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

[Matti Vaittinen <mazziesaccount@gmail.com>]

On 3/18/23 18:54, Jonathan Cameron wrote:
> On Mon, 13 Mar 2023 11:39:06 +0200
> Matti Vaittinen <mazziesaccount@gmail.com> wrote:
> 
>> On 3/12/23 17:36, Jonathan Cameron wrote:
>>> On Sun, 5 Mar 2023 14:22:51 +0200
>>> Matti Vaittinen <mazziesaccount@gmail.com> wrote:
>>>    
>>>> On 3/4/23 22:17, Jonathan Cameron wrote:
>>>>> On Thu, 2 Mar 2023 12:58:59 +0200
>>>>> Matti Vaittinen <mazziesaccount@gmail.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?
> 
> Hmm. I'm not sure that we don't have valid users of it even if they
> are unusual.  We also have some historical messes that do RAW + SCALE +
> PROCESSED so we can't really have a warning on it.
> 
> Warning generally is that the test tools that come with the kernel
> will give you the wrong reading. :)
> 
>>
>> 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.
> 
> Hmm. For the sysfs case you could use VAL_INT_PLUS_MICRO but that doesn't
> then work well with the buffered path.
> 
> It is perfectly valid to just have this as _RAW and keep your _SCALE so
> that's probably the best option
> _RAW doesn't have to mean totally raw, it just means userspace is expected
> to applying a linear conversion to get a reading in the 'base' units for the channel.

Thanks for the insight!

I'll return the scale and switch to RAW for v5 :)

Yours,
	-- Matti

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

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