Re: [PATCH 2/2] iio: light: Add APDS9160 ALS & Proximity sensor driver

[Matti Vaittinen]

On 02/12/2024 10:22, Matti Vaittinen wrote:
> Hi Jonathan & Mikael,
>
> On 01/12/2024 15:20, Jonathan Cameron wrote:
> > > > > +};
> > > > > +MODULE_DEVICE_TABLE(of, apds9160_of_match);
> > > > > +
> > > > > +static struct i2c_driver apds9160_driver = {
> > > > > +    .driver      = {
> > > > > +        .name    = APDS9160_DRIVER_NAME,
> > > > > +        .owner = THIS_MODULE,
> > > > > +        .of_match_table = apds9160_of_match,
> > > > > +    },
> > > > > +    .probe    = apds9160_probe,
> > > > > +    .remove      = apds9160_remove,
> > > > > +    .id_table = apds9160_id,
> > > > > +};
> > > First, regarding the integration time/gain/scale parameters. I took a 
> > > look at the datasheet again as there is a table
> > > provided to get lux/count (scale?) for the ALS sensor depending on 
> > > gain and integration time.
> > >
> > > It looks like the correlation in the table is almost linear but it's 
> > > not as there is a loss of precision.
> > > For example, at 1x gain with integration time 100ms the lux/count is 
> > > 0.819 but at 3x the table is stating 0.269 instead of exepected 0.273.
> > >
> > > Is it still possible to use the gts helpers in that case?
> >
> > Ah. Probably not if it goes non linear.  Matti? (+CC)
>
> Disclaimer - I didn't go through the patch and I just respond from the 
> top of my head :) So, please take my words with a pinch of salt.
>
> AFAIR, it is not required that the impact of integration time is 
> _linear_ through the range. The "multiplication factor" can be set for 
> each integration time separately. So, it is perfectly Ok to say:
>
> time 1 => multiply by 1
> time 2 => multiply by 2
> time 10 => multiply by 9 <= not linear, as linear would be 10.
> time 15 => multiply by 15
>
> ...
>
> The notable limitation of _current_ implementation is that the 
> "multiplication factor" needs to be integer. So, this may result loss of 
> accuracy.

// Snip.

I ended up re-reading this mail as a result of running some of my 
public-inbox scripts...

...and I noticed that the non linear correlation was not about 
integration time, but about gain. Eg, if I now read you right, the 
integration time is kept constant 100mS, and gain is changed from 1x => 
3x, which actually did not bring 3x gain to the lux/count values.

If this is the case, then the GTS helpers aren't likely to help you 
much. Sorry.


Yours,
	-- Matti