Re: [PATCH v9 1/8] drivers:input:tsc2007: add new common binding names, pre-calibration, flipping and rotation

From: H. Nikolaus Schaller
Date: Sat Jan 28 2017 - 16:45:18 EST

Hi Dmitry,

> Am 28.01.2017 um 20:33 schrieb Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>:
> Hi Nikolaus,
> On Wed, Dec 28, 2016 at 03:53:16PM +0100, H. Nikolaus Schaller wrote:
>> commit b98abe52fa8e ("Input: add common DT binding for touchscreens")
>> introduced common DT bindings for touchscreens [1] and a helper function to
>> parse the DT.
>> commit ed7c9870c9bc ("Input: of_touchscreen - add support for inverted / swapped axes")
>> added another helper for parsing axis inversion and swapping
>> and applying them to x and y coordinates.
>> Both helpers have been integrated to accommodate any orientation of the
>> touch panel in relation to the LCD.
>> A new feature is to introduce scaling the min/max ADC values to the screen
>> size.
>> This makes it possible to pre-calibrate the touch so that is (almost)
>> exactly matches the LCD pixel coordinates it is glued onto. This allows to
>> well enough operate the touch before a user space calibration step can
>> improve the precision.
> I question whether doing scaling in kernel is really right solution.

Since lower left corner does not exactly report [0 0] and upper right corner
does not report [4095 4095] from the ADC we need offset and steepness correction
of the ADC values.

This steepness is the scaling that must happen in kernel and I don't understand
why you want to propagate this ADC errors to user-space by avoiding scaling.

Let me iterate what we want to achieve:
* use new common bindings
* offset and steepness calibration of the ADC (called pre-calibration).
This makes a real device much more reliable to operate with factory installed
scaling factors.
* flipping and rotation

(note that touch pixel to LCD pixel scaling is not explicitly on this list!)

Now to achieve the ADC pre-calibration we must calculate

x' = (x - ti,min-x) / (ti,max-x-ti,min-x) giving a rante from 0.0 ... 1.0

This is scaled up to what is defined by touchscreen-size-x, i.e.

x' = (touchscreen-size-x * (x - ti,min-x)) / (ti,max-x-ti,min-x)

How do you want to avoid this scaling to take place? It happens automatically.
It is not even an additional line of code. And is necessary for compensating ADC
offsets and steepness.

So the only way to avoid the scaling option is to eliminate the precalibration/ADC
compensation which is essential for a device which has no means to properly
calibrate before operating the device through touch.

The other option would be to avoid common bindings and set

touchscreen-size-x = (ti,max-x-ti,min-x)

This is heavily dependent on specific ADC offsets forwarded to user-space.
IMHO the worst we can do (and the current tsc2007 driver does it that way!).

> Why do you want this?

It seems that you assume that we want to enforce 1:1 scaling between touch pixels
and LCD pixels and have designed code to achieve exactly that.

This is not the case. It is just a byproduct that you can do it.

And since it is easier to understand we have made the examples this way.

> If your touch resolution is lower than your screen
> then it might be useful, but if it is lower then you are losing data
> that can be very helpful for gesture recognition, and I hope you design
> your userspace so it can handle not only "bad" hardware, but "good" as
> well. And even with "bad" there are a lot of tricks that can be done to
> get "better" touch position in userspace.

You can just *choose* DT parameters touchscreen-size-x to match the LCD size.
This of course reduces the touch precision to full LCD pixels. For finger-
touch operated devices, subpixel precision is rarely needed.

Also, some user-spaces (e.g. older Replicant for GTA04) assume that there is
such an 1:1 mapping and they will be perfectly happy about this.

But, if you can modify your user-space easily, you can also choose a different

You can for example define touchscreen-size-x=<4096> and then you get almost
the highest precision of the ADC and won't loose any bits. Or even define a
bigger range and get steps >1 bit.

LCD driver (e.g. X11 calibration matrix) can scale it down again to LCD pixels.

So effectively we get for LCD pixels when the touch sets a mouse pointer:

x'' = user-space-scale * <<input-event<< ((touchscreen-size-x * (x - ti,min-x)) / (ti,max-x-ti,min-x))

The most simple setup would then be but others are possible:

user-space-scale = 1
touchscreen-size-x = LCD-size-x

Let me cite myself:

>> This makes it possible to pre-calibrate the touch so that is (almost)

>> Please note that the old ti,fuzz properties have been removed since they
>> are replaced by the common bindings touchscreen-fuzz-x/y/z.
>> Finally, calculate_pressure has been renamed to calculate_resistance
>> because that is what it is doing.
> That is not what your patch does though. In the presence of
> "ti,report-resistance" parameter you start reporting resistance through

Well, there is some historic confusion wether this driver reports resistance
or pressure.

The unpatched tsc2007 driver does it wrong (please test!) and we fix it on
the fly (because a separate patch is much more complex than doing it right

This ti,report-resistance property is a means to get the old (wrong) meaning back
in case someone urgently needs it and can't fix the user-space workaround which
he must be using.

AFAIK there is no mainline board using the DT except ours (and the upcoming
OMAP5-Pyra), so we shouldn't care too much. If you prefer, you can remove this
compatibility property. We don't need it for our devices.

That the function name is wrong is a second issue and this double negation might
confuse a litte.

Please test on a real device if the patched driver reports pressure now (unless
ti,report-resistance is specified).

> without any indication to the userspace that meaning of
> event changed. This is no better if than reporting it through ABS_X. You
> should not override meaning of input events.

BR and thanks,