linux-kernel - Re: [PATCH v2 5/6] iio: light: ROHM BU27034 Ambient Light Sensor

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Date:   Mon, 13 Mar 2023 11:39:06 +0200
From:   Matti Vaittinen <mazziesaccount@...il.com>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Paul Gazzillo <paul@...zz.com>,
        Zhigang Shi <Zhigang.Shi@...eon.com>,
        Shreeya Patel <shreeya.patel@...labora.com>,
        Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org
Subject: Re: [PATCH v2 5/6] iio: light: ROHM BU27034 Ambient Light Sensor

On 3/12/23 17:36, Jonathan Cameron wrote:
> On Sun, 5 Mar 2023 14:22:51 +0200
> Matti Vaittinen <mazziesaccount@...il.com> wrote:
> 
>> On 3/4/23 22:17, Jonathan Cameron wrote:
>>> On Thu, 2 Mar 2023 12:58:59 +0200
>>> Matti Vaittinen <mazziesaccount@...il.com> wrote:
>>>    

// snip

>>>> +static const struct iio_chan_spec bu27034_channels[] = {
>>>> +	{
>>>> +		.type = IIO_LIGHT,
>>>> +		.info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED) |
>>>> +				      BIT(IIO_CHAN_INFO_SCALE),
>>>
>>> What is this scale for?
>>
>> The scale is to inform users that we return data using milli lux.
>>
>>> Given the channel is computed from various different inputs, is there a
>>> clear definition of how it is scaled?  What does a write to it mean?
>>
>> Nothing. writing anything else but milli lux scale fails with -EINVAL.
>>
>> I guess I am doing something in an unusual way here :) Do you have a
>> suggestion for me?
> 
> Return data in lux?

That's what I did originally have. But then I noticed we can get 
slightly better accuracy than that. Hence I switched to mLux and added 
the scale.

>  Or return it as INFO_RAW - thus making it clear
> that the reading is not in expected units and a conversion must be
> applied by userspace.  SCALE is not applied to PROCESSED by userspace.

Ah. This makes sense then. Maybe it would be worth adding a warning to 
IIO-core if drivers set both the SCALE and PROCESSED info bits?

So, I need to select between the simplicity or better accuracy here? :/ 
I really hate ending up making choices like this without knowing all the 
real use-cases :( And it happens a lot for me. Well, I guess I'll drop 
the scale, use luxes and go with the PROCESSED data. My understanding is 
that the "thing" with the sensor is a wide-range for wavelengths, not 
the accuracy. So, maybe luxes are just good enough - and again, users 
needing something more accurate can utilize the raw intensity channels.

> 
> In the rare case where you do get SCALE and PROCESSED it's there to allow
> for changes in the underlying signal measurement that are eaten up in the
> computation needed to get to PROCESSED - that is they have no visible
> affect (beyond range changes etc).

Oh, Ok. So there is a valid case for setting both SCALE and PROCESSED. 
Then we can't add teh warning I assume :(

Yours,
	-- Matti

-- 
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

~~ When things go utterly wrong vim users can always type :help! ~~