Re: [PATCH v1 4/9] iio: afe: rescale: add offset support

[Liam Beguin]

On Mon May 31, 2021 at 10:08 AM EDT, Peter Rosin wrote:
> On 2021-05-31 15:36, Liam Beguin wrote:
> > Hi Peter,
> > 
> > On Mon May 31, 2021 at 4:52 AM EDT, Peter Rosin wrote:
> >> Hi!
> >>
> >> Thanks for the patch!
> >>
> >> On 2021-05-30 02:59, Liam Beguin wrote:
> >>> From: Liam Beguin <lvb@xxxxxxxxxx>
> >>>
> >>> This is a preparatory change required for the addition of temperature
> >>> sensing front ends.
> >>
> >> I think this is too simplistic. I think that if the upstream iio-dev has
> >> an offset, it should be dealt with (i.e. be rescaled). The rescale
> >> driver
> >> cannot ignore such an upstream offset and then throw in some other
> >> unrelated offset of its own. That would be thoroughly confusing.
> > 
> > I'm not sure I fully understand. The upstream offset should be dealt
> > with when calling iio_read_channel_processed().  That was my main
> > motivation behind using the IIO core to get a processed value.
>
> You can rescale a channel with an offset, but without using processed
> values. I.e. the upstream channel provides raw values, a scale and an
> offset. The current rescale code ignores the upstream offset. I did not
> need that when I created the driver, and at a glace it felt "difficult".
> So I punted.

I understand what you meant now.

At first, I tried to apply the upstream offset from inside the rescaler.
As you said it felt difficult and it felt like this must've been
implemented somewhere else before.

After looking around, I noticed that the code to do that was already
part of inkern.c and exposed through iio_read_channel_processed().
If the upstream channel doesn't provide a processed value, the upstream
offset and scale are automatically applied.

So with the changes in [3/9] the rescaler's raw value becomes the
upstream channel's processed value.

This seems like an easier and probably cleaner way of adding offset
support in the rescaler.

Does that make sense?

Cheers,
Liam

>
> But if the rescaler is going to start to handle offsets of any kind, it
> will get very confusing if the upstream offset is ignored. The proper
> way to do that is not something I have thought deeply about, and I
> don't know what the proper behavior is. For a processed channel, the
> offset is baked into the value that is scaled. Maybe the sane thing
> is to do that for a non-processed channel as well? But that gets a bit
> ugly, as it is counter to the simplicity, beauty and efficiency of the
> rescaler driver. In the non-processed case the driver is just adjusting
> the scale value. But since we are talking about proportional
> relationships, it should be possible to rescale a non-processed
> channel with an offset by just adjusting the offset in some way related
> to the rescale factor. Doing it with integer math is the "difficult"
> part...
>
> Cheers,
> Peter