linux-kernel - Re: [PATCH v2 1/4] iio: chemical: scd30: add core driver

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Date:   Mon, 1 Jun 2020 14:35:02 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Tomasz Duszynski <tomasz.duszynski@...akon.com>
CC:     Jonathan Cameron <jic23@...nel.org>, <linux-iio@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <robh+dt@...nel.org>, <andy.shevchenko@...il.com>,
        <pmeerw@...erw.net>
Subject: Re: [PATCH v2 1/4] iio: chemical: scd30: add core driver

...
> > > > +static const struct iio_chan_spec scd30_channels[] = {
> > > > +	{
> > > > +		.type = IIO_PRESSURE,
> > > > +		.info_mask_separate = BIT(IIO_CHAN_INFO_CALIBSCALE),
> > > > +		.info_mask_separate_available = BIT(IIO_CHAN_INFO_CALIBSCALE),
> > > > +		.scan_index = -1,
> > > > +	},
> > > > +	{
> > > > +		.type = IIO_CONCENTRATION,
> > > > +		.channel2 = IIO_MOD_CO2,
> > > > +		.address = SCD30_CONC,
> > > > +		.scan_index = SCD30_CONC,
> > > > +		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |
> > > > +				      BIT(IIO_CHAN_INFO_SCALE),
> > > > +		.info_mask_shared_by_all = BIT(IIO_CHAN_INFO_SAMP_FREQ),
> > > > +		.modified = 1,
> > > > +
> > > > +		SCD30_CHAN_SCAN_TYPE('u', 16),
> > > > +	},
> > > > +	{
> > > > +		.type = IIO_TEMP,
> > > > +		.address = SCD30_TEMP,
> > > > +		.scan_index = SCD30_TEMP,
> > > > +		.info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED) |
> > > > +				      BIT(IIO_CHAN_INFO_CALIBBIAS) |
> > > > +				      BIT(IIO_CHAN_INFO_SCALE),  
> > >
> > > Combination of processed and scale is unusual.  Normally scale provides
> > > a conversion factor or a _RAW reading.  
> >
> > Right that's pointless. Scales were for raw measurements inside buffer.
> > Somehow I failed to realize that only co2 concentration is raw.
> >  
> 
> One more thing occurred to me here. I just looked at CONCENTRATION_RAW
> description and is states that this should return *percentage* reading.
> Then after scaling what we should be left with?

It should say after application of scale and offset.  Patches welcome :)

> 
> Or perhaps scale should return just 1.0 for completeness if we want to
> live with percentages.
> 
> Though in case where percentage reading is fractional then passing
> through buffers will not work. Or am I missing something?
> 
> On the other hand if abi said nothing about percentages one would just
> push whatever raw reading sensor outputs and provide scaling info to
> userspace.
> 

> > >