[PATCH v2] iio: dac: mcp4821: add configurable gain support

[Nikhil Gautam <nikhilgtr@gmail.com>]

Add support for configuring the DAC gain using the GA bit.
Expose gain control via IIO_CHAN_INFO_CALIBSCALE.

Scale is updated dynamically based on selected gain:
- 1x gain → 2.048V full-scale
- 2x gain → 4.096V full-scale

Signed-off-by: Nikhil Gautam <nikhilgtr@gmail.com>
---
Changes in v2:
- Drop header reordering
- Move MCP4821_GAIN_ENABLE define before MCP4802_SECOND_CHAN
- Use IIO_CHAN_INFO_SCALE instead of CALIBSCALE since full scale changes
- Restore original -EINVAL handling in default cases
---
 drivers/iio/dac/mcp4821.c | 105 ++++++++++++++++++++++++++++++--------
 1 file changed, 83 insertions(+), 22 deletions(-)

diff --git a/drivers/iio/dac/mcp4821.c b/drivers/iio/dac/mcp4821.c
index 748bdca9a964..362ca12988dc 100644
--- a/drivers/iio/dac/mcp4821.c
+++ b/drivers/iio/dac/mcp4821.c
@@ -12,7 +12,6 @@
  *	MCP48x2: https://ww1.microchip.com/downloads/en/DeviceDoc/20002249B.pdf
  *
  * TODO:
- *	- Configurable gain
  *	- Regulator control
  */
 
@@ -26,12 +25,32 @@
 #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
 
-enum mcp4821_supported_drvice_ids {
+/*
+ * 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,
 	ID_MCP4802,
 	ID_MCP4811,
@@ -43,6 +62,7 @@ enum mcp4821_supported_drvice_ids {
 struct mcp4821_state {
 	struct spi_device *spi;
 	u16 dac_value[2];
+	bool gain_1x;
 };
 
 struct mcp4821_chip_info {
@@ -57,6 +77,7 @@ 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),                   \
@@ -122,8 +143,13 @@ static int mcp4821_read_raw(struct iio_dev *indio_dev,
 		state = iio_priv(indio_dev);
 		*val = state->dac_value[chan->channel];
 		return IIO_VAL_INT;
+
 	case IIO_CHAN_INFO_SCALE:
-		*val = MCP4821_2X_GAIN_VREF_MV;
+		state = iio_priv(indio_dev);
+		if (state->gain_1x)
+			*val = MCP4821_VREF_MV;
+		else
+			*val = MCP4821_VREF_MV * 2;
 		*val2 = chan->scan_type.realbits;
 		return IIO_VAL_FRACTIONAL_LOG2;
 	default:
@@ -140,34 +166,66 @@ static int mcp4821_write_raw(struct iio_dev *indio_dev,
 	__be16 write_buffer;
 	int ret;
 
-	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;
+		if (state->gain_1x)
+			write_val |= MCP4821_GAIN_ENABLE;
 
-	write_val = MCP4821_ACTIVE_MODE | val << chan->scan_type.shift;
-	if (chan->channel)
-		write_val |= MCP4802_SECOND_CHAN;
+		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:
+		if (val == 1)
+			state->gain_1x = true;
+		else if (val == 2)
+			state->gain_1x = false;
+		else
+			return -EINVAL;
+		return 0;
+	default:
+		return -EINVAL;
 	}
+}
 
-	state->dac_value[chan->channel] = val;
+static const int mcp4821_gain_avail[] = {1, 2};
 
-	return 0;
+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)
+{
+	switch (info) {
+	case IIO_CHAN_INFO_SCALE:
+		*vals = mcp4821_gain_avail;
+		*type = IIO_VAL_INT;
+		*length = 2;
+		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)
@@ -177,12 +235,15 @@ static int mcp4821_probe(struct spi_device *spi)
 	const struct mcp4821_chip_info *info;
 
 	indio_dev = devm_iio_device_alloc(&spi->dev, sizeof(*state));
-	if (indio_dev == NULL)
+	if (!indio_dev)
 		return -ENOMEM;
 
 	state = iio_priv(indio_dev);
 	state->spi = spi;
 
+	/* default gain is 2x as GA bit is active low*/
+	state->gain_1x = false;
+
 	info = spi_get_device_match_data(spi);
 	indio_dev->name = info->name;
 	indio_dev->info = &mcp4821_info;
-- 
2.43.0