[RFC 0/3] Bosch BME680 Driver

[Himanshu Jha <himanshujha199640@gmail.com>]

GSoC'2018 Project: https://summerofcode.withgoogle.com/projects/#6691473790074880
Mentor: Daniel Baluta

BME680 is a 4-in-1 sensor with temperature, pressure, humidity & gas sensor
with I2C and SPI interface support.

The sensor has various calibration parameters[1] to be used with raw adc data
in the compensation functions[2] to evaluate the correct compensated reading.
These calibration parameters are are programmed into the devices’ non-volatile
memory (NVM) during production and cannot be altered by the customer. So, we
are required to read those parameters once(suitably at probe) and use it
in the compensation functions.

But now the problem arises that these compensation functions, calibration
parameters, their addresses are *not* provided in the datasheet and instead
provided by Bosch Sensortec in their Github API[3]. The Github API has a LICENSE
file[4] around 40 lines which I presume kernel community won't entertain.

Therefore, we contacted Bosch Sensortec thrice 2 weeks ago and got no response
so far. We stated all the problems relating to Lincensing & missing calibration
info from the datasheet. I got to know about these missing information from the
Github API and also looking at the various sensors supplied by the company
such as BMP280, BME280 etc.

After using their compensation function in my driver I get correct readings.
But we can't simply use these function without their permission!

For instance:
I got a temperature reading of:

	3254

and if this sensor has a similar resolution as that of BMP280 which 0.01 degC
then it is absolutely correct.

	3254 * 0.01 = 32.54 degC

This 0.01 degC resolution was mentioned in the BMP280 datasheet but
there is no such information in the BME680 datasheet nor in the Github API.

This is what I have assumed since after testing pressure & humidity also
revealed similar readings and there is some possibilty that they could
have same output resolution as they belong to the same family of sensors.

For now I have placed the sensor in the IMU but this is not an IMU and there
is no such sensor(4-in-1) in the whole IIO subsystem. So, where should it
be placed ?
Would it be worthwhile to create a new subdirectory "environmental" since
it is a environmental sensor ?

The sensor also has an IIR filter to remove short term fluctuations from
the temperature and pressure readings. It has various filter coefficients
and I have chosen the middle valued coefficient(15) from the table for now.

What filter coefficient is most appropriate for the device ?

Again, I asked this question too from Bosch since they discuss all these
optimal selection of filter in "3.4 Filter selection" Pg 14 BMP280.

I have used regmap API  and therefore we don't need the LSB address
of data registers as we only care about the MSB address of the data register
and regmap_bulk_read() handles everything fine.

So, would it be worthwhile to add these LSB macros definition too ?
They certainly will never be used!

For now, this series doesn't include any of the copyrighted code from
Bosch and I have just added them as a dummy function.

I have mentioned more detailed summary in my blog[5] and would love
any feedback on it.

If anyone has contacts to Bosch Sensortec GmbH then please let me
know.

[1] https://github.com/BoschSensortec/BME680_driver/blob/313a58a9c57ad86c8df06c98521579c6cb695999/bme680_defs.h#L410
[2] https://github.com/BoschSensortec/BME680_driver/blob/master/bme680.c#L876
[3] https://github.com/BoschSensortec/BME680_driver
[4] https://github.com/BoschSensortec/BME680_driver/blob/master/LICENSE
[5] https://himanshujha199640.wordpress.com/2018/06/14/0x01bme680-temperature-channel/

Himanshu Jha (3):
  iio: imu: bme680: Add initial support for Bosch BME680
  iio: imu: bme680: Add temperaure, pressure & humidity channels
  iio: imu: bme680: Add ACPI support

 drivers/iio/imu/Kconfig              |   1 +
 drivers/iio/imu/Makefile             |   1 +
 drivers/iio/imu/bme680/Kconfig       |  32 +++
 drivers/iio/imu/bme680/Makefile      |   6 +
 drivers/iio/imu/bme680/bme680.h      |  11 +
 drivers/iio/imu/bme680/bme680_core.c | 542 +++++++++++++++++++++++++++++++++++
 drivers/iio/imu/bme680/bme680_i2c.c  |  62 ++++
 drivers/iio/imu/bme680/bme680_spi.c  |  49 ++++
 8 files changed, 704 insertions(+)
 create mode 100644 drivers/iio/imu/bme680/Kconfig
 create mode 100644 drivers/iio/imu/bme680/Makefile
 create mode 100644 drivers/iio/imu/bme680/bme680.h
 create mode 100644 drivers/iio/imu/bme680/bme680_core.c
 create mode 100644 drivers/iio/imu/bme680/bme680_i2c.c
 create mode 100644 drivers/iio/imu/bme680/bme680_spi.c

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2.7.4