Re: [PATCH] iio: afe: rescale: Fix logic bug

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 24 Aug 2023 10:28:01 +0200
Linus Walleij <linus.walleij@linaro.org> wrote:

> On Thu, Aug 24, 2023 at 10:24 AM Linus Walleij <linus.walleij@linaro.org> wrote:
> 
> > This should definitely be reflected in the scale attribute for the
> > raw channel instead,  
> 
> Actually both IIO_CHAN_INFO_SCALE and IIO_CHAN_INFO_OFFSET
> as it looks.
> 
> I usually use tools/iio/iio_generic_buffer.c to test the result after
> applied scale and offset as it takes those into account.
> 
> Yours,
> Linus Walleij

Not 100% the follow is relevant as I've lost track of the discussion
but maybe it is useful.

Worth noting there are a few reasons why RAW and PROCESSED can coexist
in drivers and indeed why SCALE can be absent.. (OFFSET is much the same)

1) If SCALE = 1.0 the driver is allowed not to provide it - the channel
   might still be raw if OFFSET != 0
2) If the channel has a horrible non linear and none invertable conversion
   to standard units and events support the you might need PROCESSED to
   provide the useful value, but RAW to give you clue what the current value
   is for setting an event (light sensors are usual place we see this).
3) Historical ABI errors.  If we first had RAW and no scale or offset because
   we had no known values for them.  Then later we discovered that there
   was a non linear transform involved (often when someone found a magic
   calibration code somewhere).  Given the RAW interface might be in use
   and isn't a bug as such, we can't easily remove it.  The new PROCESSED
   interface needs to be there because of the non linear transform..

Odd corner cases...  In this particular case the original code made no
sense but might have allowed for case 3 by accident?

Jonathan