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Subject: Re: [PATCH v3] iio: chemical: Add support for Bosch BME680 sensor
To:     Himanshu Jha <himanshujha199640@gmail.com>, jic23@kernel.org
Cc:     knaack.h@gmx.de, lars@metafoo.de, pmeerw@pmeerw.net,
        linux-kernel@vger.kernel.org, linux-iio@vger.kernel.org,
        Daniel Baluta <daniel.baluta@gmail.com>
References: <1531311229-3586-1-git-send-email-himanshujha199640@gmail.com>
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Date:   Fri, 13 Jul 2018 13:42:35 -0700
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Hi Himanshu Jha,

First a bit of background.  I'm working on a device which will contain a 
bme680 sensor.  A colleague of mine started work on a Linux kernel 
driver for the chip a little while ago.  The (WIP) driver can be found 
here: 
https://github.com/mangOH/mangOH/tree/master/linux_kernel_modules/bme680

This driver is written targeting an older kernel (3.18.x) because that's 
the kernel we're stuck on for now.  Rather than writing the driver from 
scratch, what we did was write the Linux kernel driver as a wrapper 
around the Bosch code.  My theory at the time was that Bosch made the 
chip, so they probably know what they're doing when it comes to writing 
a driver library.  After having looked at the code in more detail, I'm 
less confident that our approach was the best one.  I'm not attempting 
to upstream the driver built by my colleague and I'm not trying to 
request review of this code either.  I simply want to make you aware of 
it so that you can look at it to get some ideas.

I have included a number of comments on your driver below.  Keep up the 
good work!


On 7/11/2018 5:13 AM, Himanshu Jha wrote:
> Bosch BME680 is a 4-in-1 sensor with temperature, pressure, humidity
> and gas sensing capability. It supports both I2C and SPI communication
> protocol for effective data communication.
> 
> The device supports two modes:
> 
> 1. Sleep mode
> 2. Forced mode
> 
> The measurements only takes place when forced mode is triggered and a
> single TPHG cycle is performed by the sensor. The sensor automatically
> goes to sleep after afterwards.
> 
> The device has various calibration constants/parameters programmed into
> devices' non-volatile memory(NVM) during production and can't be altered
> by the user. These constants are used in the compensation functions to
> get the required compensated readings along with the raw data. The
> compensation functions/algorithms are provided by Bosch Sensortec GmbH
> via their API[1]. As these don't change during the measurement cycle,
> therefore we read and store them at the probe. The default configs
> supplied by Bosch are also set at probe.
> 
> 0-day tested with build success.
> 
> GSoC-2018: https://summerofcode.withgoogle.com/projects/#6691473790074880
> Mentor: Daniel Baluta
> [1] https://github.com/BoschSensortec/BME680_driver
> Datasheet:
> https://ae-bst.resource.bosch.com/media/_tech/media/datasheets/BST-BME680-DS001-00.pdf
> 
> Cc: Daniel Baluta <daniel.baluta@gmail.com>
> Signed-off-by: Himanshu Jha <himanshujha199640@gmail.com>
> ---
> 
> v3:
>     -moved files to chemical directory instead of a dedicated directory.
>     -read calibration parameters serially with endian conversions.
>     -drop some return ret.
>     -removed few unnecessary casts safely.
>     -added 'u' suffix to explicitly specify unsigned for large values
>      and thereby fixing comiler warning.
>     -left aligned all comments.
>     -added a comment explaining heater stability failure.
> 
> v2:
>     -Used devm_add_action() to add a generic remove method for
>      both I2C & SPI driver.
>     -Introduction of compensation functions.
>     -chip initialisation routines moved to respective I2C and SPI
>      driver.
>     -Introduction of gas sensing rountines.
>     -Simplified Kconfig to reduce various options.
> 
>   drivers/iio/chemical/Kconfig       |  25 +
>   drivers/iio/chemical/Makefile      |   3 +
>   drivers/iio/chemical/bme680.h      |  99 ++++
>   drivers/iio/chemical/bme680_core.c | 946 +++++++++++++++++++++++++++++++++++++
>   drivers/iio/chemical/bme680_i2c.c  |  83 ++++
>   drivers/iio/chemical/bme680_spi.c  | 123 +++++
>   6 files changed, 1279 insertions(+)
>   create mode 100644 drivers/iio/chemical/bme680.h
>   create mode 100644 drivers/iio/chemical/bme680_core.c
>   create mode 100644 drivers/iio/chemical/bme680_i2c.c
>   create mode 100644 drivers/iio/chemical/bme680_spi.c
> 
> diff --git a/drivers/iio/chemical/Kconfig b/drivers/iio/chemical/Kconfig
> index 5cb5be7..24790a8 100644
> --- a/drivers/iio/chemical/Kconfig
> +++ b/drivers/iio/chemical/Kconfig
> @@ -21,6 +21,31 @@ config ATLAS_PH_SENSOR
>   	 To compile this driver as module, choose M here: the
>   	 module will be called atlas-ph-sensor.
>   
> +config BME680
> +	tristate "Bosch Sensortec BME680 sensor driver"
> +	depends on (I2C || SPI)
> +	select REGMAP
> +	select BME680_I2C if (I2C)
> +	select BME680_SPI if (SPI)
> +	help
> +	  Say yes here to build support for Bosch Sensortec BME680 sensor with
> +	  temperature, pressure, humidity and gas sensing capability.
> +
> +	  This driver can also be built as a module. If so, the module for I2C
> +	  would be called bme680_i2c and bme680_spi for SPI support.
> +
> +config BME680_I2C
> +	tristate
> +	depends on BME680
> +	depends on I2C
> +	select REGMAP_I2C
> +
> +config BME680_SPI
> +	tristate
> +	depends on BME680
> +	depends on SPI
> +	select REGMAP_SPI
> +
>   config CCS811
>   	tristate "AMS CCS811 VOC sensor"
>   	depends on I2C
> diff --git a/drivers/iio/chemical/Makefile b/drivers/iio/chemical/Makefile
> index a629b29..2f4c4ba 100644
> --- a/drivers/iio/chemical/Makefile
> +++ b/drivers/iio/chemical/Makefile
> @@ -4,6 +4,9 @@
>   
>   # When adding new entries keep the list in alphabetical order
>   obj-$(CONFIG_ATLAS_PH_SENSOR)	+= atlas-ph-sensor.o
> +obj-$(CONFIG_BME680) += bme680_core.o
> +obj-$(CONFIG_BME680_I2C) += bme680_i2c.o
> +obj-$(CONFIG_BME680_SPI) += bme680_spi.o
>   obj-$(CONFIG_CCS811)		+= ccs811.o
>   obj-$(CONFIG_IAQCORE)		+= ams-iaq-core.o
>   obj-$(CONFIG_VZ89X)		+= vz89x.o
> diff --git a/drivers/iio/chemical/bme680.h b/drivers/iio/chemical/bme680.h
> new file mode 100644
> index 0000000..80c4190
> --- /dev/null
> +++ b/drivers/iio/chemical/bme680.h
> @@ -0,0 +1,99 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef BME680_H_
> +#define BME680_H_
> +
> +#define BME680_REG_CHIP_I2C_ID			0xD0
> +#define BME680_REG_CHIP_SPI_ID			0x50
> +#define BME680_CHIP_ID_VAL			0x61
Try to be consistent with the indenting of the defines.  I think this 
would be clearest:
#define BME680_REG_X			0x00
#define   BME680_X_FOO_EN_MASK		BIT(0)
#define   BME680_X_BAR_MASK		GENMASK(3, 1)
#define     BME680_BAR_VAL1		3
#define     BME680_BAR_VAL2		7

This way the register, field definition and field values are all 
visually distinctive.

> +#define BME680_REG_SOFT_RESET			0xE0
The datasheet says that the soft reset register differs for I2C and SPI. 
  For I2C it is 0xE0 and for SPI it is 0x60 when page 0 is selected.

> +#define BME680_CMD_SOFTRESET			0xB6
> +#define BME680_REG_STATUS			0x73
> +#define   BME680_SPI_MEM_PAGE_BIT		BIT(4)
> +#define   BME680_SPI_MEM_PAGE_1_VAL		1
> +
> +#define BME680_OSRS_TEMP_X(osrs_t)		((osrs_t) << 5)
> +#define BME680_OSRS_PRESS_X(osrs_p)		((osrs_p) << 2)
> +#define BME680_OSRS_HUMID_X(osrs_h)		((osrs_h) << 0)
You could use the FIELD_PREP macro from <linux/bitfield.h> to eliminate 
the need for these macros.  For example:
ctrl_meas_reg = FIELD_PREP(BME680_OSRS_TEMP_MASK, temp_val) |
                 FIELD_PREP(BME680_OSRS_PRESS_MASK, press_val) |
                 FIELD_PREP(BME880_MODE_MASK, mode_val);

> +
> +#define BME680_REG_TEMP_MSB			0x22
> +#define BME680_REG_PRESS_MSB			0x1F
> +#define BM6880_REG_HUMIDITY_MSB			0x25
> +#define BME680_REG_GAS_MSB			0x2A
> +#define BME680_REG_GAS_R_LSB			0x2B
> +#define   BME680_GAS_STAB_BIT			BIT(4)
> +
> +#define BME680_REG_CTRL_HUMIDITY		0x72
> +#define   BME680_OSRS_HUMIDITY_MASK		GENMASK(2, 0)
> +
> +#define BME680_REG_CTRL_MEAS			0x74
> +#define   BME680_OSRS_TEMP_MASK			GENMASK(7, 5)
> +#define   BME680_OSRS_PRESS_MASK		GENMASK(4, 2)
> +#define   BME680_MODE_MASK			GENMASK(1, 0)
> +
> +#define BME680_MODE_FORCED			BIT(0)
> +#define BME680_MODE_SLEEP			0
This should be:
#define BME680_MODE_SLEEP			0
#define BME680_MODE_FORCED			1

> +
> +#define BME680_REG_CONFIG			0x75
> +#define   BME680_FILTER_MASK			GENMASK(4, 2)
> +#define   BME680_FILTER_COEFF			BIT(1)
> +
> +/* TEMP/PRESS/HUMID reading skipped */
> +#define BME680_MEAS_SKIPPED			0x8000
> +
> +#define BME680_MAX_OVERFLOW_VAL			0x40000000
> +#define BME680_HUM_REG_SHIFT_VAL		4
> +#define BME680_BIT_H1_DATA_MSK			0x0F
> +
> +#define BME680_REG_RES_HEAT_RANGE		0x02
> +#define BME680_RHRANGE_MSK			0x30
> +#define BME680_REG_RES_HEAT_VAL			0x00
> +#define BME680_REG_RANGE_SW_ERR			0x04
> +#define BME680_RSERROR_MSK			0xF0
> +#define BME680_REG_RES_HEAT_0			0x5A
> +#define BME680_REG_GAS_WAIT_0			0x64
> +#define BME680_GAS_RANGE_MASK			0x0F
> +#define BME680_ADC_GAS_RES_SHIFT		6
> +#define BME680_AMB_TEMP				25
> +
> +#define BME680_REG_CTRL_GAS_1			0x71
> +#define   BME680_RUN_GAS_MASK			BIT(4)
> +#define   BME680_NB_CONV_MASK			GENMASK(3, 0)
> +#define	BME680_RUN_GAS_EN			BIT(4)
> +#define BME680_NB_CONV_0			0
> +
> +#define BME680_REG_MEAS_STAT_0			0x1D
> +#define   BME680_GAS_MEAS_BIT			BIT(6)
> +
> +/* Calibration Parameters */
> +#define BME680_T2_LSB_REG	0x8A
> +#define BME680_T3_REG		0x8C
> +#define BME680_P1_LSB_REG	0x8E
> +#define BME680_P2_LSB_REG	0x90
> +#define BME680_P3_REG		0x92
> +#define BME680_P4_LSB_REG	0x94
> +#define BME680_P5_LSB_REG	0x96
> +#define BME680_P7_REG		0x98
> +#define BME680_P6_REG		0x99
> +#define BME680_P8_LSB_REG	0x9C
> +#define BME680_P9_LSB_REG	0x9E
> +#define BME680_P10_REG		0xA0
> +#define BME680_H2_LSB_REG	0xE2
> +#define BME680_H2_MSB_REG	0xE1
> +#define BME680_H1_MSB_REG	0xE3
> +#define BME680_H1_LSB_REG	0xE2
> +#define BME680_H3_REG		0xE4
> +#define BME680_H4_REG		0xE5
> +#define BME680_H5_REG		0xE6
> +#define BME680_H6_REG		0xE7
> +#define BME680_H7_REG		0xE8
> +#define BME680_T1_LSB_REG	0xE9
> +#define BME680_GH2_LSB_REG	0xEB
> +#define BME680_GH1_REG		0xED
> +#define BME680_GH3_REG		0xEE
> +
> +extern const struct regmap_config bme680_regmap_config;
> +
> +int bme680_core_probe(struct device *dev, struct regmap *regmap,
> +		      const char *name);
> +
> +#endif  /* BME680_H_ */
> diff --git a/drivers/iio/chemical/bme680_core.c b/drivers/iio/chemical/bme680_core.c
> new file mode 100644
> index 0000000..8dd789e
> --- /dev/null
> +++ b/drivers/iio/chemical/bme680_core.c
> @@ -0,0 +1,946 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Bosch BME680 - Temperature, Pressure, Humidity & Gas Sensor
> + *
> + * Copyright (C) 2017 - 2018 Bosch Sensortec GmbH
> + * Copyright (C) 2018 Himanshu Jha <himanshujha199640@gmail.com>
> + */
> +#include <linux/acpi.h>
> +#include <linux/device.h>
> +#include <linux/module.h>
> +#include <linux/log2.h>
> +#include <linux/regmap.h>
> +#include <linux/iio/iio.h>
> +#include <linux/iio/sysfs.h>
> +
> +#include "bme680.h"
> +
> +struct bme680_calib {
> +	u16 par_t1;
> +	s16 par_t2;
> +	s8  par_t3;
> +	u16 par_p1;
> +	s16 par_p2;
> +	s8  par_p3;
> +	s16 par_p4;
> +	s16 par_p5;
> +	s8  par_p6;
> +	s8  par_p7;
> +	s16 par_p8;
> +	s16 par_p9;
> +	u8  par_p10;
> +	u16 par_h1;
> +	u16 par_h2;
> +	s8  par_h3;
> +	s8  par_h4;
> +	s8  par_h5;
> +	s8  par_h6;
> +	s8  par_h7;
> +	s8  par_gh1;
> +	s16 par_gh2;
> +	s8  par_gh3;
> +	u8  res_heat_range;
> +	s8  res_heat_val;
> +	s8  range_sw_err;
> +};
> +
> +struct bme680_data {
> +	struct regmap *regmap;
> +	struct bme680_calib bme680;
> +	u8 oversampling_temp;
> +	u8 oversampling_press;
> +	u8 oversampling_humid;
> +	u16 heater_dur;
> +	u16 heater_temp;
> +	/*
> +	 * Carryover value from temperature conversion, used in pressure
> +	 * and humidity compensation calculations.
> +	 */
> +	s32 t_fine;
> +};
> +
> +const struct regmap_config bme680_regmap_config = {
> +	.reg_bits = 8,
> +	.val_bits = 8,
> +};
> +EXPORT_SYMBOL(bme680_regmap_config);
> +
> +static const struct iio_chan_spec bme680_channels[] = {
> +	{
> +		.type = IIO_TEMP,
> +		.info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED) |
> +				      BIT(IIO_CHAN_INFO_OVERSAMPLING_RATIO),
> +	},
> +	{
> +		.type = IIO_PRESSURE,
> +		.info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED) |
> +				      BIT(IIO_CHAN_INFO_OVERSAMPLING_RATIO),
> +	},
> +	{
> +		.type = IIO_HUMIDITYRELATIVE,
> +		.info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED) |
> +				      BIT(IIO_CHAN_INFO_OVERSAMPLING_RATIO),
> +	},
> +	{
> +		.type = IIO_RESISTANCE,
> +		.info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED),
> +	},
> +};
> +
> +static const int bme680_oversampling_avail[] = { 1, 2, 4, 8, 16 };
> + > +static int bme680_read_calib(struct bme680_data *data,
> +			     struct bme680_calib *calib)
> +{
> +	struct device *dev = regmap_get_device(data->regmap);
> +	unsigned int tmp, tmp_msb, tmp_lsb;
> +	int ret;
> +	__le16 buf;
> +
> +	/* Temperature related coefficients */
> +	ret = regmap_bulk_read(data->regmap, BME680_T1_LSB_REG,
> +			       (u8 *) &buf, 2);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_T1_LSB_REG\n");
> +		return ret;
> +	}
> +	calib->par_t1 = le16_to_cpu(buf);
> +
> +	ret = regmap_bulk_read(data->regmap, BME680_T2_LSB_REG,
> +			       (u8 *) &buf, 2);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_T2_LSB_REG\n");
> +		return ret;
> +	}
> +	calib->par_t2 = le16_to_cpu(buf);
> +
> +	ret = regmap_read(data->regmap, BME680_T3_REG, &tmp);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_T3_REG\n");
> +		return ret;
> +	}
> +	calib->par_t3 = tmp;
> +
> +	/* Pressure related coefficients */
> +	ret = regmap_bulk_read(data->regmap, BME680_P1_LSB_REG,
> +			       (u8 *) &buf, 2);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_P1_LSB_REG\n");
> +		return ret;
> +	}
> +	calib->par_p1 = le16_to_cpu(buf);
> +
> +	ret = regmap_bulk_read(data->regmap, BME680_P2_LSB_REG,
> +			       (u8 *) &buf, 2);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_P2_LSB_REG\n");
> +		return ret;
> +	}
> +	calib->par_p2 = le16_to_cpu(buf);
> +
> +	ret = regmap_read(data->regmap, BME680_P3_REG, &tmp);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_P3_REG\n");
> +		return ret;
> +	}
> +	calib->par_p3 = tmp;
> +
> +	ret = regmap_bulk_read(data->regmap, BME680_P4_LSB_REG,
> +			       (u8 *) &buf, 2);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_P4_LSB_REG\n");
> +		return ret;
> +	}
> +	calib->par_p4 = le16_to_cpu(buf);
> +
> +	ret = regmap_bulk_read(data->regmap, BME680_P5_LSB_REG,
> +			       (u8 *) &buf, 2);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_P5_LSB_REG\n");
> +		return ret;
> +	}
> +	calib->par_p5 = le16_to_cpu(buf);
> +
> +	ret = regmap_read(data->regmap, BME680_P6_REG, &tmp);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_P6_REG\n");
> +		return ret;
> +	}
> +	calib->par_p6 = tmp;
> +
> +	ret = regmap_read(data->regmap, BME680_P7_REG, &tmp);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_P7_REG\n");
> +		return ret;
> +	}
> +	calib->par_p7 = tmp;
> +
> +	ret = regmap_bulk_read(data->regmap, BME680_P8_LSB_REG,
> +			       (u8 *) &buf, 2);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_P8_LSB_REG\n");
> +		return ret;
> +	}
> +	calib->par_p8 = le16_to_cpu(buf);
> +
> +	ret = regmap_bulk_read(data->regmap, BME680_P9_LSB_REG,
> +			       (u8 *) &buf, 2);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_P9_LSB_REG\n");
> +		return ret;
> +	}
> +	calib->par_p9 = le16_to_cpu(buf);
> +
> +	ret = regmap_read(data->regmap, BME680_P10_REG, &tmp);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_P10_REG\n");
> +		return ret;
> +	}
> +	calib->par_p10 = tmp;
> +
> +	/* Humidity related coefficients */
> +	ret = regmap_read(data->regmap, BME680_H1_MSB_REG, &tmp_msb);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_H1_MSB_REG\n");
> +		return ret;
> +	}
> +
> +	ret = regmap_read(data->regmap, BME680_H1_LSB_REG, &tmp_lsb);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_H1_LSB_REG\n");
> +		return ret;
> +	}
> +
> +	calib->par_h1 = (tmp_msb << BME680_HUM_REG_SHIFT_VAL) |
> +				(tmp_lsb & BME680_BIT_H1_DATA_MSK);
> +
> +	ret = regmap_read(data->regmap, BME680_H2_MSB_REG, &tmp_msb);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_H2_MSB_REG\n");
> +		return ret;
> +	}
> +
> +	ret = regmap_read(data->regmap, BME680_H2_LSB_REG, &tmp_lsb);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_H2_LSB_REG\n");
> +		return ret;
> +	}
> +
> +	calib->par_h2 = (tmp_msb << BME680_HUM_REG_SHIFT_VAL) |
> +				(tmp_lsb >> BME680_HUM_REG_SHIFT_VAL);
> +
> +	ret = regmap_read(data->regmap, BME680_H3_REG, &tmp);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_H3_REG\n");
> +		return ret;
> +	}
> +	calib->par_h3 = tmp;
> +
> +	ret = regmap_read(data->regmap, BME680_H4_REG, &tmp);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_H4_REG\n");
> +		return ret;
> +	}
> +	calib->par_h4 = tmp;
> +
> +	ret = regmap_read(data->regmap, BME680_H5_REG, &tmp);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_H5_REG\n");
> +		return ret;
> +	}
> +	calib->par_h5 = tmp;
> +
> +	ret = regmap_read(data->regmap, BME680_H6_REG, &tmp);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_H6_REG\n");
> +		return ret;
> +	}
> +	calib->par_h6 = tmp;
> +
> +	ret = regmap_read(data->regmap, BME680_H7_REG, &tmp);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_H7_REG\n");
> +		return ret;
> +	}
> +	calib->par_h7 = tmp;
> +
> +	/* Gas heater related coefficients */
> +	ret = regmap_read(data->regmap, BME680_GH1_REG, &tmp);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_GH1_REG\n");
> +		return ret;
> +	}
> +	calib->par_gh1 = tmp;
> +
> +	ret = regmap_bulk_read(data->regmap, BME680_GH2_LSB_REG,
> +			       (u8 *) &buf, 2);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_GH2_LSB_REG\n");
> +		return ret;
> +	}
> +	calib->par_gh2 = le16_to_cpu(buf);
> +
> +	ret = regmap_read(data->regmap, BME680_GH3_REG, &tmp);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read BME680_GH3_REG\n");
> +		return ret;
> +	}
> +	calib->par_gh3 = tmp;
> +
> +	/* Other coefficients */
> +	ret = regmap_read(data->regmap, BME680_REG_RES_HEAT_RANGE, &tmp);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read resistance heat range\n");
> +		return ret;
> +	}
> +	calib->res_heat_range = (tmp & BME680_RHRANGE_MSK) / 16;
> +
> +	ret = regmap_read(data->regmap, BME680_REG_RES_HEAT_VAL, &tmp);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read resistance heat value\n");
> +		return ret;
> +	}
> +	calib->res_heat_val = tmp;
> +
> +	ret = regmap_read(data->regmap, BME680_REG_RANGE_SW_ERR, &tmp);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read range software error\n");
> +		return ret;
> +	}
> +	calib->range_sw_err = (tmp & BME680_RSERROR_MSK) / 16;
> +
> +	return 0;
> +}
> +
> +/* Taken from Bosch BME680 API */

I think there should be a link to the Bosch code 
(https://github.com/BoschSensortec/BME680_driver/) somewhere within the 
comments of this file.  Maybe it belongs at the top of this file?

> +static s32 bme680_compensate_temp(struct bme680_data *data,
> +				  s32 adc_temp)
> +{
> +	struct bme680_calib *calib = &data->bme680;
> +	s64 var1, var2, var3, calc_temp;
> +
> +	var1 = ((s32) adc_temp >> 3) - ((s32) calib->par_t1 << 1);
> +	var2 = (var1 * (s32) calib->par_t2) >> 11;
> +	var3 = ((var1 >> 1) * (var1 >> 1)) >> 12;
> +	var3 = ((var3) * ((s32) calib->par_t3 << 4)) >> 14;
> +	data->t_fine = (s32) (var2 + var3);
> +	calc_temp = (s16) (((data->t_fine * 5) + 128) >> 8);
> +
> +	return calc_temp;
I think the meaning of the s32 returned should be documented.  Based on 
code elsewhere in the driver I'm guessing it's degrees celcius * 100. 
The same comment applies to the other compensate functions.

> +}
> +
> +/* Taken from Bosch BME680 API */
> +static u32 bme680_compensate_press(struct bme680_data *data,
> +				   u32 adc_press)
> +{
> +	struct bme680_calib *calib = &data->bme680;
> +	s32 var1, var2, var3, press_comp;
> +
> +	var1 = (((s32)data->t_fine) >> 1) - 64000;
> +	var2 = ((((var1 >> 2) * (var1 >> 2)) >> 11) * (s32)calib->par_p6) >> 2;
> +	var2 = var2 + ((var1 * (s32)calib->par_p5) << 1);
> +	var2 = (var2 >> 2) + ((s32)calib->par_p4 << 16);
> +	var1 = (((((var1 >> 2) * (var1 >> 2)) >> 13) *
> +			((s32)calib->par_p3 << 5)) >> 3) +
> +			(((s32)calib->par_p2 * var1) >> 1);
> +	var1 = var1 >> 18;
> +	var1 = ((32768 + var1) * (s32)calib->par_p1) >> 15;
> +	press_comp = 1048576 - adc_press;
> +	press_comp = ((press_comp - (var2 >> 12)) * 3125);
> +
> +	if (press_comp >= BME680_MAX_OVERFLOW_VAL)
> +		press_comp = ((press_comp / (u32)var1) << 1);
> +	else
> +		press_comp = ((press_comp << 1) / (u32)var1);
> +
> +	var1 = ((s32)calib->par_p9 * (((press_comp >> 3) *
> +				     (press_comp >> 3)) >> 13)) >> 12;
> +	var2 = ((press_comp >> 2) * (s32)calib->par_p8) >> 13;
> +	var3 = ((press_comp >> 8) * (press_comp >> 8) *
> +			(press_comp >> 8) * calib->par_p10) >> 17;
> +
> +	press_comp += ((var1 + var2 + var3 + ((s32)calib->par_p7 << 7)) >> 4);
> +
> +	return press_comp;
> +}
> +
> +/* Taken from Bosch BME680 API */
> +static u32 bme680_compensate_humid(struct bme680_data *data,
> +				   u16 adc_humid)
> +{
> +	struct bme680_calib *calib = &data->bme680;
> +	s32 var1, var2, var3, var4, var5, var6, temp_scaled, calc_hum;
> +
> +	temp_scaled = (((s32) data->t_fine * 5) + 128) >> 8;
> +	var1 = (adc_humid - ((s32) ((s32) calib->par_h1 * 16))) -
> +			(((temp_scaled * (s32) calib->par_h3) / 100) >> 1);
> +	var2 = ((s32) calib->par_h2 * (((temp_scaled * (s32) calib->par_h4) /
> +			((s32) 100)) + (((temp_scaled * ((temp_scaled *
> +			(s32) calib->par_h5) / 100)) >> 6) / 100) +
> +			(s32) (1 << 14))) >> 10;
> +	var3 = var1 * var2;
> +	var4 = (s32) calib->par_h6 << 7;
> +	var4 = (var4 + ((temp_scaled * (s32) calib->par_h7) / 100)) >> 4;
> +	var5 = ((var3 >> 14) * (var3 >> 14)) >> 10;
> +	var6 = (var4 * var5) >> 1;
> +	calc_hum = (((var3 + var6) >> 10) * 1000) >> 12;
> +
> +	if (calc_hum > 100000) /* Cap at 100%rH */
> +		calc_hum = 100000;
> +	else if (calc_hum < 0)
> +		calc_hum = 0;
> +
> +	return calc_hum;
> +}
> +
> +/* Taken from Bosch BME680 API */
> +static u32 bme680_compensate_gas(struct bme680_data *data, u16 gas_res_adc,
> +				 u8 gas_range)
> +{
> +	struct bme680_calib *calib = &data->bme680;
> +	s64 var1;
> +	u64 var2;
> +	s64 var3;
> +	u32 calc_gas_res;
> +
> +	/* Look up table 1 for the possible gas range values */
> +	u32 lookupTable1[16] = {2147483647u, 2147483647u, 2147483647u,
> +				2147483647u, 2147483647u, 2126008810u,
> +				2147483647u, 2130303777u, 2147483647u,
> +				2147483647u, 2143188679u, 2136746228u,
> +				2147483647u, 2126008810u, 2147483647u,
> +				2147483647u};
> +	/* Look up table 2 for the possible gas range values */
> +	u32 lookupTable2[16] = {4096000000u, 2048000000u, 1024000000u,
> +				512000000u, 255744255u, 127110228u, 64000000u,
> +				32258064u, 16016016u, 8000000u, 4000000u,
> +				2000000u, 1000000u, 500000u, 250000u, 125000u};
> +
> +	var1 = ((1340 + (5 * (s64) calib->range_sw_err)) *
> +			((s64) lookupTable1[gas_range])) >> 16;
> +	var2 = (((s64) ((s64) gas_res_adc << 15) - 16777216) + var1);
> +	var3 = (((s64) lookupTable2[gas_range] * (s64) var1) >> 9);
> +	calc_gas_res = (u32) ((var3 + ((s64) var2 >> 1)) / (s64) var2);
> +
> +	return calc_gas_res;
> +}
> +
> +/* Taken from Bosch BME680 API */
> +static u8 bme680_calc_heater_res(struct bme680_data *data, u16 temp)
> +{
> +	struct bme680_calib *calib = &data->bme680;
> +	s32 var1, var2, var3, var4, var5, heatr_res_x100;
> +	u8 heatr_res;
> +
> +	if (temp > 400) /* Cap temperature */
> +		temp = 400;
> +
> +	var1 = (((s32) BME680_AMB_TEMP * calib->par_gh3) / 1000) * 256;
> +	var2 = (calib->par_gh1 + 784) * (((((calib->par_gh2 + 154009) *
> +						temp * 5) / 100)
> +						+ 3276800) / 10);
> +	var3 = var1 + (var2 / 2);
> +	var4 = (var3 / (calib->res_heat_range + 4));
> +	var5 = (131 * calib->res_heat_val) + 65536;
> +	heatr_res_x100 = ((var4 / var5) - 250) * 34;
> +	heatr_res = (heatr_res_x100 + 50) / 100;
> +
> +	return heatr_res;
> +}
> +
> +/* Taken from Bosch BME680 API */
> +static u8 bme680_calc_heater_dur(u16 dur)
> +{
> +	u8 durval, factor = 0;
> +
> +	if (dur >= 0xfc0) {
> +		durval = 0xff; /* Max duration */
> +	} else {
> +		while (dur > 0x3F) {
> +			dur = dur / 4;
> +			factor += 1;
> +		}
> +		durval = dur + (factor * 64);
> +	}
> +
> +	return durval;
> +}
> +
> +static int bme680_set_mode(struct bme680_data *data, bool mode)
> +{
> +	struct device *dev = regmap_get_device(data->regmap);
> +	int ret;
> +
> +	if (mode) {
> +		ret = regmap_write_bits(data->regmap, BME680_REG_CTRL_MEAS,
> +					BME680_MODE_MASK, BME680_MODE_FORCED);
> +		if (ret < 0)
> +			dev_err(dev, "failed to set forced mode\n");
> +
> +	} else {
> +		ret = regmap_write_bits(data->regmap, BME680_REG_CTRL_MEAS,
> +					BME680_MODE_MASK, BME680_MODE_SLEEP);
> +		if (ret < 0)
> +			dev_err(dev, "failed to set sleep mode\n");
> +
> +	}
> +
> +	return ret;
> +}
> +
> +static int bme680_chip_config(struct bme680_data *data)
> +{
> +	struct device *dev = regmap_get_device(data->regmap);
> +	int ret;
> +	u8 osrs = BME680_OSRS_HUMID_X(data->oversampling_humid + 1);
> +	/*
> +	 * Highly recommended to set oversampling of humidity before
> +	 * temperature/pressure oversampling.
> +	 */
I think you are referring to this snippet from the datasheet: "It is 
highly recommended to set first osrs_h<2:0> followed by osrs_t<2:0> and 
osrs_p<2:0> in one write command (see Section 3.3)."  My interpretation 
of this is that they are saying that you should do one bulk write rather 
than writing the fields individually.  Maybe they're just recommending 
this for efficiency reasons.  I'm not really sure though.  I read 
through section 3.3 and I couldn't find any justification to backup this 
rather prominent suggestion.

> +	ret = regmap_update_bits(data->regmap, BME680_REG_CTRL_HUMIDITY,
> +				 BME680_OSRS_HUMIDITY_MASK, osrs);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to write ctrl_hum register\n");
> +		return ret;
> +	}
> +
> +	/* IIR filter settings */
> +	ret = regmap_update_bits(data->regmap, BME680_REG_CONFIG,
> +				 BME680_FILTER_MASK,
> +				 BME680_FILTER_COEFF);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to write config register\n");
> +		return ret;
> +	}
> +
> +	osrs = BME680_OSRS_TEMP_X(data->oversampling_temp + 1) |
> +		BME680_OSRS_PRESS_X(data->oversampling_press + 1);
> +
> +	ret = regmap_write_bits(data->regmap, BME680_REG_CTRL_MEAS,
> +				BME680_OSRS_TEMP_MASK |
> +				BME680_OSRS_PRESS_MASK,
> +				osrs);
> +	if (ret < 0)
> +		dev_err(dev, "failed to write ctrl_meas register\n");
> +
> +	return ret;
> +}
> +
> +static int bme680_gas_config(struct bme680_data *data)
> +{
> +	struct device *dev = regmap_get_device(data->regmap);
> +	int ret;
> +	u8 heatr_res, heatr_dur;
> +
> +	heatr_res = bme680_calc_heater_res(data, data->heater_temp);
> +
> +	/* set target heater temperature */
> +	ret = regmap_write(data->regmap, BME680_REG_RES_HEAT_0, heatr_res);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to write res_heat_0 register\n");
> +		return ret;
> +	}
> +
> +	heatr_dur = bme680_calc_heater_dur(data->heater_dur);
> +
> +	/* set target heating duration */
> +	ret = regmap_write(data->regmap, BME680_REG_GAS_WAIT_0, heatr_dur);
> +	if (ret < 0) {
> +		dev_err(dev, "failted to write gas_wait_0 register\n");
> +		return ret;
> +	}
> +
> +	/* Selecting the runGas and NB conversion settings for the sensor */
> +	ret = regmap_update_bits(data->regmap, BME680_REG_CTRL_GAS_1,
> +				 BME680_RUN_GAS_MASK | BME680_NB_CONV_MASK,
> +				 BME680_RUN_GAS_EN | BME680_NB_CONV_0);
> +	if (ret < 0)
> +		dev_err(dev, "failed to write ctrl_gas_1 register\n");
> +
> +	return ret;
> +}
> +
> +/* Outputs temperature measurement in degC */
> +static int bme680_read_temp(struct bme680_data *data,
> +			    int *val, int *val2)
> +{
> +	struct device *dev = regmap_get_device(data->regmap);
> +	int ret = 0;
> +	__be32 tmp = 0;
> +	s32 adc_temp, comp_temp;
> +
> +	/* set forced mode to trigger measurement */
> +	ret = bme680_set_mode(data, true);
> +	if (ret < 0)
> +		return ret;
> +
> +	ret = regmap_bulk_read(data->regmap, BME680_REG_TEMP_MSB,
> +			       (u8 *) &tmp, 3);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read temperature\n");
> +		return ret;
> +	}
I think the value you're reading from the register may actually be from 
a previous request.  I think you need to poll meas_status_0 (0x1D) field 
new_data_0 (bit 7) to wait for new data after setting the mode to 
forced.  You can see that's what Bosch's code does:
https://github.com/BoschSensortec/BME680_driver/blob/63bb5336db4659519860832be2738c685133aa33/bme680.c#L1227

> +
> +	adc_temp = be32_to_cpu(tmp) >> 12;
> +	if (adc_temp == BME680_MEAS_SKIPPED) {
> +		/* reading was skipped */
> +		dev_err(dev, "reading temperature skipped\n");
> +		return -EINVAL;
> +	}
> +	comp_temp = bme680_compensate_temp(data, adc_temp);
> +	/*
> +	 * val might be NULL if we're called by the read_press/read_humid
> +	 * routine which is callled to get t_fine value used in
> +	 * compensate_press/compensate_humid to get compensated
> +	 * pressure/humidity readings.
> +	 */
> +	if (val && val2) {
> +		*val = comp_temp;
> +		*val2 = 100;
> +		return IIO_VAL_FRACTIONAL;
> +	}
> +
> +	return ret;
> +}
> +
> +/* Outputs pressure measurement in hPa */
> +static int bme680_read_press(struct bme680_data *data,
> +			     int *val, int *val2)
> +{
> +	struct device *dev = regmap_get_device(data->regmap);
> +	int ret;
> +	__be32 tmp = 0;
> +	s32 adc_press;
> +
> +	/* Read and compensate temperature to get a reading of t_fine */
> +	ret = bme680_read_temp(data, NULL, NULL);
> +	if (ret < 0)
> +		return ret;
> +
> +	ret = regmap_bulk_read(data->regmap, BME680_REG_PRESS_MSB,
> +			       (u8 *) &tmp, 3);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read pressure\n");
> +		return ret;
> +	}
> +
> +	adc_press = be32_to_cpu(tmp) >> 12;
> +	if (adc_press == BME680_MEAS_SKIPPED) {
> +		/* reading was skipped */
> +		dev_err(dev, "reading pressure skipped\n");
> +		return -EINVAL;
> +	}
> +
> +	*val = bme680_compensate_press(data, adc_press);
> +	*val2 = 100;
> +	return IIO_VAL_FRACTIONAL;
> +}
> +
> +/* Outputs humidity measurement in %r.H */
> +static int bme680_read_humid(struct bme680_data *data,
> +			     int *val, int *val2)
> +{
> +	struct device *dev = regmap_get_device(data->regmap);
> +	int ret;
> +	__be16 tmp = 0;
> +	s32 adc_humidity;
> +	u32 comp_humidity;
> +
> +	/* Read and compensate temperature so we get a reading of t_fine */
> +	ret = bme680_read_temp(data, NULL, NULL);
> +	if (ret < 0)
> +		return ret;
> +
> +	ret = regmap_bulk_read(data->regmap, BM6880_REG_HUMIDITY_MSB,
> +			       (u8 *) &tmp, 2);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read humidity\n");
> +		return ret;
> +	}
> +
> +	adc_humidity = be16_to_cpu(tmp);
> +	if (adc_humidity == BME680_MEAS_SKIPPED) {
> +		/* reading was skipped */
> +		dev_err(dev, "reading humidity skipped\n");
> +		return -EINVAL;
> +	}
> +	comp_humidity = bme680_compensate_humid(data, adc_humidity);
> +
> +	*val = comp_humidity;
> +	*val2 = 1000;
> +	return IIO_VAL_FRACTIONAL;
> +}
> +
> +/* Outputs gas measurement in ohm */
> +static int bme680_read_gas(struct bme680_data *data,
> +			   int *val)
> +{
> +	struct device *dev = regmap_get_device(data->regmap);
> +	int ret;
> +	__be16 tmp = 0;
> +	unsigned int check;
> +	u16 adc_gas_res;
> +	u8 gas_range;
> +
> +	/* Set heater settings */
> +	ret = bme680_gas_config(data);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to set gas config\n");
> +		return ret;
> +	}
> +
> +	/* set forced mode to trigger a single measurement */
> +	ret = bme680_set_mode(data, true);
> +	if (ret < 0)
> +		return ret;
> +
> +	ret = regmap_read(data->regmap, BME680_REG_MEAS_STAT_0, &check);
> +	if (check & BME680_GAS_MEAS_BIT) {
> +		dev_err(dev, "gas measurement incomplete\n");
> +		return -EBUSY;
> +	}
> +
> +	ret = regmap_read(data->regmap, BME680_REG_GAS_R_LSB, &check);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read gas_r_lsb register\n");
> +		return ret;
> +	}
> +
> +	if ((check & BME680_GAS_STAB_BIT) == 0) {
> +	/*
> +	 * occurs if either the gas heating duration was insuffient
> +	 * to reach the target heater temperature or the target
> +	 * heater temperature was too high for the heater sink to
> +	 * reach.
> +	 */
> +		dev_err(dev, "heater failed to reach the target temperature\n");
> +		return -EINVAL;
> +	}
> +
> +	ret = regmap_bulk_read(data->regmap, BME680_REG_GAS_MSB,
> +			       (u8 *) &tmp, 2);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to read gas resistance\n");
> +		return ret;
> +	}
> +
> +	gas_range = check & BME680_GAS_RANGE_MASK;
> +	adc_gas_res = be16_to_cpu(tmp) >> BME680_ADC_GAS_RES_SHIFT;
> +
> +	*val = bme680_compensate_gas(data, adc_gas_res, gas_range);
> +	return IIO_VAL_INT;
> +}
> +
> +static int bme680_read_raw(struct iio_dev *indio_dev,
> +			   struct iio_chan_spec const *chan,
> +			   int *val, int *val2, long mask)
> +{
> +	struct bme680_data *data = iio_priv(indio_dev);
> +
> +	switch (mask) {
> +	case IIO_CHAN_INFO_PROCESSED:
> +		switch (chan->type) {
> +		case IIO_TEMP:
> +			return bme680_read_temp(data, val, val2);
> +		case IIO_PRESSURE:
> +			return bme680_read_press(data, val, val2);
> +		case IIO_HUMIDITYRELATIVE:
> +			return bme680_read_humid(data, val, val2);
> +		case IIO_RESISTANCE:
> +			return bme680_read_gas(data, val);
> +		default:
> +			return -EINVAL;
> +		}
> +	case IIO_CHAN_INFO_OVERSAMPLING_RATIO:
> +		switch (chan->type) {
> +		case IIO_TEMP:
> +			*val = 1 << data->oversampling_temp;
> +			return IIO_VAL_INT;
> +		case IIO_PRESSURE:
> +			*val = 1 << data->oversampling_press;
> +			return IIO_VAL_INT;
> +		case IIO_HUMIDITYRELATIVE:
> +			*val = 1 << data->oversampling_humid;
> +			return IIO_VAL_INT;
> +		default:
> +			return -EINVAL;
> +		}
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	return -EINVAL;
> +}
> +
> +static int bme680_write_oversampling_ratio_temp(struct bme680_data *data,
> +						int val)
> +{
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(bme680_oversampling_avail); ++i) {
> +		if (bme680_oversampling_avail[i] == val) {
> +			data->oversampling_temp = ilog2(val);
> +
> +			return bme680_chip_config(data);
> +		}
> +	}
> +
> +	return -EINVAL;
> +}
> +
> +static int bme680_write_oversampling_ratio_press(struct bme680_data *data,
> +						 int val)
> +{
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(bme680_oversampling_avail); ++i) {
> +		if (bme680_oversampling_avail[i] == val) {
> +			data->oversampling_press = ilog2(val);
> +
> +			return bme680_chip_config(data);
> +		}
> +	}
> +
> +	return -EINVAL;
> +}
> +
> +static int bme680_write_oversampling_ratio_humid(struct bme680_data *data,
> +						 int val)
> +{
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(bme680_oversampling_avail); ++i) {
> +		if (bme680_oversampling_avail[i] == val) {
> +			data->oversampling_humid = ilog2(val);
> +
> +			return bme680_chip_config(data);
> +		}
> +	}
> +
> +	return -EINVAL;
> +}
> +
> +static int bme680_write_raw(struct iio_dev *indio_dev,
> +			    struct iio_chan_spec const *chan,
> +			    int val, int val2, long mask)
> +{
> +	struct bme680_data *data = iio_priv(indio_dev);
> +
> +	switch (mask) {
> +	case IIO_CHAN_INFO_OVERSAMPLING_RATIO:
> +		switch (chan->type) {
> +		case IIO_TEMP:
> +			return bme680_write_oversampling_ratio_temp(data, val);
> +		case IIO_PRESSURE:
> +			return bme680_write_oversampling_ratio_press(data, val);
> +		case IIO_HUMIDITYRELATIVE:
> +			return bme680_write_oversampling_ratio_humid(data, val);
> +		default:
> +			return -EINVAL;
> +		}
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	return -EINVAL;
> +}
> +
> +static const char bme680_oversampling_ratio_show[] = "1 2 4 8 16";
> +
> +static IIO_CONST_ATTR(oversampling_ratio_available,
> +		      bme680_oversampling_ratio_show);
> +
> +static struct attribute *bme680_attributes[] = {
> +	&iio_const_attr_oversampling_ratio_available.dev_attr.attr,
> +	NULL,
> +};
> +
> +static const struct attribute_group bme680_attribute_group = {
> +	.attrs = bme680_attributes,
> +};
> +
> +static const struct iio_info bme680_info = {
> +	.read_raw = &bme680_read_raw,
> +	.write_raw = &bme680_write_raw,
> +	.attrs = &bme680_attribute_group,
> +};
> +
> +static const char *bme680_match_acpi_device(struct device *dev)
> +{
> +	const struct acpi_device_id *id;
> +
> +	id = acpi_match_device(dev->driver->acpi_match_table, dev);
> +	if (!id)
> +		return NULL;
> +
> +	return dev_name(dev);
> +}
> +
> +static void bme680_core_remove(void *arg)
> +{
> +	iio_device_unregister(arg);
> +}
> +
> +int bme680_core_probe(struct device *dev, struct regmap *regmap,
> +		      const char *name)
> +{
> +	struct iio_dev *indio_dev;
> +	struct bme680_data *data;
> +	int ret;
> +
> +	indio_dev = devm_iio_device_alloc(dev, sizeof(*data));
> +	if (!indio_dev)
> +		return -ENOMEM;
> +
> +	ret = devm_add_action(dev, bme680_core_remove, indio_dev);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to register remove action\n");
> +		return ret;
> +	}
> +
> +	if (!name && ACPI_HANDLE(dev))
> +		name = bme680_match_acpi_device(dev);
> +
> +	data = iio_priv(indio_dev);
> +	dev_set_drvdata(dev, indio_dev);
> +	data->regmap = regmap;
> +	indio_dev->dev.parent = dev;
> +	indio_dev->name = name;
> +	indio_dev->channels = bme680_channels;
> +	indio_dev->num_channels = ARRAY_SIZE(bme680_channels);
> +	indio_dev->info = &bme680_info;
> +	indio_dev->modes = INDIO_DIRECT_MODE;
> +
> +	/* default values for the sensor */
> +	data->oversampling_humid = ilog2(2); /* 2X oversampling rate */
> +	data->oversampling_press = ilog2(4); /* 4X oversampling rate */
> +	data->oversampling_temp = ilog2(8);  /* 8X oversampling rate */
> +	data->heater_temp = 320; /* degree Celsius */
> +	data->heater_dur = 150;  /* milliseconds */
> +
> +	ret = bme680_chip_config(data);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to set chip_config data\n");
> +		return ret;
> +	}
> +
> +	ret = bme680_gas_config(data);
> +	if (ret < 0) {
> +		dev_err(dev, "failed to set gas config data\n");
> +		return ret;
> +	}
> +
> +	ret = bme680_read_calib(data, &data->bme680);
> +	if (ret < 0) {
> +		dev_err(dev,
> +			"failed to read calibration coefficients at probe\n");
> +		return ret;
> +	}
> +
> +	return iio_device_register(indio_dev);
> +}
> +EXPORT_SYMBOL_GPL(bme680_core_probe);
> +
> +MODULE_AUTHOR("Himanshu Jha <himanshujha199640@gmail.com>");
> +MODULE_DESCRIPTION("Bosch BME680 Driver");
> +MODULE_LICENSE("GPL v2");
> diff --git a/drivers/iio/chemical/bme680_i2c.c b/drivers/iio/chemical/bme680_i2c.c
> new file mode 100644
> index 0000000..a3a77cf
> --- /dev/null
> +++ b/drivers/iio/chemical/bme680_i2c.c
> @@ -0,0 +1,83 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * BME680 - I2C Driver
> + *
> + * Copyright (C) 2018 Himanshu Jha <himanshujha199640@gmail.com>
> + *
> + * 7-Bit I2C slave address is:
> + *	- 0x76 if SDO is pulled to GND
> + *	- 0x77 if SDO is pulled to VDDIO
> + *
> + * Note: SDO pin cannot be left floating otherwise I2C address
> + *	 will be undefined.
> + */
> +#include <linux/acpi.h>
> +#include <linux/i2c.h>
> +#include <linux/module.h>
> +#include <linux/regmap.h>
> +
> +#include "bme680.h"
> +
> +static int bme680_i2c_probe(struct i2c_client *client,
> +			    const struct i2c_device_id *id)
> +{
> +	struct regmap *regmap;
> +	const char *name = NULL;
> +	unsigned int val;
> +	int ret;
> +
> +	regmap = devm_regmap_init_i2c(client, &bme680_regmap_config);
> +	if (IS_ERR(regmap)) {
> +		dev_err(&client->dev, "Failed to register i2c regmap %d\n",
> +				(int)PTR_ERR(regmap));
> +		return PTR_ERR(regmap);
> +	}
> +
> +	ret = regmap_write(regmap, BME680_REG_SOFT_RESET,
> +			   BME680_CMD_SOFTRESET);
> +	if (ret < 0)
> +		return ret;
> +
> +	ret = regmap_read(regmap, BME680_REG_CHIP_I2C_ID, &val);
> +	if (ret < 0) {
> +		dev_err(&client->dev, "Error reading I2C chip ID\n");
> +		return ret;
> +	}
> +
> +	if (val != BME680_CHIP_ID_VAL) {
> +		dev_err(&client->dev, "Wrong chip ID, got %x expected %x\n",
> +				val, BME680_CHIP_ID_VAL);
> +		return -ENODEV;
> +	}
> +
> +	if (id)
> +		name = id->name;
> +
> +	return bme680_core_probe(&client->dev, regmap, name);
> +}
> +
> +static const struct i2c_device_id bme680_i2c_id[] = {
> +	{"bme680", 0},
> +	{},
> +};
> +MODULE_DEVICE_TABLE(i2c, bme680_i2c_id);
> +
> +static const struct acpi_device_id bme680_acpi_match[] = {
> +	{"BME0680", 0},
> +	{},
> +};
> +MODULE_DEVICE_TABLE(acpi, bme680_acpi_match);
> +
> +static struct i2c_driver bme680_i2c_driver = {
> +	.driver = {
> +		.name			= "bme680_i2c",
> +		.acpi_match_table       = ACPI_PTR(bme680_acpi_match),
> +	},
> +	.probe = bme680_i2c_probe,
> +	.id_table = bme680_i2c_id,
> +};
> +module_i2c_driver(bme680_i2c_driver);
> +
> +MODULE_AUTHOR("Himanshu Jha <himanshujha199640@gmail.com>");
> +MODULE_DESCRIPTION("BME680 I2C driver");
> +MODULE_LICENSE("GPL v2");
> diff --git a/drivers/iio/chemical/bme680_spi.c b/drivers/iio/chemical/bme680_spi.c
> new file mode 100644
> index 0000000..1fb3466
> --- /dev/null
> +++ b/drivers/iio/chemical/bme680_spi.c
> @@ -0,0 +1,123 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * BME680 - SPI Driver
> + *
> + * Copyright (C) 2018 Himanshu Jha <himanshujha199640@gmail.com>
> + */
> +#include <linux/acpi.h>
> +#include <linux/module.h>
> +#include <linux/regmap.h>
> +#include <linux/spi/spi.h>
> +
> +#include "bme680.h"
> +
> +static int bme680_regmap_spi_write(void *context, const void *data,
> +				   size_t count)
> +{
> +	struct spi_device *spi = context;
> +	u8 buf[2];
> +
> +	memcpy(buf, data, 2);
> +	/*
> +	 * The SPI register address (= full register address without bit 7)
> +	 * and the write command (bit7 = RW = '0')
> +	 */
> +	buf[0] &= ~0x80;
> +
> +	return spi_write_then_read(spi, buf, 2, NULL, 0);
> +}
> +
> +static int bme680_regmap_spi_read(void *context, const void *reg,
> +				  size_t reg_size, void *val, size_t val_size)
> +{
> +	struct spi_device *spi = context;
> +
> +	return spi_write_then_read(spi, reg, reg_size, val, val_size);
> +}
> +
> +static struct regmap_bus bme680_regmap_bus = {
> +	.write = bme680_regmap_spi_write,
> +	.read = bme680_regmap_spi_read,
> +	.reg_format_endian_default = REGMAP_ENDIAN_BIG,
> +	.val_format_endian_default = REGMAP_ENDIAN_BIG,
> +};
> +
> +static int bme680_spi_probe(struct spi_device *spi)
> +{
> +	const struct spi_device_id *id = spi_get_device_id(spi);
> +	struct regmap *regmap;
> +	unsigned int val;
> +	int ret;
> +
> +	spi->bits_per_word = 8;
> +	ret = spi_setup(spi);
> +	if (ret < 0) {
> +		dev_err(&spi->dev, "spi_setup failed!\n");
> +		return ret;
> +	}
> +
> +	regmap = devm_regmap_init(&spi->dev, &bme680_regmap_bus,
> +				  &spi->dev, &bme680_regmap_config);
> +	if (IS_ERR(regmap)) {
> +		dev_err(&spi->dev, "Failed to register spi regmap %d\n",
> +				(int)PTR_ERR(regmap));
> +		return PTR_ERR(regmap);
> +	}
> +
> +	ret = regmap_write(regmap, BME680_REG_SOFT_RESET,
> +			   BME680_CMD_SOFTRESET);
> +	if (ret < 0)
> +		return ret;
> +
> +	/* after power-on reset, Page 0(0x80-0xFF) of spi_mem_page is active */
> +	ret = regmap_read(regmap, BME680_REG_CHIP_SPI_ID, &val);
> +	if (ret < 0) {
> +		dev_err(&spi->dev, "Error reading SPI chip ID\n");
> +		return ret;
> +	}
> +
> +	if (val != BME680_CHIP_ID_VAL) {
> +		dev_err(&spi->dev, "Wrong chip ID, got %x expected %x\n",
> +				val, BME680_CHIP_ID_VAL);
> +		return -ENODEV;
> +	}
> +	/*
> +	 * select Page 1 of spi_mem_page to enable access to
> +	 * to registers from address 0x00 to 0x7F.
> +	 */
> +	ret = regmap_write_bits(regmap, BME680_REG_STATUS,
> +				BME680_SPI_MEM_PAGE_BIT,
> +				BME680_SPI_MEM_PAGE_1_VAL);
> +	if (ret < 0) {
> +		dev_err(&spi->dev, "failed to set page 1 of spi_mem_page\n");
> +		return ret;
> +	}
> +
> +	return bme680_core_probe(&spi->dev, regmap, id->name);
> +}
> +
> +static const struct spi_device_id bme680_spi_id[] = {
> +	{"bme680", 0},
> +	{},
> +};
> +MODULE_DEVICE_TABLE(spi, bme680_spi_id);
> +
> +static const struct acpi_device_id bme680_acpi_match[] = {
> +	{"BME0680", 0},
> +	{},
> +};
> +MODULE_DEVICE_TABLE(acpi, bme680_acpi_match);
> +
> +static struct spi_driver bme680_spi_driver = {
> +	.driver = {
> +		.name			= "bme680_spi",
> +		.acpi_match_table	= ACPI_PTR(bme680_acpi_match),
> +	},
> +	.probe = bme680_spi_probe,
> +	.id_table = bme680_spi_id,
> +};
> +module_spi_driver(bme680_spi_driver);
> +
> +MODULE_AUTHOR("Himanshu Jha <himanshujha199640@gmail.com>");
> +MODULE_DESCRIPTION("Bosch BME680 SPI driver");
> +MODULE_LICENSE("GPL v2");
>