[PATCH 3/3] iio: adc: ltc2497: add temperature sensor support

[Yusuf Alper Bilgin <y.alperbilgin@gmail.com>]

The LTC2495 and LTC2499 include an internal temperature sensor. This
patch adds support for reading it via a standard IIO temperature
channel.

Signed-off-by: Yusuf Alper Bilgin <y.alperbilgin@gmail.com>
---
 drivers/iio/adc/ltc2496.c      |  1 +
 drivers/iio/adc/ltc2497-core.c | 80 ++++++++++++++++++++++++++++++++++++++----
 drivers/iio/adc/ltc2497.c      | 33 ++++++++++++++---
 drivers/iio/adc/ltc2497.h      |  7 +++-
 4 files changed, 109 insertions(+), 12 deletions(-)

diff --git a/drivers/iio/adc/ltc2496.c b/drivers/iio/adc/ltc2496.c
index f06dd0b9a85819935abea6496abe2a808ac549c6..909dbd5f9aee877aafe423b47581ff4f36a76c52 100644
--- a/drivers/iio/adc/ltc2496.c
+++ b/drivers/iio/adc/ltc2496.c
@@ -90,6 +90,7 @@ static void ltc2496_remove(struct spi_device *spi)
 static const struct ltc2497_chip_info ltc2496_info = {
 	.resolution = 16,
 	.name = NULL,
+	.has_temp_channel = false,
 };
 
 static const struct of_device_id ltc2496_of_match[] = {
diff --git a/drivers/iio/adc/ltc2497-core.c b/drivers/iio/adc/ltc2497-core.c
index 400f4fe5af30e8e16b75506726235f10f2a4237f..938259df4d27e9e26dea2be4112bef355adaaafb 100644
--- a/drivers/iio/adc/ltc2497-core.c
+++ b/drivers/iio/adc/ltc2497-core.c
@@ -72,8 +72,8 @@ static int ltc2497core_read(struct ltc2497core_driverdata *ddata, u8 address, in
 }
 
 static int ltc2497core_read_raw(struct iio_dev *indio_dev,
-			    struct iio_chan_spec const *chan,
-			    int *val, int *val2, long mask)
+				struct iio_chan_spec const *chan,
+				int *val, int *val2, long mask)
 {
 	struct ltc2497core_driverdata *ddata = iio_priv(indio_dev);
 	int ret;
@@ -93,10 +93,52 @@ static int ltc2497core_read_raw(struct iio_dev *indio_dev,
 		if (ret < 0)
 			return ret;
 
-		*val = ret / 1000;
-		*val2 = ddata->chip_info->resolution + 1;
+		switch (chan->type) {
+		case IIO_VOLTAGE:
+			*val = ret / 1000;
+			*val2 = ddata->chip_info->resolution + 1;
+
+			return IIO_VAL_FRACTIONAL_LOG2;
+
+		case IIO_TEMP:
+			if (!ddata->chip_info->has_temp_channel)
+				return -EINVAL;
+
+			/*
+			 * The datasheet formula to get Temperature in Celsius is:
+			 * Temp_C = (Conversion * Vref / temp_scale) - 273
+			 *
+			 * To match the IIO framework's model of (raw + offset) * scale,
+			 * and to get the final result in millidegrees Celsius:
+			 *
+			 * = ((Conversion * Vref / temp_scale) - 273) * 1000
+			 * = (Conversion - (273 * temp_scale / Vref)) * 1000 * Vref / temp_scale
+			 *
+			 * This gives us if the Vref is in mV:
+			 * scale  = Vref * 1000 / temp_scale
+			 * offset = -273 * temp_scale / Vref
+			 */
+			*val = ret;
+			*val2 = ddata->chip_info->temp_scale;
+
+			return IIO_VAL_FRACTIONAL;
+
+		default:
+			return -EINVAL;
+		}
+	case IIO_CHAN_INFO_OFFSET:
+		if (chan->type != IIO_TEMP)
+			return -EINVAL;
+
+		/* see the calculation above. Offset with (-273 * temp_scale / Vref) */
+		ret = regulator_get_voltage(ddata->ref);
+		if (ret < 0)
+			return ret;
 
-		return IIO_VAL_FRACTIONAL_LOG2;
+		*val = -273 * ddata->chip_info->temp_scale;
+		*val2 = ret / 1000;
+
+		return IIO_VAL_FRACTIONAL;
 
 	default:
 		return -EINVAL;
@@ -124,6 +166,14 @@ static int ltc2497core_read_raw(struct iio_dev *indio_dev,
 	.differential = 1, \
 }
 
+#define LTC2497_T_CHAN() {									\
+	.type = IIO_TEMP,									\
+	.channel = 0,										\
+	.address = (LTC2497_TEMP_CMD_ADDR),							\
+	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),						\
+	.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE) | BIT(IIO_CHAN_INFO_OFFSET),	\
+}
+
 static const struct iio_chan_spec ltc2497core_channel[] = {
 	LTC2497_CHAN(0, LTC2497_SGL, "CH0"),
 	LTC2497_CHAN(1, LTC2497_SGL, "CH1"),
@@ -159,6 +209,8 @@ static const struct iio_chan_spec ltc2497core_channel[] = {
 	LTC2497_CHAN_DIFF(7, LTC2497_DIFF | LTC2497_SIGN),
 };
 
+static const struct iio_chan_spec ltc2497_temp_channel = LTC2497_T_CHAN();
+
 static const struct iio_info ltc2497core_info = {
 	.read_raw = ltc2497core_read_raw,
 };
@@ -180,8 +232,22 @@ int ltc2497core_probe(struct device *dev, struct iio_dev *indio_dev)
 
 	indio_dev->info = &ltc2497core_info;
 	indio_dev->modes = INDIO_DIRECT_MODE;
-	indio_dev->channels = ltc2497core_channel;
-	indio_dev->num_channels = ARRAY_SIZE(ltc2497core_channel);
+
+	/* Dynamically create the channel list */
+	if (ddata->chip_info->has_temp_channel) {
+		/* Allocate space for common channels + 1 temp channel */
+		indio_dev->num_channels = ARRAY_SIZE(ltc2497core_channel) + 1;
+		indio_dev->channels = devm_kmemdup(dev, ltc2497core_channel,
+						   sizeof(ltc2497core_channel), GFP_KERNEL);
+		if (!indio_dev->channels)
+			return -ENOMEM;
+
+		memcpy((void *)&indio_dev->channels[ARRAY_SIZE(ltc2497core_channel)],
+		       &ltc2497_temp_channel, sizeof(ltc2497_temp_channel));
+	} else {
+		indio_dev->channels = ltc2497core_channel;
+		indio_dev->num_channels = ARRAY_SIZE(ltc2497core_channel);
+	}
 
 	ret = ddata->result_and_measure(ddata, LTC2497_CONFIG_DEFAULT, NULL);
 	if (ret < 0)
diff --git a/drivers/iio/adc/ltc2497.c b/drivers/iio/adc/ltc2497.c
index 8f4665547b5b0d32084599f8557c40102c37a4ce..3f8515ef465d3fda7572d63ef3a265dec89079ea 100644
--- a/drivers/iio/adc/ltc2497.c
+++ b/drivers/iio/adc/ltc2497.c
@@ -86,11 +86,31 @@ static int ltc2497_result_and_measure(struct ltc2497core_driverdata *ddata,
 			return 0;
 	}
 
-	ret = i2c_smbus_write_byte(st->client,
-				   LTC2497_ENABLE | address);
+	/*
+	 * Chips with temperature sensor support (e.g., LTC2495/LTC2499)
+	 * require a two-byte command format to select any channel.
+	 *
+	 * To read the internal temperature, LTC2497_TEMP_CMD_ADDR is sent as
+	 * the second byte. To read a voltage channel, LTC2497_EN2 is sent,
+	 * which sets the default configuration: simultaneous 50/60Hz
+	 * rejection, 1x speed, and gain=1.
+	 *
+	 * Chips without this feature use a standard single-byte command.
+	 */
+	if (ddata->chip_info->has_temp_channel) {
+		if (address == LTC2497_TEMP_CMD_ADDR)
+			ret = i2c_smbus_write_byte_data(st->client, LTC2497_ENABLE,
+							LTC2497_TEMP_CMD_ADDR);
+		else
+			ret = i2c_smbus_write_byte_data(st->client, LTC2497_ENABLE | address,
+							LTC2497_EN2);
+
+	} else {
+		ret = i2c_smbus_write_byte(st->client, LTC2497_ENABLE | address);
+	}
+
 	if (ret)
-		dev_err(&st->client->dev, "i2c transfer failed: %pe\n",
-			ERR_PTR(ret));
+		dev_err(&st->client->dev, "i2c transfer failed: %pe\n", ERR_PTR(ret));
 	return ret;
 }
 
@@ -135,14 +155,19 @@ static const struct ltc2497_chip_info ltc2497_info[] = {
 	[TYPE_LTC2495] = {
 		.resolution = 16,
 		.name = "ltc2495",
+		.has_temp_channel = true,
+		.temp_scale = 12250, /* 12.250 V per degree C -> 12250 mV */
 	},
 	[TYPE_LTC2497] = {
 		.resolution = 16,
 		.name = NULL,
+		.has_temp_channel = false,
 	},
 	[TYPE_LTC2499] = {
 		.resolution = 24,
 		.name = "ltc2499",
+		.has_temp_channel = true,
+		.temp_scale = 1570000, /* 1570 V per degree C -> 1570000 mV */
 	},
 };
 
diff --git a/drivers/iio/adc/ltc2497.h b/drivers/iio/adc/ltc2497.h
index f2139f260c3fe4e8772c6db9c46331de775dcd5c..c133a4fb8092b1c6a66a7336016dd361783f531c 100644
--- a/drivers/iio/adc/ltc2497.h
+++ b/drivers/iio/adc/ltc2497.h
@@ -1,10 +1,15 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
-#define LTC2497_ENABLE	0xA0
+#define LTC2497_ENABLE		0xA0
+#define LTC2497_EN2		0x80
+#define LTC2497_IM		0x40
+#define LTC2497_TEMP_CMD_ADDR	(LTC2497_EN2 | LTC2497_IM)
 
 struct ltc2497_chip_info {
 	u32 resolution;
 	const char *name;
+	bool has_temp_channel;
+	u32 temp_scale; /* in mV, for temp conversion */
 };
 
 struct ltc2497core_driverdata {

-- 
2.43.0