On 07/06/2018 12:21 PM, Jonathan Cameron wrote:
On Mon, 2 Jul 2018 22:48:35 -0400
William Breathitt Gray <vilhelm.gray@xxxxxxxxx> wrote:
On Mon, Jul 02, 2018 at 01:13:40PM -0500, David Lechner wrote:
On 06/21/2018 04:06 PM, William Breathitt Gray wrote:
I decided to strip down these devices to arrive at the core essence of
what constitutes a "counter device" and therefore design a "generic
counter" abstraction to better represent these devices and prevent the
ambiguity we discovered with the existing IIO Counter interface. This
abstraction became the Generic Counter paradigm, which is explained in
detail within the Documentation/driver-api/generic-counter.rst file
introduced by this patchset.
I'm curious if you have given any thought to the time aspect of counters.
I am interested in the rate at which the counters are counting (e.g. how
many counts per second). I realize that you can calculate this in
userspace or in the kernel using the system timer, but it is not very
accurate since Linux is not a realtime OS. So, I would like to get the
rate directly from the hardware. For example, the TI eQEP[1], like the
one found in BeagleBones, has a couple ways of measuring time (see link
for details).
[1]: http://www.ti.com/lit/ug/sprug05a/sprug05a.pdf
Cool in eQEP if that is what you are targetting - been wanting to see
nice kernel support implemented for that for a long time, but never
got the spare cycles to do it myself!
Ah yes, I see you initially attempted adding a frequency channel type to
the IIO code. I agree with you that this calculation is best kept away
from the operating system, not just because of the realtime requirement
considerations, but also because the hardware likely knows best its own
data, so let's expose it!
Regarding the Generic Counter interface, a frequency attribute can be
added quite easily in a technical sense, so this discussion should be
focused more on the warrant for exposing this data. I understand from
the discussion thread on your initial patch submission that you're
working with hot-swappable encoder wheels and would like to expose a
counting rate since you have trouble otherwise knowing the number of
counts equal to one revolution due to the various possible encoder
wheels that could be installed -- do I understand this correctly?
Luckily the Generic Counter interface is a more abstract paradigm, so
the hot-swappable encoder wheels should not be a problem for us as long
as we nail down a consistent and thorough definition for this attribute.
To that end, since the Generic Counter paradigm is designed to abstract
away specifics of counter devices in order to present the final data of
interest to users (e.g. the count value, the mode of operation, etc.),
let's make sure frequency is actually what we want to expose and not
just a middle-step datum on the path to the final result.
What is the real life use-case for this information (are you tracking
position)?
We are attempting to control the speed of the motors well enough to make
a balancing (inverted pendulum) robot and also synchronize two motors
when driving a robot (e.g. if one gets "stuck", the other slows down and
waits for the first to catch up).
Does the relevant hardware report the number of counts equal
to one revolution, or are you calculating this from frequency?
Not exactly. For the most part, the motors are the same, so we assume
that as default, but have a way for changing the parameters from
userspace.
Are you
using this frequency to simply track the number of revolutions?
No, we are using it to calculate the rotational speed of the motor.
Should
revolution count also be exposed?
I don't think so. The raw count value is good enough.
Is frequency useful for other
applications on its own (perhaps velocity of an automobile device
equipped with an encoder wheel for some reason or other)?
Since we are just dealing with counters here, I think we should call
it "rate" instead of "frequency". At least that seems to be the common
name in industrial automation.
Another possible use case for "rate" would be flow meters. Some
flow meters generate a pulse every X gallons. Assuming that the
counter also has a rate output, then you can scale the rate (e.g.
counts/second) into flow in gallons per minute.
Once we figure out how this data is used, we can determine the best
design and place to introduce it into the Generic Counter interface,
then move on to integration from there.
Great - as long as this fits reasonably well in ABI wise (whatever the
details) sounds like we don't need to solve it today. I'm anxious not
to delay merging this counter subsystem for another cycle.
Certainly don't delay things on account of me. I'm just trying to get
a feel for where things are headed since I missed the earlier discussions.
I don't see any major problems with the current state of things.
Once this lands, I may have a go at the eQEP and see how it looks.
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