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

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Date: Sun, 2 Jun 2024 21:00:15 +0200
From: Vasileios Amoiridis <vassilisamir@...il.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Vasileios Amoiridis <vassilisamir@...il.com>, lars@...afoo.de,
	himanshujha199640@...il.com, linux-iio@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 04/17] iio: chemical: bme680: Fix sensor data read
 operation

On Sun, Jun 02, 2024 at 01:41:06PM +0100, Jonathan Cameron wrote:
> On Mon, 27 May 2024 20:37:52 +0200
> Vasileios Amoiridis <vassilisamir@...il.com> wrote:
> 
> > 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@...il.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 */
> I'd break oversampling up
> 
> 	/* (Oversampling ratio * time per reading) ...
> or something along those lines because it's related to oversampling but isn't
> of itself oversampling.
> 

Ok I see , makes sense.

> > +	return (meas * 1936) + (477 * 4) + (477 * 5) + 1000 + (dur * 1000);
> 
> Trivial but I think we can rely on precedence both for correctness and readability
> and hence don't need the brackets
> 

The reason I used the parentheses was because in my eyes it's easier to read what's
happening exactly. But I can also remove them, there is no problem.

> > +}
> > +
> > +static void bme680_wait_for_eoc(struct bme680_data *data)
> Don't call it wait as that implies something is being checked.
> 
> bme680_conversion_sleep() or something like that.
> 

This sesnor has a sleep mode, so I think calling a function like that might
make it a bit confusing, since we are not putting the sensor into sleeping
mode but rather actually wait for the eoc.

> > +{
> > +	int wait_eoc = bme680_conversion_time_us(data->oversampling_temp +
> > +						 data->oversampling_press +
> > +						 data->oversampling_press,
> > +						 data->heater_dur);
> 
> I'd pull the calculation inline in here unless you are going to use it elsewhere
> in later patches.
> 

Ok, I could merge them into one, I don't think there is a problem.

> > +
> > +	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");
>