Re: [PATCH] iio: add channel type for frequency

[David Lechner]

On 07/02/2018 08:03 AM, Jonathan Cameron wrote:
> On Sun, 1 Jul 2018 17:24:34 -0500
> David Lechner <david@xxxxxxxxxxxxxx> wrote:
>
> > On 07/01/2018 02:18 AM, Lars-Peter Clausen wrote:
> > > On 07/01/2018 04:59 AM, David Lechner wrote:
> > > > This adds a new type for frequency to the IIO channel type enumeration.
> > > >
> > > > Units are in Hz.
> > > >   
> > >
> > > Documentation?
> >
> > I take it that you mean Documentation/ABI/testing/sysfs-bus-iio? Or
> > somewhere else too?
>
> Yes.
>
> > > We already have the altvoltage channel type with the frequency attribute.
> > > Difficult to say if there are any overlaps without documentation on how this
> > > new attribute is supposed to be used.
> >
> > I'm basically trying to implement a quadrature encoder in iio. I want to be
> > able to use it in-kernel to get a rotational speed value for a motor.
>
> Have you seen the counters subsystem work that is pretty much ready to merge?

I hadn't seen that. Thanks. It sounds like the way to go since I am dealing with
a quadrature encoder.

> https://lkml.org/lkml/2018/6/21/659
>
> > The motors (and encoder wheels) are hot-swapable, so we don't know how many
> > counts from the quadrature encoder equals one rotation because it depends on
> > which motor is being used. So using IIO_ANGL_VEL doesn't work for this case.
>
> Hmm. This always ugly when we have hotswappable external devices.  Ideal is
> to describe them with device tree or similar none the less as this stuff isn't
> really something userspace should have to figure out.
>
> > It seems to me that the proper generic unit for "speed" from the quadrature
> > encoder would be counts per second, hence the suggestion of a frequency unit.
>
> It is probably not that clear cut, as what is it frequency of?  All depends on
> what mode the quadrature counter is working in x4 or x1 for simplest options.
>
>
> > I'm not sure if voltage frequency works here since a "count" on a quadrature
> > encoder is derived from two different voltage signals (and may vary depending
> > on how the encoder determines what one count it).
> >
> > Also, unrelated to my quadrature encoder project, I was thinking that
> > frequency counters would use this unit as well (although the voltage alt
> > makes more sense for this case).
> >
> > There are also sound sensors that measure frequency that could use this unit.
>
> I have not fundamental issue with having a frequency channel, but only
> if we have a clear cut use case.  I suspect the right option here is to
> look at what extensions are needed to William's counter subsystem instead
> of in IIO (which frankly failed to stretch far enough to support counters
> well).

Agreed. Let's drop this patch.