linux-kernel - Re: [PATCH 2/3] iio: light: add support for veml6046x00 RGBIR color sensor

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Message-ID: <Z_I-qwzUrTNz1DZp@mail.your-server.de>
Date: Sun, 6 Apr 2025 10:43:23 +0200
From: Andreas Klinger <ak@...klinger.de>
To: Jonathan Cameron <jic23@...nel.org>
Cc: robh@...nel.org, krzk+dt@...nel.org, conor+dt@...nel.org,
	lars@...afoo.de, linux-iio@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	javier.carrasco.cruz@...il.com, mazziesaccount@...il.com,
	subhajit.ghosh@...aklogic.com, muditsharma.info@...il.com,
	arthur.becker@...tec.com, ivan.orlov0322@...il.com
Subject: Re: [PATCH 2/3] iio: light: add support for veml6046x00 RGBIR color
 sensor

Hi Jonathan,

I need to pick up the meaning of scale once again for clarification.

Jonathan Cameron <jic23@...nel.org> schrieb am Mo, 17. Mär 11:50:
> On Sun, 16 Mar 2025 12:31:30 +0100
> Andreas Klinger <ak@...klinger.de> wrote:
> 
> > +static int veml6046x00_get_scale(struct veml6046x00_data *data,
> > +				 int *val, int *val2)
> 
> How is this related to integration time?  I'd normally expect
> to see that read in here somewhere as well as doubling integration
> time tends to double scale.

In the documentation file "sysfs-bus-iio" it says:
"
What:           /sys/.../iio:deviceX/in_illuminanceY_raw
[...]
Description:
                Illuminance measurement, units after application of scale
                                and offset are lux.
"

This means that the scale should be the real factor and not the gain multiplied
by photodiode size (PDDIV) as i implemented it so far.

This means also that doubling integration time should halve the scale. The
higher raw value should lead to the same lux value.

The documentation of the sensor (veml6046x00.pdf) provides us the calculation
between raw green values and lux.
Wouldn't it be better to give the user the real factor to be able to get lux?

The fact that only the green channel can be used for calculation could be
documented in the driver.

Then i found the "in_illuminance_hardwaregain" property. It seems that this is
exactly what the combination of gain and PDDIV is used for.

So what is the scale at the moment could become the hardwaregain and the scale
the factor from raw value to lux.

To sum up the suggested interface under /sys/bus/iio/devices/iio\:deviceX would
be something like:

in_illuminance_hardwaregain --> set and get gain and PDDIV on the sensor

integration_time            --> set and get integration time on the sensor

integration_time_available  --> show available integration time values

scale                       --> (only) get real calculation value, taken from
                                sensor documenation, e.g. 1.3440

scale_available             --> not existing anymore

What do you think?

Andreas

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