[PATCH v1 04/17] iio: chemical: bme680: Fix sensor data read operation

[Vasileios Amoiridis <vassilisamir@gmail.com>]

A read operation is happening as follows:

a) Set sensor to forced mode
b) Sensor measures values and update data registers and sleeps again
c) Read data registers

In the current implementation the read operation happens immediately
after the sensor is set to forced mode so the sensor does not have
the time to update properly the registers. This leads to the following
2 problems:

1) The first ever value which is read by the register is always wrong
2) Every read operation, puts the register into forced mode and reads
the data that were calculated in the previous conversion.

This behaviour was tested in 2 ways:

1) The internal meas_status_0 register was read before and after every
read operation in order to verify that the data were ready even before
the register was set to forced mode and also to check that after the
forced mode was set the new data were not yet ready.

2) Physically changing the temperature and measuring the temperature

This commit adds the waiting time in between the set of the forced mode
and the read of the data. The function is taken from the Bosch BME68x
Sensor API [1].

[1]: https://github.com/boschsensortec/BME68x_SensorAPI/blob/v4.4.8/bme68x.c#L490
Fixes: 1b3bd8592780 ("iio: chemical: Add support for Bosch BME680 sensor")
Signed-off-by: Vasileios Amoiridis <vassilisamir@gmail.com>
---
 drivers/iio/chemical/bme680_core.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/drivers/iio/chemical/bme680_core.c b/drivers/iio/chemical/bme680_core.c
index 5db48f6d646c..dd2cd11b6dd3 100644
--- a/drivers/iio/chemical/bme680_core.c
+++ b/drivers/iio/chemical/bme680_core.c
@@ -10,6 +10,7 @@
  */
 #include <linux/acpi.h>
 #include <linux/bitfield.h>
+#include <linux/delay.h>
 #include <linux/device.h>
 #include <linux/module.h>
 #include <linux/log2.h>
@@ -532,6 +533,26 @@ static u8 bme680_oversampling_to_reg(u8 val)
 	return ilog2(val) + 1;
 }
 
+/*
+ * Taken from Bosch BME680 API:
+ * https://github.com/boschsensortec/BME68x_SensorAPI/blob/v4.4.8/bme68x.c#L490
+ */
+static int bme680_conversion_time_us(u8 meas, u8 dur)
+{
+	/* Oversampling + TPH meas + Gas meas + Forced mode + heater duration */
+	return (meas * 1936) + (477 * 4) + (477 * 5) + 1000 + (dur * 1000);
+}
+
+static void bme680_wait_for_eoc(struct bme680_data *data)
+{
+	int wait_eoc = bme680_conversion_time_us(data->oversampling_temp +
+						 data->oversampling_press +
+						 data->oversampling_press,
+						 data->heater_dur);
+
+	usleep_range(wait_eoc, wait_eoc + 100);
+}
+
 static int bme680_chip_config(struct bme680_data *data)
 {
 	struct device *dev = regmap_get_device(data->regmap);
@@ -622,6 +643,8 @@ static int bme680_read_temp(struct bme680_data *data, int *val)
 	if (ret < 0)
 		return ret;
 
+	bme680_wait_for_eoc(data);
+
 	ret = regmap_bulk_read(data->regmap, BME680_REG_TEMP_MSB,
 			       &tmp, 3);
 	if (ret < 0) {
@@ -738,6 +761,8 @@ static int bme680_read_gas(struct bme680_data *data,
 	if (ret < 0)
 		return ret;
 
+	bme680_wait_for_eoc(data);
+
 	ret = regmap_read(data->regmap, BME680_REG_MEAS_STAT_0, &check);
 	if (check & BME680_GAS_MEAS_BIT) {
 		dev_err(dev, "gas measurement incomplete\n");
-- 
2.25.1