "ltc2378-20",
 	.chan = LTC2378_DIFF_CHANNEL(20),
+	.offload_chan = LTC2378_OFFLOAD_DIFF_CHANNEL(20),
+	.max_sample_rate_Hz = 1 * HZ_PER_MHZ,
+	.tconv_ns = 675,
 };
 
 static const struct ltc2378_chip_info ltc2379_18_chip_info = {
 	.name = "ltc2379-18",
 	.chan = LTC2378_DIFF_CHANNEL(18),
+	.offload_chan = LTC2378_OFFLOAD_DIFF_CHANNEL(18),
+	.max_sample_rate_Hz = 1600 * HZ_PER_KHZ,
+	.tconv_ns = 412,
 };
 
 static const struct ltc2378_chip_info ltc2380_16_chip_info = {
 	.name = "ltc2380-16",
 	.chan = LTC2378_DIFF_CHANNEL(16),
+	.offload_chan = LTC2378_OFFLOAD_DIFF_CHANNEL(16),
+	.max_sample_rate_Hz = 2 * HZ_PER_MHZ,
+	.tconv_ns = 322,
 };
 
 static int ltc2378_convert_and_acquire(struct ltc2378_state *st)
@@ -257,7 +374,137 @@ static int ltc2378_read_raw(struct iio_dev *indio_dev,
 			*val2 = chan->scan_type.realbits;
 
 		return IIO_VAL_FRACTIONAL_LOG2;
+	case IIO_CHAN_INFO_SAMP_FREQ:
+		*val = st->cnv_Hz;
+		return IIO_VAL_INT;
+	default:
+		return -EINVAL;
+	}
+}
 
+static int ltc2378_read_avail(struct iio_dev *indio_dev,
+			      struct iio_chan_spec const *chan,
+			      const int **vals, int *type, int *length, long mask)
+{
+	struct ltc2378_state *st = iio_priv(indio_dev);
+
+	switch (mask) {
+	case IIO_CHAN_INFO_SAMP_FREQ:
+		*vals = st->sample_freq_range;
+		*type = IIO_VAL_INT;
+		return IIO_AVAIL_RANGE;
+	default:
+		return -EINVAL;
+	}
+}
+
+/*
+ * SPI offload wiring schema
+ *
+ *     +-------------+         +-------------+
+ *     |         CNV |<-----+--| GPIO        |
+ *     |             |      +--| PWM0        |
+ *     |             |         |             |
+ *     |             |      +--| PWM1        |
+ *     |             |      |  +-------------+
+ *     |             |      +->| TRIGGER     |
+ *     |             |         |             |
+ *     |     ADC     |         |    SPI      |
+ *     |             |         | controller  |
+ *     |             |         |             |
+ *     |         SDI |<--------| SDO         |
+ *     |         SDO |-------->| SDI         |
+ *     |        SCLK |<--------| SCLK        |
+ *     +-------------+         +-------------+
+ *
+ */
+static int ltc2378_update_conversion_rate(struct ltc2378_state *st, int freq_Hz)
+{
+	struct spi_offload_trigger_config config = st->offload_trigger_config;
+	unsigned int min_read_offset, offload_period_ns;
+	struct pwm_waveform cnv_wf = { };
+	u64 target = LTC2378_TCNV_HIGH_NS;
+	unsigned int count;
+	u64 offload_offset_ns;
+	int ret;
+
+	if (freq_Hz == 0)
+		return -EINVAL;
+
+	if (!in_range(freq_Hz, 1, st->info->max_sample_rate_Hz))
+		return -ERANGE;
+
+	/* Configure CNV PWM waveform */
+	cnv_wf.period_length_ns = DIV_ROUND_CLOSEST(NSEC_PER_SEC, freq_Hz);
+
+	/*
+	 * Ensure CNV high time meets minimum requirement (20ns). The PWM
+	 * hardware may round the duty cycle, so iterate until we get at least
+	 * the minimum required high time (or reach a try count limit).
+	 */
+	count = 100;
+	do {
+		cnv_wf.duty_length_ns = target;
+		ret = pwm_round_waveform_might_sleep(st->cnv_trigger, &cnv_wf);
+		if (ret)
+			return ret;
+		target += 10;  /* Increment by PWM duty cycle period */
+	} while (count-- && cnv_wf.duty_length_ns < LTC2378_TCNV_HIGH_NS);
+
+	/* Check the minimum CNV high time is met */
+	if (cnv_wf.duty_length_ns < LTC2378_TCNV_HIGH_NS)
+		return -EDOM;
+
+	/*
+	 * 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_u64(HZ_PER_GHZ, offload_period_ns);
+	min_read_offset = LTC2378_TBUSYLH_NS + st->info->tconv_ns + LTC2378_TDSDOBUSYL_NS;
+	offload_offset_ns = min_read_offset;
+	count = 100;
+	do {
+		config.periodic.offset_ns = offload_offset_ns;
+		ret = spi_offload_trigger_validate(st->offload_trigger, &config);
+		if (ret)
+			return ret;
+		offload_offset_ns += 10;
+	} while (count-- && config.periodic.offset_ns < min_read_offset);
+
+	/* Check the minimum CNV to SCLK delay is met */
+	if (config.periodic.offset_ns < min_read_offset)
+		return -EDOM;
+
+	/* Check the PWM periods remain the same */
+	offload_period_ns = div64_u64(HZ_PER_GHZ, config.periodic.frequency_hz);
+	if (cnv_wf.period_length_ns != offload_period_ns)
+		return -EDOM;
+
+	st->offload_trigger_config = config;
+	st->cnv_wf = cnv_wf;
+	st->cnv_Hz = DIV_ROUND_CLOSEST_ULL(HZ_PER_GHZ, cnv_wf.period_length_ns);
+
+	return 0;
+}
+
+static int ltc2378_write_raw(struct iio_dev *indio_dev,
+			     struct iio_chan_spec const *chan,
+			     int val, int val2, long mask)
+{
+	struct ltc2378_state *st = iio_priv(indio_dev);
+
+	IIO_DEV_ACQUIRE_DIRECT_MODE(indio_dev, claim);
+	if (IIO_DEV_ACQUIRE_FAILED(claim))
+		return -EBUSY;
+
+	switch (mask) {
+	case IIO_CHAN_INFO_SAMP_FREQ:
+		return ltc2378_update_conversion_rate(st, val);
 	default:
 		return -EINVAL;
 	}
@@ -267,6 +514,111 @@ static const struct iio_info ltc2378_iio_info = {
 	.read_raw = &ltc2378_read_raw,
 };
 
+static const struct iio_info ltc2378_offload_iio_info = {
+	.read_raw = &ltc2378_read_raw,
+	.read_avail = &ltc2378_read_avail,
+	.write_raw = &ltc2378_write_raw,
+};
+
+static int ltc2378_offload_buffer_postenable(struct iio_dev *indio_dev)
+{
+	struct ltc2378_state *st = iio_priv(indio_dev);
+	int ret;
+
+	ret = pwm_set_waveform_might_sleep(st->cnv_trigger, &st->cnv_wf, true);
+	if (ret)
+		return ret;
+
+	ret = spi_offload_trigger_enable(st->offload, st->offload_trigger,
+					 &st->offload_trigger_config);
+	if (ret)
+		goto out_pwm_disable;
+
+	return 0;
+
+out_pwm_disable:
+	pwm_disable(st->cnv_trigger);
+	return ret;
+}
+
+static int ltc2378_offload_buffer_predisable(struct iio_dev *indio_dev)
+{
+	struct ltc2378_state *st = iio_priv(indio_dev);
+
+	spi_offload_trigger_disable(st->offload, st->offload_trigger);
+	pwm_disable(st->cnv_trigger);
+
+	return 0;
+}
+
+static const struct iio_buffer_setup_ops ltc2378_offload_buffer_ops = {
+	.postenable = &ltc2378_offload_buffer_postenable,
+	.predisable = &ltc2378_offload_buffer_predisable,
+};
+
+static int ltc2378_prepare_offload_message(struct device *dev,
+					   struct ltc2378_state *st)
+{
+	unsigned int resolution = st->info->offload_chan.scan_type.realbits;
+
+	st->offload_xfer.bits_per_word = resolution;
+	st->offload_xfer.len = spi_bpw_to_bytes(resolution);
+	st->offload_xfer.offload_flags = SPI_OFFLOAD_XFER_RX_STREAM;
+
+	/* Initialize message with offload */
+	spi_message_init_with_transfers(&st->offload_msg, &st->offload_xfer, 1);
+	st->offload_msg.offload = st->offload;
+
+	return devm_spi_optimize_message(dev, st->spi, &st->offload_msg);
+}
+
+static int ltc2378_spi_offload_setup(struct iio_dev *indio_dev,
+				     struct ltc2378_state *st)
+{
+	struct device *dev = &st->spi->dev;
+	struct dma_chan *rx_dma;
+
+	indio_dev->setup_ops = &ltc2378_offload_buffer_ops;
+
+	st->offload_trigger = devm_spi_offload_trigger_get(dev, st->offload,
+							   SPI_OFFLOAD_TRIGGER_PERIODIC);
+	if (IS_ERR(st->offload_trigger))
+		return dev_err_probe(dev, PTR_ERR(st->offload_trigger),
+				     "failed to get offload trigger\n");
+
+	st->offload_trigger_config.type = SPI_OFFLOAD_TRIGGER_PERIODIC;
+
+	rx_dma = devm_spi_offload_rx_stream_request_dma_chan(dev, st->offload);
+	if (IS_ERR(rx_dma))
+		return dev_err_probe(dev, PTR_ERR(rx_dma), "failed to get offload RX DMA\n");
+
+	return devm_iio_dmaengine_buffer_setup_with_handle(dev, indio_dev, rx_dma,
+							   IIO_BUFFER_DIRECTION_IN);
+}
+
+static int ltc2378_pwm_get(struct ltc2378_state *st)
+{
+	struct device *dev = &st->spi->dev;
+
+	st->cnv_trigger = devm_pwm_get(dev, NULL);
+	if (IS_ERR(st->cnv_trigger))
+		return dev_err_probe(dev, PTR_ERR(st->cnv_trigger),
+				     "failed to get cnv pwm\n");
+
+	/*
+	 * Disable the PWM connected to CNV in case it was left running by
+	 * something else.
+	 */
+	pwm_disable(st->cnv_trigger);
+
+	return 0;
+}
+
+static const struct spi_offload_config ltc2378_offload_config = {
+	.capability_flags = SPI_OFFLOAD_CAP_TRIGGER |
+			    SPI_OFFLOAD_CAP_RX_STREAM_DMA,
+};
+
 static int ltc2378_ref_setup(struct device *dev, struct ltc2378_state *st)
 {
 	int ret;
@@ -307,7 +659,6 @@ static int ltc2378_probe(struct spi_device *spi)
 		return ret;
 
 	indio_dev->name = st->info->name;
-	indio_dev->info = &ltc2378_iio_info;
 	indio_dev->modes = INDIO_DIRECT_MODE;
 
 	st->cnv_gpio = devm_gpiod_get(dev, "cnv", GPIOD_OUT_LOW);
@@ -315,8 +666,53 @@ static int ltc2378_probe(struct spi_device *spi)
 		return dev_err_probe(dev, PTR_ERR(st->cnv_gpio),
 				     "failed to get CNV GPIO");
 
-	indio_dev->channels = &st->info->chan;
-	indio_dev->num_channels = 1;
+	st->offload = devm_spi_offload_get(dev, spi, &ltc2378_offload_config);
+	ret = PTR_ERR_OR_ZERO(st->offload);
+	/* Fall back to low speed usage when no SPI offload is available. */
+	if (ret == -ENODEV) {
+		indio_dev->info = &ltc2378_iio_info;
+		indio_dev->channels = &st->info->chan;
+		indio_dev->num_channels = 1;
+	} else if (ret) {
+		return dev_err_probe(dev, ret, "failed to get offload\n");
+	} else {
+		indio_dev->info = &ltc2378_offload_iio_info;
+		indio_dev->channels = &st->info->offload_chan;
+		indio_dev->num_channels = 1;
+		ret = ltc2378_spi_offload_setup(indio_dev, st);
+		if (ret)
+			return dev_err_probe(dev, ret,
+					     "failed to setup SPI offload\n");
+
+		ret = ltc2378_pwm_get(st);
+		if (ret)
+			return dev_err_probe(dev, ret, "failed to get PWM\n");
+
+		st->sample_freq_range[0] = 1; /* min */
+		st->sample_freq_range[1] = 1; /* step */
+		st->sample_freq_range[2] = st->info->max_sample_rate_Hz; /* max */
+
+		/*
+		 * Start with a slower sampling rate so there is some room for
+		 * adjusting the sample averaging and the sampling frequency
+		 * without hitting the maximum conversion rate.
+		 */
+		ret = ltc2378_update_conversion_rate(st, st->info->max_sample_rate_Hz >> 4);
+		if (ret)
+			return dev_err_probe(dev, ret,
+					     "failed to set offload samp freq\n");
+
+		ret = ltc2378_prepare_offload_message(&spi->dev, st);
+		if (ret)
+			return dev_err_probe(dev, ret, "failed to optimize SPI message\n");
+
+		/*
+		 * Set single-read transfer bits_per_word so the SPI subsystem
+		 * rearranges data to CPU endianness, enabling us to reuse
+		 * offload_chan specifications for single-shot reads.
+		 */
+		st->xfer.bits_per_word = st->info->offload_chan.scan_type.realbits;
+	}
 
 	st->xfer.rx_buf = &st->scan.data;
 	st->xfer.len = spi_bpw_to_bytes(indio_dev->channels[0].scan_type.realbits);
@@ -385,3 +781,5 @@ module_spi_driver(ltc2378_driver);
 MODULE_AUTHOR("Marcelo Schmitt <marcelo.schmitt@analog.com>");
 MODULE_DESCRIPTION("Analog Devices LTC2378 ADC series driver");
 MODULE_LICENSE("GPL");
+MODULE_IMPORT_NS("IIO_DMAENGINE_BUFFER");
+MODULE_IMPORT_NS("SPI_OFFLOAD");
-- 
2.53.0