[PATCH v2 3/4] iio: adc: ltc2497: add temperature sensor support

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

Support for reading the internal temperature sensor on LTC2495 and
LTC2499 via a standard IIO temperature channel.

Signed-off-by: Yusuf Alper Bilgin <y.alperbilgin@gmail.com>
---
 drivers/iio/adc/ltc2497-core.c | 140 ++++++++++++++++++++++++++++++-----------
 drivers/iio/adc/ltc2497.c      |  28 ++++++++-
 drivers/iio/adc/ltc2497.h      |  17 ++++-
 3 files changed, 145 insertions(+), 40 deletions(-)

diff --git a/drivers/iio/adc/ltc2497-core.c b/drivers/iio/adc/ltc2497-core.c
index 2dc5c704426949a4ec62c42591d6c2c40ffb79cc..0e21d5f59e6108be8b5d79bd706425278043b44d 100644
--- a/drivers/iio/adc/ltc2497-core.c
+++ b/drivers/iio/adc/ltc2497-core.c
@@ -18,6 +18,7 @@
 #define LTC2497_SGL			BIT(4)
 #define LTC2497_DIFF			0
 #define LTC2497_SIGN			BIT(3)
+#define LTC2497_KELVIN_TO_CELCIUS	273
 
 static int ltc2497core_wait_conv(struct ltc2497core_driverdata *ddata)
 {
@@ -95,10 +96,53 @@ 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 = (DATAOUT * Vref_V / temp_scale) - 273
+			 *
+			 * To match the IIO framework's model of (raw + offset) * scale,
+			 * and to get the final result in millidegrees Celsius:
+			 *
+			 * Temp_mC = ((DATAOUT * Vref_mV / temp_scale_mV) - 273) * 1000
+			 * Temp_mC = (DATAOUT - (273 * temp_scale_mV / Vref_mv)) * 1000 *
+			 *           Vref_mV / temp_scale_mV
+			 *
+			 * This gives us:
+			 * scale  = Vref_mV * 1000 / temp_scale_mV
+			 * offset = -273 * temp_scale / Vref_mV
+			 */
+			*val = ret;
+			*val2 = ddata->chip_info->temp_scale_mV;
+
+			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 = -LTC2497_KELVIN_TO_CELCIUS * ddata->chip_info->temp_scale_mV;
+		*val2 = ret / 1000;
+
+		return IIO_VAL_FRACTIONAL;
 
 	default:
 		return -EINVAL;
@@ -126,39 +170,55 @@ static int ltc2497core_read_raw(struct iio_dev *indio_dev,
 	.differential = 1, \
 }
 
+#define LTC2497_T_CHAN {									\
+	.type = IIO_TEMP,									\
+	.channel = 0,										\
+	.address = (LTC2497_ENABLE_TEMPERATURE_CONV),						\
+	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),						\
+	.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE) | BIT(IIO_CHAN_INFO_OFFSET),	\
+}
+
+#define LTC2497_CHANNELS \
+	LTC2497_CHAN(0, LTC2497_SGL, "CH0"),			\
+	LTC2497_CHAN(1, LTC2497_SGL, "CH1"),			\
+	LTC2497_CHAN(2, LTC2497_SGL, "CH2"),			\
+	LTC2497_CHAN(3, LTC2497_SGL, "CH3"),			\
+	LTC2497_CHAN(4, LTC2497_SGL, "CH4"),			\
+	LTC2497_CHAN(5, LTC2497_SGL, "CH5"),			\
+	LTC2497_CHAN(6, LTC2497_SGL, "CH6"),			\
+	LTC2497_CHAN(7, LTC2497_SGL, "CH7"),			\
+	LTC2497_CHAN(8, LTC2497_SGL, "CH8"),			\
+	LTC2497_CHAN(9, LTC2497_SGL, "CH9"),			\
+	LTC2497_CHAN(10, LTC2497_SGL, "CH10"),			\
+	LTC2497_CHAN(11, LTC2497_SGL, "CH11"),			\
+	LTC2497_CHAN(12, LTC2497_SGL, "CH12"),			\
+	LTC2497_CHAN(13, LTC2497_SGL, "CH13"),			\
+	LTC2497_CHAN(14, LTC2497_SGL, "CH14"),			\
+	LTC2497_CHAN(15, LTC2497_SGL, "CH15"),			\
+	LTC2497_CHAN_DIFF(0, LTC2497_DIFF),			\
+	LTC2497_CHAN_DIFF(1, LTC2497_DIFF),			\
+	LTC2497_CHAN_DIFF(2, LTC2497_DIFF),			\
+	LTC2497_CHAN_DIFF(3, LTC2497_DIFF),			\
+	LTC2497_CHAN_DIFF(4, LTC2497_DIFF),			\
+	LTC2497_CHAN_DIFF(5, LTC2497_DIFF),			\
+	LTC2497_CHAN_DIFF(6, LTC2497_DIFF),			\
+	LTC2497_CHAN_DIFF(7, LTC2497_DIFF),			\
+	LTC2497_CHAN_DIFF(0, LTC2497_DIFF | LTC2497_SIGN),	\
+	LTC2497_CHAN_DIFF(1, LTC2497_DIFF | LTC2497_SIGN),	\
+	LTC2497_CHAN_DIFF(2, LTC2497_DIFF | LTC2497_SIGN),	\
+	LTC2497_CHAN_DIFF(3, LTC2497_DIFF | LTC2497_SIGN),	\
+	LTC2497_CHAN_DIFF(4, LTC2497_DIFF | LTC2497_SIGN),	\
+	LTC2497_CHAN_DIFF(5, LTC2497_DIFF | LTC2497_SIGN),	\
+	LTC2497_CHAN_DIFF(6, LTC2497_DIFF | LTC2497_SIGN),	\
+	LTC2497_CHAN_DIFF(7, LTC2497_DIFF | LTC2497_SIGN)
+
 static const struct iio_chan_spec ltc2497core_channel[] = {
-	LTC2497_CHAN(0, LTC2497_SGL, "CH0"),
-	LTC2497_CHAN(1, LTC2497_SGL, "CH1"),
-	LTC2497_CHAN(2, LTC2497_SGL, "CH2"),
-	LTC2497_CHAN(3, LTC2497_SGL, "CH3"),
-	LTC2497_CHAN(4, LTC2497_SGL, "CH4"),
-	LTC2497_CHAN(5, LTC2497_SGL, "CH5"),
-	LTC2497_CHAN(6, LTC2497_SGL, "CH6"),
-	LTC2497_CHAN(7, LTC2497_SGL, "CH7"),
-	LTC2497_CHAN(8, LTC2497_SGL, "CH8"),
-	LTC2497_CHAN(9, LTC2497_SGL, "CH9"),
-	LTC2497_CHAN(10, LTC2497_SGL, "CH10"),
-	LTC2497_CHAN(11, LTC2497_SGL, "CH11"),
-	LTC2497_CHAN(12, LTC2497_SGL, "CH12"),
-	LTC2497_CHAN(13, LTC2497_SGL, "CH13"),
-	LTC2497_CHAN(14, LTC2497_SGL, "CH14"),
-	LTC2497_CHAN(15, LTC2497_SGL, "CH15"),
-	LTC2497_CHAN_DIFF(0, LTC2497_DIFF),
-	LTC2497_CHAN_DIFF(1, LTC2497_DIFF),
-	LTC2497_CHAN_DIFF(2, LTC2497_DIFF),
-	LTC2497_CHAN_DIFF(3, LTC2497_DIFF),
-	LTC2497_CHAN_DIFF(4, LTC2497_DIFF),
-	LTC2497_CHAN_DIFF(5, LTC2497_DIFF),
-	LTC2497_CHAN_DIFF(6, LTC2497_DIFF),
-	LTC2497_CHAN_DIFF(7, LTC2497_DIFF),
-	LTC2497_CHAN_DIFF(0, LTC2497_DIFF | LTC2497_SIGN),
-	LTC2497_CHAN_DIFF(1, LTC2497_DIFF | LTC2497_SIGN),
-	LTC2497_CHAN_DIFF(2, LTC2497_DIFF | LTC2497_SIGN),
-	LTC2497_CHAN_DIFF(3, LTC2497_DIFF | LTC2497_SIGN),
-	LTC2497_CHAN_DIFF(4, LTC2497_DIFF | LTC2497_SIGN),
-	LTC2497_CHAN_DIFF(5, LTC2497_DIFF | LTC2497_SIGN),
-	LTC2497_CHAN_DIFF(6, LTC2497_DIFF | LTC2497_SIGN),
-	LTC2497_CHAN_DIFF(7, LTC2497_DIFF | LTC2497_SIGN),
+	LTC2497_CHANNELS,
+};
+
+static const struct iio_chan_spec ltc2497core_channel_with_temperature[] = {
+	LTC2497_CHANNELS,
+	LTC2497_T_CHAN,
 };
 
 static const struct iio_info ltc2497core_info = {
@@ -182,8 +242,14 @@ 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);
+
+	if (ddata->chip_info->has_temp_channel) {
+		indio_dev->channels = ltc2497core_channel_with_temperature;
+		indio_dev->num_channels = ARRAY_SIZE(ltc2497core_channel_with_temperature);
+	} 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..07fced79aeead3778964b114d479fdcb643c16df 100644
--- a/drivers/iio/adc/ltc2497.c
+++ b/drivers/iio/adc/ltc2497.c
@@ -86,8 +86,28 @@ 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_ENABLE_TEMPERATURE_CONV
+	 * 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_ENABLE_TEMPERATURE_CONV)
+			ret = i2c_smbus_write_byte_data(st->client, LTC2497_ENABLE,
+							LTC2497_ENABLE_TEMPERATURE_CONV);
+		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));
@@ -135,6 +155,8 @@ static const struct ltc2497_chip_info ltc2497_info[] = {
 	[TYPE_LTC2495] = {
 		.resolution = 16,
 		.name = "ltc2495",
+		.has_temp_channel = true,
+		.temp_scale_mV = 12250,
 	},
 	[TYPE_LTC2497] = {
 		.resolution = 16,
@@ -143,6 +165,8 @@ static const struct ltc2497_chip_info ltc2497_info[] = {
 	[TYPE_LTC2499] = {
 		.resolution = 24,
 		.name = "ltc2499",
+		.has_temp_channel = true,
+		.temp_scale_mV = 1570000,
 	},
 };
 
diff --git a/drivers/iio/adc/ltc2497.h b/drivers/iio/adc/ltc2497.h
index 64e81c95a3dd05911b6717c09ac0560c9f47f304..7f53bf8b8469d2371bb7ae64ce90a61beadba9ba 100644
--- a/drivers/iio/adc/ltc2497.h
+++ b/drivers/iio/adc/ltc2497.h
@@ -3,10 +3,25 @@
 #define LTC2497_ENABLE			0xA0
 #define LTC2497_CONFIG_DEFAULT		LTC2497_ENABLE
 #define LTC2497_CONVERSION_TIME_MS	150ULL
+#define LTC2497_EN2			BIT(7)
+/* Enable the internal temperature sensor */
+#define LTC2497_IM                      BIT(6)
+/* Second command byte value to initiate a temperature conversion */
+#define LTC2497_ENABLE_TEMPERATURE_CONV	(LTC2497_EN2 | LTC2497_IM)
 
 struct ltc2497_chip_info {
-	u32 resolution;
 	const char *name;
+	u32 resolution;
+	/*
+	 * Represents the datasheet constant from the temperature formula:
+	 * T_Kelvin = (DATAOUT * Vref) / temp_scale, where Vref is in Volts.
+	 *
+	 * To allow the driver to use Vref in millivolts for the calculation
+	 * and also to avoid floating points, this stored value represents the
+	 * datasheet constant scaled by 1000.
+	 */
+	u32 temp_scale_mV;
+	bool has_temp_channel;
 };
 
 struct ltc2497core_driverdata {

-- 
2.43.0