[PATCH v6 3/3] iio: dac: mcp4821: add configurable gain support

Nikhil Gautam
2026-04-14T17:03:45Z

Add support for configuring the DAC gain using the GA bit

The MCP4821 supports two gain settings:
- 1x gain → 2.048V full-scale
- 2x gain → 4.096V full-scale

Scale write support is added in the IIO interface. Only scale
values advertised via the scale_available attribute are accepted,
ensuring consistency between the configured gain and exposed
scale values.

Signed-off-by: Nikhil Gautam <nikhilgtr@gmail.com>
---
drivers/iio/dac/mcp4821.c | 128 ++++++++++++++++++++++++++++++++------
1 file changed, 108 insertions(+), 20 deletions(-)

diff --git a/drivers/iio/dac/mcp4821.c b/drivers/iio/dac/mcp4821.c
index 102ae3718514..18b5934fb8a2 100644
--- a/drivers/iio/dac/mcp4821.c
+++ b/drivers/iio/dac/mcp4821.c
@@ -12,13 +12,13 @@
* MCP48x2: https://ww1.microchip.com/downloads/en/DeviceDoc/20002249B.pdf
*
* TODO:
- * - Configurable gain
* - Regulator control
*/

#include <linux/module.h>
#include <linux/mod_devicetable.h>
#include <linux/spi/spi.h>
+#include <linux/units.h>

#include <linux/iio/iio.h>
#include <linux/iio/types.h>
@@ -26,10 +26,30 @@
#include <linux/unaligned.h>

#define MCP4821_ACTIVE_MODE BIT(12)
+#define MCP4821_GAIN_ENABLE BIT(13)
#define MCP4802_SECOND_CHAN BIT(15)

-/* DAC uses an internal Voltage reference of 4.096V at a gain of 2x */
-#define MCP4821_2X_GAIN_VREF_MV 4096
+/* DAC uses an internal Voltage reference of 2.048V */
+#define MCP4821_VREF_MV 2048
+
+/*
+ * MCP48xx DAC output:
+ *
+ * Vout = (Vref * D / 2^N) * G
+ *
+ * where:
+ * - Vref = 2.048V (internal reference)
+ * - N = DAC resolution (12 bits for MCP4821)
+ * - G = gain selection:
+ * 1x when GA bit = 1
+ * 2x when GA bit = 0 (default)
+ *
+ * Therefore full-scale voltage is:
+ * - 1x gain: 2.048V
+ * - 2x gain: 4.096V
+ *
+ * Scale = Vfull-scale / 2^N
+ */

enum mcp4821_supported_device_ids {
ID_MCP4801,
@@ -43,6 +63,8 @@ enum mcp4821_supported_device_ids {
struct mcp4821_state {
struct spi_device *spi;
u16 dac_value[2];
+ int gain;
+ int scale_avail[4];
};

struct mcp4821_chip_info {
@@ -57,6 +79,7 @@ struct mcp4821_chip_info {
.channel = (channel_id), \
.info_mask_separate = BIT(IIO_CHAN_INFO_RAW), \
.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE), \
+ .info_mask_shared_by_type_available = BIT(IIO_CHAN_INFO_SCALE), \
.scan_type = { \
.realbits = (resolution), \
.shift = 12 - (resolution), \
@@ -121,8 +144,9 @@ static int mcp4821_read_raw(struct iio_dev *indio_dev,
case IIO_CHAN_INFO_RAW:
*val = state->dac_value[chan->channel];
return IIO_VAL_INT;
+
case IIO_CHAN_INFO_SCALE:
- *val = MCP4821_2X_GAIN_VREF_MV;
+ *val = MCP4821_VREF_MV * state->gain;
*val2 = chan->scan_type.realbits;
return IIO_VAL_FRACTIONAL_LOG2;
default:
@@ -130,6 +154,17 @@ static int mcp4821_read_raw(struct iio_dev *indio_dev,
}
}

+static void mcp4821_calc_scale(int vref_mv, int resolution,
+ int *val, int *val2)
+{
+ s64 tmp;
+ int micro;
+
+ tmp = (s64)vref_mv * MICRO >> resolution;
+ *val = div_s64_rem(tmp, MICRO, &micro);
+ *val2 = micro;
+}
+
static int mcp4821_write_raw(struct iio_dev *indio_dev,
struct iio_chan_spec const *chan, int val,
int val2, long mask)
@@ -138,35 +173,85 @@ static int mcp4821_write_raw(struct iio_dev *indio_dev,
u16 write_val;
__be16 write_buffer;
int ret;
+ int v, v2;

- if (val2 != 0)
- return -EINVAL;
+ switch (mask) {
+ case IIO_CHAN_INFO_RAW:

- if (val < 0 || val >= BIT(chan->scan_type.realbits))
- return -EINVAL;
+ if (val2 != 0)
+ return -EINVAL;

- if (mask != IIO_CHAN_INFO_RAW)
- return -EINVAL;
+ if (val < 0 || val >= BIT(chan->scan_type.realbits))
+ return -EINVAL;
+
+ write_val = MCP4821_ACTIVE_MODE | val << chan->scan_type.shift;
+ if (chan->channel)
+ write_val |= MCP4802_SECOND_CHAN;

- write_val = MCP4821_ACTIVE_MODE | val << chan->scan_type.shift;
- if (chan->channel)
- write_val |= MCP4802_SECOND_CHAN;
+ /* GA bit = 1 -> 1x gain */
+ if (state->gain == 1)
+ write_val |= MCP4821_GAIN_ENABLE;

- write_buffer = cpu_to_be16(write_val);
- ret = spi_write(state->spi, &write_buffer, sizeof(write_buffer));
- if (ret) {
- dev_err(&state->spi->dev, "Failed to write to device: %d", ret);
- return ret;
+ write_buffer = cpu_to_be16(write_val);
+ ret = spi_write(state->spi, &write_buffer, sizeof(write_buffer));
+ if (ret) {
+ dev_err(&state->spi->dev, "Failed to write to device: %d", ret);
+ return ret;
+ }
+
+ state->dac_value[chan->channel] = val;
+ return 0;
+
+ case IIO_CHAN_INFO_SCALE:
+ mcp4821_calc_scale(MCP4821_VREF_MV, chan->scan_type.realbits, &v, &v2);
+ if (val == v && val2 == v2) {
+ state->gain = 1;
+ return 0;
+ }
+
+ mcp4821_calc_scale(MCP4821_VREF_MV * 2,
+ chan->scan_type.realbits, &v, &v2);
+ if (val == v && val2 == v2) {
+ state->gain = 2;
+ return 0;
+ }
+ return -EINVAL;
+ default:
+ return -EINVAL;
}
+}

- state->dac_value[chan->channel] = val;
+static inline void mcp4821_init_avail_gain(struct mcp4821_state *state,
+ int resolution)
+{
+ state->scale_avail[0] = MCP4821_VREF_MV;
+ state->scale_avail[1] = resolution;
+ state->scale_avail[2] = MCP4821_VREF_MV * 2;
+ state->scale_avail[3] = resolution;
+}
+
+static int mcp4821_read_avail(struct iio_dev *indio_dev,
+ struct iio_chan_spec const *chan,
+ const int **vals, int *type, int *length,
+ long info)
+{
+ struct mcp4821_state *state = iio_priv(indio_dev);

- return 0;
+ switch (info) {
+ case IIO_CHAN_INFO_SCALE:
+ *vals = state->scale_avail;
+ *type = IIO_VAL_FRACTIONAL_LOG2;
+ *length = ARRAY_SIZE(state->scale_avail);
+ return IIO_AVAIL_LIST;
+ default:
+ return -EINVAL;
+ }
}

static const struct iio_info mcp4821_info = {
.read_raw = &mcp4821_read_raw,
.write_raw = &mcp4821_write_raw,
+ .read_avail = &mcp4821_read_avail,
};

static int mcp4821_probe(struct spi_device *spi)
@@ -182,12 +267,15 @@ static int mcp4821_probe(struct spi_device *spi)
state = iio_priv(indio_dev);
state->spi = spi;

+ /* default gain is 2x */
+ state->gain = 2;
info = spi_get_device_match_data(spi);
indio_dev->name = info->name;
indio_dev->info = &mcp4821_info;
indio_dev->modes = INDIO_DIRECT_MODE;
indio_dev->channels = info->channels;
indio_dev->num_channels = info->num_channels;
+ mcp4821_init_avail_gain(state, info->channels[0].scan_type.realbits);

return devm_iio_device_register(&spi->dev, indio_dev);
}
--
2.43.0