[PATCH] input/touchscreen: add S3C24XX SoC touchscreen input driver

[Nelson Castillo]

On Tue, Oct 20, 2009 at 2:39 AM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Mon, Oct 19, 2009 at 11:21:40PM -0500, Nelson Castillo wrote:
>> There is one case where "whatever way" fails here. If you get a set of
>> points and you notice they have noise you can schedule more
>> conversions _right_away_ without latency to get new points from the
>> ADC. That's what the "group filter" does and you can not do that from
>> tslib and I say it after learning how to write tslib filters.
>
> In which case people aren't implementing their kernel drivers correctly.
> The kernel is supposed to produce a stream of samples when the pen is
> touched, and those samples should be delivered to tslib.  tslib's filters
> then has the responsibility to read groups of samples, and filter them
> using whatever method that the user wants.

The group filter (that mixes information from X and Y) could be ported
to user-space but we would need points at a much higher rate.

Can the s3c driver send the events at full speed? (Maximum possible ADC rate?).

Even if it's possible I think we would monopolize the adc driver
needlessly because we don't need all that conversions when there is
not much noise. Please read on.

> I also don't buy your comparison on your web page since there's almost
> zero details of what you're comparing.  You don't give details of what
> filters you're using with the userspace method - for all the reader
> knows, you're not using tslib at all.

Blame lack of time.

Most of the time tslib filtering worked well when we have good pressure.

We have problems when we don't have much pressure and in this case
there is a lot of noise and the events that are reported by the driver
are actually noise. This noise would be more common at pen-down before
the adc readings settle and at pen-up.

This is the noise we filtered in most cases.

I remember I wrote a tslib filter to skip the N first and the M last
points but it was not enough.

http://svn.arhuaco.org/svn/src/openmoko/touchscreen/tslib-skip_module.patch

I should report some data here so that you can see how bad the noise
problem is. It might be a hardware problem. I've heard of other boards
that also have a lot of noise.

I did read this message last month in tslib:

https://lists.berlios.de/pipermail/tslib-general/2009-September/000228.html

  I've taken the tack that we when we use the resistive
  sensors we have to take sloppy positioning and mis-clicks
  into the user interface design.

Really? We managed to fix a similar problem. Perhaps it can also be
fixed with tslib but we tried and we could not. This was the last
message in the thread so I took it as a conclusion.

>> Most people who hear about the filters blindly say "this should be
>> done in user-space" without considering this fact.
>
> I'm the original author of tslib.  Don't think for a moment that I haven't
> any experience in this area.

I already knew, and I know, I know :-)

Perhaps I need to replace talk with data but I don't really feel like
doing it now. I could do it later.

If everyone solves the noise problem in resistive touchscreens with
tslib then there is no need for in-kernel filtering. But then I see
the reply I quoted from the tslib ML where it is stated that one has
to learn to live with the "sloppy positioning and mis-clicks". I don't
think this is acceptable if there is a solution.

As you wrote before it will not work in all cases, but I've seen it's
working for a few s3c-based projects.