iio: chemical: scd4x: Add pressure compensation

This patch adds pressure compensation to the scd4x driver. The pressure can
be written to the sensor in hPa. The pressure will be compensated
internally by the sensor.
Signed-off-by: Roan van Dijk <roan@protonic.nl>
Link: https://lore.kernel.org/r/20230711101419.2065107-1-roan@protonic.nlSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/chemical/scd4x.c

@@ -36,6 +36,8 @@
 #define SCD4X_WRITE_BUF_SIZE 5
 #define SCD4X_FRC_MIN_PPM 0
 #define SCD4X_FRC_MAX_PPM 2000
+#define SCD4X_PRESSURE_COMP_MIN_MBAR 700
+#define SCD4X_PRESSURE_COMP_MAX_MBAR 1200
 #define SCD4X_READY_MASK 0x01
 
 /*Commands SCD4X*/
@@ -45,6 +47,8 @@ enum scd4x_cmd {
 	CMD_STOP_MEAS           = 0x3f86,
 	CMD_SET_TEMP_OFFSET     = 0x241d,
 	CMD_GET_TEMP_OFFSET     = 0x2318,
+	CMD_SET_AMB_PRESSURE	= 0xe000,
+	CMD_GET_AMB_PRESSURE	= 0xe000,
 	CMD_FRC                 = 0x362f,
 	CMD_SET_ASC             = 0x2416,
 	CMD_GET_ASC             = 0x2313,
@@ -137,7 +141,8 @@ static int scd4x_read(struct scd4x_state *state, enum scd4x_cmd cmd,
 	 * Measurement needs to be stopped before sending commands.
 	 * Except for reading measurement and data ready command.
 	 */
-	if ((cmd != CMD_GET_DATA_READY) && (cmd != CMD_READ_MEAS)) {
+	if ((cmd != CMD_GET_DATA_READY) && (cmd != CMD_READ_MEAS) &&
+	    (cmd != CMD_GET_AMB_PRESSURE)) {
 		ret = scd4x_send_command(state, CMD_STOP_MEAS);
 		if (ret)
 			return ret;
@@ -166,7 +171,8 @@ static int scd4x_read(struct scd4x_state *state, enum scd4x_cmd cmd,
 	}
 
 	/* start measurement */
-	if ((cmd != CMD_GET_DATA_READY) && (cmd != CMD_READ_MEAS)) {
+	if ((cmd != CMD_GET_DATA_READY) && (cmd != CMD_READ_MEAS) &&
+	    (cmd != CMD_GET_AMB_PRESSURE)) {
 		ret = scd4x_send_command(state, CMD_START_MEAS);
 		if (ret)
 			return ret;
@@ -188,9 +194,11 @@ static int scd4x_write(struct scd4x_state *state, enum scd4x_cmd cmd, uint16_t a
 	buf[4] = crc;
 
 	/* measurement needs to be stopped before sending commands */
-	ret = scd4x_send_command(state, CMD_STOP_MEAS);
-	if (ret)
-		return ret;
+	if (cmd != CMD_SET_AMB_PRESSURE) {
+		ret = scd4x_send_command(state, CMD_STOP_MEAS);
+		if (ret)
+			return ret;
+	}
 
 	/* execution time */
 	msleep_interruptible(500);
@@ -200,7 +208,7 @@ static int scd4x_write(struct scd4x_state *state, enum scd4x_cmd cmd, uint16_t a
 		return ret;
 
 	/* start measurement, except for forced calibration command */
-	if (cmd != CMD_FRC) {
+	if ((cmd != CMD_FRC) && (cmd != CMD_SET_AMB_PRESSURE)) {
 		ret = scd4x_send_command(state, CMD_START_MEAS);
 		if (ret)
 			return ret;
@@ -338,6 +346,18 @@ static int scd4x_read_raw(struct iio_dev *indio_dev,
 
 	switch (mask) {
 	case IIO_CHAN_INFO_RAW:
+		if (chan->output) {
+			mutex_lock(&state->lock);
+			ret = scd4x_read(state, CMD_GET_AMB_PRESSURE, &tmp, sizeof(tmp));
+			mutex_unlock(&state->lock);
+
+			if (ret)
+				return ret;
+
+			*val = be16_to_cpu(tmp);
+			return IIO_VAL_INT;
+		}
+
 		ret = iio_device_claim_direct_mode(indio_dev);
 		if (ret)
 			return ret;
@@ -386,6 +406,25 @@ static int scd4x_read_raw(struct iio_dev *indio_dev,
 	}
 }
 
+static const int scd4x_pressure_calibbias_available[] = {
+	SCD4X_PRESSURE_COMP_MIN_MBAR, 1, SCD4X_PRESSURE_COMP_MAX_MBAR,
+};
+
+static int scd4x_read_avail(struct iio_dev *indio_dev, struct iio_chan_spec const *chan,
+			    const int **vals, int *type, int *length, long mask)
+{
+	switch (mask) {
+	case IIO_CHAN_INFO_RAW:
+		*vals = scd4x_pressure_calibbias_available;
+		*type = IIO_VAL_INT;
+
+		return IIO_AVAIL_RANGE;
+	}
+
+	return -EINVAL;
+}
+
+
 static int scd4x_write_raw(struct iio_dev *indio_dev, struct iio_chan_spec const *chan,
 				int val, int val2, long mask)
 {
@@ -399,6 +438,21 @@ static int scd4x_write_raw(struct iio_dev *indio_dev, struct iio_chan_spec const
 		mutex_unlock(&state->lock);
 
 		return ret;
+	case IIO_CHAN_INFO_RAW:
+		switch (chan->type) {
+		case IIO_PRESSURE:
+			if (val < SCD4X_PRESSURE_COMP_MIN_MBAR ||
+			    val > SCD4X_PRESSURE_COMP_MAX_MBAR)
+				return -EINVAL;
+
+			mutex_lock(&state->lock);
+			ret = scd4x_write(state, CMD_SET_AMB_PRESSURE, val);
+			mutex_unlock(&state->lock);
+
+			return ret;
+		default:
+			return -EINVAL;
+		}
 	default:
 		return -EINVAL;
 	}
@@ -503,9 +557,22 @@ static const struct iio_info scd4x_info = {
 	.attrs = &scd4x_attr_group,
 	.read_raw = scd4x_read_raw,
 	.write_raw = scd4x_write_raw,
+	.read_avail = scd4x_read_avail,
 };
 
 static const struct iio_chan_spec scd4x_channels[] = {
+	{
+		/*
+		 * this channel is special in a sense we are pretending that
+		 * sensor is able to change measurement chamber pressure but in
+		 * fact we're just setting pressure compensation value
+		 */
+		.type = IIO_PRESSURE,
+		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
+		.info_mask_separate_available = BIT(IIO_CHAN_INFO_RAW),
+		.output = 1,
+		.scan_index = -1,
+	},
 	{
 		.type = IIO_CONCENTRATION,
 		.channel2 = IIO_MOD_CO2,