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

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Date:   Sat, 18 Mar 2023 16:57:58 +0000
From:   Jonathan Cameron <jic23@...nel.org>
To:     Matti Vaittinen <mazziesaccount@...il.com>
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 Tue, 14 Mar 2023 11:39:22 +0200
Matti Vaittinen <mazziesaccount@...il.com> wrote:

> 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 :)

Ah. Looks like the code doesn't have any check on whether the sysfs
read is _raw or _processed which is kind of understandable.

This may be the first case where those have both applied on a channel
that is available via buffered route.

Given processed channels are rarely IIO_VAL_INT which is kind of necessary
to poke it in the buffered route, that may well be true.

Jonathan

> 
> Yours,
> 	-- Matti
>