iio: dac: mcp4821: add configurable gain support

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>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

diff --git a/drivers/iio/dac/mcp4821.c b/drivers/iio/dac/mcp4821.c
index 102ae37185148b3c1e4643c23bffb1980afff84a..18b5934fb8a20a73a9b356980b2a02f44d4e8c7e 100644 (file)
--- 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);
 }