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

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Date:   Tue, 14 Mar 2023 11:39:22 +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?  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.
> 

I just noticed a thing. I used the iio_generic_buffer to test the 
changes - and it got the channel values scaled to luxes even for the 
PROCESSED channel. So, it seems to me the iio_generic_buffer does apply 
the scale for PROCESSED channels too. I think that is slightly 
misleading. Oh, and this is not intended to be a complaint - just a 
report that there might be some room for an improvement :)

Yours,
	-- Matti

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

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