Re: BME-280 humidity over-/underflow

[Tomasz Duszynski <tomasz.duszynski@octakon.com>]

Hello Hendrik,

On Wed, Jun 10, 2020 at 03:45:07PM +0200, Hendrik Hoeth wrote:
> Hi,
>
> I've discovered an over-/underflow problem for relative humidity
> measurements in drivers/iio/pressure/bmp280-core.c. At very low
> humidities the value measured by the sensor can fluctuate into the
> negative humidity range. The kernel driver then reports this as
> extremely large value (e.g. 4193085, corresponding to 4193% RH). The
> same probably happens at high humidity values, but with our climate
> chamber I was only able to look at the low end.
>
> To avoid this, the reference code in the BME-280 datasheet catches
> values < 0% RH and > 100% RH. These two lines are currently missing in
> the kernel driver.
>
> Attached you find a patch that fixed the problem for us. You can find
> the sensor datasheet at
> https://ae-bst.resource.bosch.com/media/_tech/media/datasheets/BST-BME280-DS002.pdf
> The code I'm talking about is in section 4.2.3 on page 25/26.
>
> Thanks a lot,
>
>    Hendrik

Seems this particular issue has been already fixed here:
dee2dabc0e4115b80945fe2c91603e634f4b4686 iio: bmp280: fix compensation of humidity

>
> --
> Garrecht Avionik GmbH | Wieslocher Str. 38 | 69190 Walldorf
> https://www.air-avionics.com/
> Tel.: +49 6224 98 96 999

> Fix underflow / overflow for humidity measurements
>
> diff --git a/drivers/iio/pressure/bmp280-core.c b/drivers/iio/pressure/bmp280-core.c
> index 29c209cc1108..d1f36b044f05 100644
> --- a/drivers/iio/pressure/bmp280-core.c
> +++ b/drivers/iio/pressure/bmp280-core.c
> @@ -270,6 +270,8 @@ static u32 bmp280_compensate_humidity(struct bmp280_data *data,
>  		* (((var * (s32)calib->H3) >> 11) + (s32)32768)) >> 10)
>  		+ (s32)2097152) * calib->H2 + 8192) >> 14);
>  	var -= ((((var >> 15) * (var >> 15)) >> 7) * (s32)calib->H1) >> 4;
> +	var = ((var < 0) ? 0 : var);
> +	var = ((var > 419430400) ? 419430400 : var);
>
>  	return var >> 12;
>  };