Re: [RFC PATCH v3 1/9] iio: pressure: bmp280: correct meas_time_us calculation

[Jonathan Cameron <jic23@kernel.org>]

On Sun, 28 Sep 2025 19:26:28 +0200
Achim Gratz <Achim.Gratz@Stromeko.DE> wrote:

> Correction of meas_time_us initialization based on an observation and
> partial patch by David Lechner.
> 
> The constant part of the measurement time (as described in the
> datasheet and implemented in the BM(P/E)2 Sensor API) was apparently
> forgotten (it was already correctly applied for the BMP380) and is now
> used.
> 
> There was also another thinko in bmp280_wait_conv:
> data->oversampling_humid can actually have a value of 0 (for an
> oversampling_ratio of 1), so it can not be used to detect the presence
> of the humidity measurement capability.  Use
> data->chip_info->oversampling_humid_avail instead, which is NULL for
> chips that cannot measure humidity and therefore must skip that part
> of the calculation.
> 
> Closes: https://lore.kernel.org/linux-iio/875xgfg0wz.fsf@Gerda.invalid/
> Fixes: 26ccfaa9ddaa ("iio: pressure: bmp280: Use sleep and forced mode for oneshot captures")
> Suggested-by: David Lechner <dlechner@baylibre.com>
> Tested-by: Achim Gratz <Achim.Gratz@Stromeko.DE>
> Signed-off-by: Achim Gratz <Achim.Gratz@Stromeko.DE>

Applied this to the fixes-togreg branch of iio.git.  I'll send a pull request
for that shortly after rc1.  The others will have to wait until that is available upstream.

> 
> ---
> 
> Notes
> =====
> 
> Since the BMx280 device support was added, oversampling=0 actually is
> a valid register setting (meaning that measurement channel is off),
> but actually allowing that setting to be used by changing the data
> structure to hold the actual value instead of its ilog2 would require
> more extensive changes elsewhere in the code.
> 
> Further changes would be necesary to avoid the awkward use of -1 as
> the value that would currently be necessary to achieve a setting of 0
> in the register.
> 
> Datasheet values and median actual measurement times through sysfs for
> a single reading with my BME280 connected to a 400kHz I²C bus provided
> by the i915 IGP VGA port:
> 
> |--------------+---------+---------+----------+----------|
> | Oversampling | Typical | Maximum | Measured | Overhead |
> |        Ratio |    [ms] |    [ms] |     [ms] |     [ms] |
> |--------------+---------+---------+----------+----------|
> |           16 |    98.0 |   112.8 |    122.1 |      9.3 |
> |            8 |    50.0 |    57.6 |     63.6 |      6.0 |
> |            4 |    26.0 |    30.0 |     34.6 |      4.6 |
> |            2 |    14.0 |    16.2 |     21.7 |      5.5 |
> |            1 |     8.0 |     9.3 |     14.8 |      5.5 |
> |--------------+---------+---------+----------+----------|
> 
> Reading all three channels via sysfs triples those times, including
> the overhead of course as each read triggers a new measurement on all
> enabled channels.  It is therefore also impossible to obtain all
> channel values from the same measurement cycle with MODE_FORCED, which
> is in a way a limitation of the sysfs interface and can only be
> obtained by using buffered reads.
> ---
>  drivers/iio/pressure/bmp280-core.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/iio/pressure/bmp280-core.c b/drivers/iio/pressure/bmp280-core.c
> index 6cdc8ed53520..2078b810745b 100644
> --- a/drivers/iio/pressure/bmp280-core.c
> +++ b/drivers/iio/pressure/bmp280-core.c
> @@ -1042,13 +1042,16 @@ static int bmp280_wait_conv(struct bmp280_data *data)
>  	unsigned int reg, meas_time_us;
>  	int ret;
>  
> -	/* Check if we are using a BME280 device */
> -	if (data->oversampling_humid)
> -		meas_time_us = BMP280_PRESS_HUMID_MEAS_OFFSET +
> -				BIT(data->oversampling_humid) * BMP280_MEAS_DUR;
> +	/* Constant part of the measurement time */
> +	meas_time_us = BMP280_MEAS_OFFSET;
>  
> -	else
> -		meas_time_us = 0;
> +	/*
> +	 * Check if we are using a BME280 device,
> +	 * Humidity measurement time
> +	 */
> +	if (data->chip_info->oversampling_humid_avail)
> +		meas_time_us += BMP280_PRESS_HUMID_MEAS_OFFSET +
> +				BIT(data->oversampling_humid) * BMP280_MEAS_DUR;
>  
>  	/* Pressure measurement time */
>  	meas_time_us += BMP280_PRESS_HUMID_MEAS_OFFSET +