[PATCH v4 3/3] iio: dac: mcp4821: add configurable gain and fix scale handling

[Nikhil Gautam <nikhilgtr@gmail.com>]

Add support for configuring the DAC gain using the GA bit and
update scale handling to follow the IIO ABI.

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

Scale is now exposed via IIO_CHAN_INFO_SCALE and reflects the
selected gain. The driver returns scale using
IIO_VAL_FRACTIONAL_LOG2 and provides scale_available with the
corresponding values.

Writes to the scale attribute are validated and mapped to the
appropriate gain setting. Only supported scale values are accepted,
ensuring consistency between scale and scale_available.

Signed-off-by: Nikhil Gautam <nikhilgtr@gmail.com>
---
v4:
- Split changes into separate patches
- Fix scale handling to comply with IIO ABI
- Use IIO_VAL_FRACTIONAL_LOG2 for scale and scale_available
- Ensure scale_available matches scale
- Handle sysfs write inputs correctly (INT+MICRO)
- Reject invalid scale values

v3:
- Restore NULL check in indio_dev allocation

v2:
- Use IIO_CHAN_INFO_SCALE instead of CALIBSCALE
- Fix error handling and cleanup
---
 drivers/iio/dac/mcp4821.c | 103 ++++++++++++++++++++++++++++++--------
 1 file changed, 83 insertions(+), 20 deletions(-)

diff --git a/drivers/iio/dac/mcp4821.c b/drivers/iio/dac/mcp4821.c
index 6b732db1cf49..db659b5be8f8 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
  */
 
@@ -26,10 +25,30 @@
 #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 +62,7 @@ enum mcp4821_supported_device_ids {
 struct mcp4821_state {
 	struct spi_device *spi;
 	u16 dac_value[2];
+	int gain;
 };
 
 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 +143,9 @@ 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:
@@ -140,34 +162,72 @@ 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;
 
-	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:
+
+		if (val == 0 && val2 == 500000)
+			state->gain = 1;
+		else if (val == 1 && val2 == 0)
+			state->gain = 2;
+		else
+			return -EINVAL;
+		return 0;
+
+	default:
+		return -EINVAL;
 	}
+}
 
-	state->dac_value[chan->channel] = val;
+static const int mcp4821_gain_avail[] = {
+	MCP4821_VREF_MV, 12,
+	MCP4821_VREF_MV * 2, 12,};
 
-	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_FRACTIONAL_LOG2;
+		*length = ARRAY_SIZE(mcp4821_gain_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)
@@ -183,6 +243,9 @@ static int mcp4821_probe(struct spi_device *spi)
 	state = iio_priv(indio_dev);
 	state->spi = spi;
 
+	/* default gain is 2x as GA bit is active low*/
+	state->gain = 2;
+
 	info = spi_get_device_match_data(spi);
 	indio_dev->name = info->name;
 	indio_dev->info = &mcp4821_info;
-- 
2.43.0