On Tue, 9 Mar 2021 12:40:46 +0100
Oleksij Rempel [off-list ref] wrote:

Hi Jonathan,

On Sat, Mar 06, 2021 at 02:59:59PM +0000, Jonathan Cameron wrote:

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On Sat, 6 Mar 2021 14:28:52 +0100
Oleksij Rempel [off-list ref] wrote:
  

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On Fri, Mar 05, 2021 at 07:02:39PM +0000, Jonathan Cameron wrote:  

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On Fri, 5 Mar 2021 14:38:13 +0100
Oleksij Rempel [off-list ref] wrote:
    

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Basically the TI TSC2046 touchscreen controller is 8 channel ADC optimized for
the touchscreen use case. By implementing it as IIO ADC device, we can
make use of resistive-adc-touch and iio-hwmon drivers.

So far, this driver was tested with custom version of resistive-adc-touch driver,
since it need to be extended to make use of Z1 and Z2 channels. The X/Y
are working without additional changes.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>    

Hi Oleksij,

To consider this as a possible long term route instead of just making this
a touchscreen driver, we'll want to see those mods to the resistive-adc-touch.
Of course that doesn't stop review of this in the meantime.    

ok.

I had following issues with the existing resistive-adc-touch driver:
- the buffer layout is not configurable over DT or i didn't understood
  how to properly configure it
- the "pressure" channel provide pre processed data driver or
  controller, this information cannot be extracted directly from the
  touchscreen plates.

I did following changes to make it work for my use case:

--- a/drivers/input/touchscreen/resistive-adc-touch.c
+++ b/drivers/input/touchscreen/resistive-adc-touch.c

@@ -44,15 +44,34 @@ static int grts_cb(const void *data, void *private)
 {
 	const u16 *touch_info = data;
 	struct grts_state *st = private;
-	unsigned int x, y, press = 0x0;
+	unsigned int x, y, press = 0x0, z1, z2;
+	unsigned int Rt;
 
 	/* channel data coming in buffer in the order below */
-	x = touch_info[0];
-	y = touch_info[1];
+	// TODO: make sure we get buffers in proper order  

Ah.  So to figure this out we'll need to read some more info about the
channels.  The phandle order for the touchscreen binding
should probably be specified (if it's not already) and that should let
us establish the ordering of channels.   

Ack. So this should be done in the touchscreen driver and can be done
later?

Sure. I don't recall if we have handy look up functions to make this easy
but we can add some as needed for the touchscreen side of things.

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+	x = touch_info[3];
+	z2 = touch_info[2];
+	z1 = touch_info[1];
+	y = touch_info[0];
+
+	if (z1) {
+		Rt = z2;  

So for this we are going to need to define it in a generic fashion - probably
via a mode + coefficients in DT?  

I assume, mode will be needed any way, coefficients can stay as is and
if we get some different use case add an overwrite binding to the
devicetree.

Ok, we can use these as default values.

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+		Rt -= z1;
+		Rt *= 800;
+		//Rt *= ts->x_plate_ohms;
+		Rt = DIV_ROUND_CLOSEST(Rt, 16);
+		Rt *= x;
+		Rt /= z1;
+		Rt = DIV_ROUND_CLOSEST(Rt, 256);
+	} else
+		Rt = 0x400;
+
 	if (st->pressure)
-		press = touch_info[2];
+		press = Rt;
 
-	if ((!x && !y) || (st->pressure && (press < st->pressure_min))) {
+	//printk("%s:%i: x: %x, y %x, z1: %x, z2: %x, press: %x\n", __func__, __LINE__, x, y, z1, z2, press);
+	//if ((!x && !y) || (st->pressure && (press < st->pressure_min))) {
+	if ((!x && !y) || (st->pressure && (press > 0x350))) {
 		/* report end of touch */
 		input_report_key(st->input, BTN_TOUCH, 0);
 		input_sync(st->input);

@@ -116,7 +135,7 @@ static int grts_probe(struct platform_device *pdev)
 	}
 
 	chan = &st->iio_chans[0];
-	st->pressure = false;
+	st->pressure = true;
 	while (chan && chan->indio_dev) {
 

  

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There are quite a few things in here that feel pretty specific to the touchscreen
usecase. That makes me wonder if this is a sensible approach or not.    

I'm sure it is the right way to go. Here is why:

A typical resistive touchscreen can be described as 2 resistors (plates)
shorted to each other on pressure:

          o Y+
          |
          #
	  #
          # /---- shorted on pressure
          |/
o---###---+---###--o
X-        |        X+
          #
          #
          #
          |
          o Y-


To find the location of shorted circuit (finger position) we need to
measure voltage on different points of the circuit:
- to get X-position, apply voltage on X+/X- and measure voltage on Y+
- to get Y-position, apply voltage on Y+/Y- and measure voltage on X+

Measuring the "pressure" is a bit more tricky:
- apply voltage on X-/Y+ and measure on X+ and Y-, so we will get Z1 and
  Z2
- will need to know real plate resistance to do following calculation:
  Rtouch = Rx-plate * (X-position / 4096) * (Z2/Z1 - 1)

There is is still more points which share all resistive touchscreens:
- they have parasitic capacitance, so it take some time between
  switching to voltage on and usable measurements
- they act as antenna, so we measure different kind of electrical noise
- we have low-frequency mechanical waves on the plates which can trigger
  some bounce artifacts
- the results will change depending on the temperature and the supply
  voltage. So we need to monitor both of them to adjust our results.

To handle this issues we need to skip some samples until voltage is
settled, we need to apply some simple digital low-pass filter to
reduce the noise and add some corrections if we are able to measure
system temperature and voltage.

All of described measurements are more or less touchscreen and not
touch controller specific. IMO, resistive-adc-touch provide proper
abstraction separation and should be a long therm - way to go :)

Now is the question, what is TSC2046? Please see the block diagram on
page 10:
https://www.ti.com/lit/ds/symlink/tsc2046.pdf

If I oversimplify this diagram, this controller is an ADC with 3 pin
muxes:
- ADC input mux
- termistor mux chained after ADC mux
- voltage output mux

all touchscreen specific code within this driver can be moved to the
resistive-adc-touch, but need some optimization or discussion on how
this should be done properly.   

ok.  There is definitely an argument in favour of generic code.  Whether
the method of IIO provider and touch screen driver consumer makes sense
is a little less clear but it's certainly a reasonable option.
  

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---
+/*
+ * Default settling time. This time depends on the:
+ * - PCB design
+ * - touch plates size, temperature, etc
+ * - initial power state of the ADC
+ *
+ * Since most values higher than 100us seems to be good, it make sense to
+ * have some default value. These values were measuring get by testing on a
+ * PLYM2M board at 2MHz SPI CLK rate.
+ *
+ * Sometimes there are extra signal filter capacitors on the touchscreen
+ * signals, which make it 10 or 100 times worse.    

Sounds like something that makes sense to expose in dt?    

yes. we need to discuss what is the proper place. Is it possible to
grub enough buffers in CPU effective way withing one SPI transfer and
then hand over it to the resistive-adc-touch driver?

I placed it within ADC driver for optimization. IMO, proper place should
be resistive-adc-touch driver.  

It isn't that unusual to have a generic ADC that needs some settling time
before a good result is obtained, so it might be fine to keep an element of
that in the ADC driver, perhaps even defining a standard interface to do
it.   It is a somewhat similar requirement to oversampling, just that you
drop the first N samples rather than averaging them.  

ok, sounds good. 

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+	 */
+	if (ret < 0 && ret != -EBUSY) {
+		dev_err_ratelimited(dev, "%s: Filed to push scan buffer %pe\n",    

failed    

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+				    __func__, ERR_PTR(ret));
+
+		return ret;
+	}
+
+	return valid ? 0 : -EAGAIN;    

If not valid, why are we pushing it to the buffer?    

The touchscreen driver will classify this buffer as finger-up event.
and the ADC driver need to know it to stop sampling.  

That is a fairly specific thing to do.  Does the existing 
touch screen driver do that, or it that a proposed change?  

existing driver is already doing it, the proposed change change only
change the way how this information is extracted. The ADC driver needs
only provide enough samples, so that touchscreen driver can detect it.

Ok. I'm fine with that in the touchscreen side of things.