[PATCH v2 5/5] iio: chemical: sgp40: Implement turn_heater_off-command

[Jaakko Koivisto <jmatko@utu.fi>]

Add functionality for turning the SGP40 sensor heating element
on or off manually. After turning the heater off, the sensor enters
idle mode. Track the state of the heater in the driver, as the device
does not expose current heater state via any register.

Turning the heater off saves approx. 2.5 mA compared to regular
operation. The heating element is automatically turned back on
when measurement is performed.

Signed-off-by: Jaakko Koivisto <jmatko@utu.fi>
---
 drivers/iio/chemical/sgp40.c | 62 +++++++++++++++++++++++++++++++++++-
 1 file changed, 61 insertions(+), 1 deletion(-)

diff --git a/drivers/iio/chemical/sgp40.c b/drivers/iio/chemical/sgp40.c
index 278390bc6b04..87df8d79fe6e 100644
--- a/drivers/iio/chemical/sgp40.c
+++ b/drivers/iio/chemical/sgp40.c
@@ -54,7 +54,8 @@ struct sgp40_data {
 	int			rht;
 	int			temp;
 	int			res_calibbias;
-	/* Prevent concurrent access to rht, tmp, calibbias */
+	bool			heater_enabled;
+	/* Prevent concurrent access to rht, tmp, calibbias, heater_enabled */
 	struct mutex		lock;
 };
 
@@ -263,6 +264,27 @@ static int sgp40_execute_self_test(struct sgp40_data *data)
 	}
 }
 
+static int sgp40_turn_heater_off(struct sgp40_data *data)
+{
+	int ret;
+	struct i2c_client *client = data->client;
+	struct sgp40_tg_measure turn_off = { .command = {0x36, 0x15} };
+
+	guard(mutex)(&data->lock);
+
+	ret = i2c_master_send(client, (char *)&turn_off, sizeof(turn_off.command));
+	if (ret < 0)
+		return ret;
+	if (ret != sizeof(turn_off.command))
+		return dev_err_probe(data->dev, -EIO,
+		       "i2c_master_send ret: %d, expected %zu", ret, sizeof(turn_off.command));
+
+	fsleep(1000); /* Max duration for command turn_heater_off. Datasheet table 8. */
+	data->heater_enabled = false;
+
+	return 0;
+}
+
 static int sgp40_measure_resistance_raw(struct sgp40_data *data, u16 *resistance_raw)
 {
 	int ret;
@@ -293,6 +315,7 @@ static int sgp40_measure_resistance_raw(struct sgp40_data *data, u16 *resistance
 		return -EIO;
 	}
 	msleep(30);
+	data->heater_enabled = true;
 
 	ret = i2c_master_recv(client, (u8 *)&tgres, sizeof(tgres));
 	if (ret < 0)
@@ -423,10 +446,47 @@ static ssize_t serialnumber_show(struct device *dev,
 	return sysfs_emit_at(buf, 0, "%llu\n", serial_number);
 }
 
+static ssize_t heater_enable_store(struct device *dev,
+				   struct device_attribute *attr,
+				   const char *buf, size_t len)
+{
+	int ret;
+	u16 __ignore;
+	bool val;
+	struct sgp40_data *data = iio_priv(dev_to_iio_dev(dev));
+
+	ret = kstrtobool(buf, &val);
+	if (ret)
+		return ret;
+
+	if (val)
+		ret = sgp40_measure_resistance_raw(data, &__ignore);
+	else
+		ret = sgp40_turn_heater_off(data);
+
+	if (ret)
+		return ret;
+
+	return len;
+}
+
+static ssize_t heater_enable_show(struct device *dev,
+				  struct device_attribute *attr,
+				  char *buf)
+{
+	struct sgp40_data *data = iio_priv(dev_to_iio_dev(dev));
+
+	guard(mutex)(&data->lock);
+
+	return sysfs_emit(buf, "%d\n", data->heater_enabled);
+}
+
 static IIO_DEVICE_ATTR_RO(serialnumber, 0);
+static IIO_DEVICE_ATTR_RW(heater_enable, 0);
 
 static struct attribute *sgp40_attributes[] = {
 	&iio_dev_attr_serialnumber.dev_attr.attr,
+	&iio_dev_attr_heater_enable.dev_attr.attr,
 	NULL
 };
 
-- 
2.56.0