View source for Calibration of resistive touchscreens

On devices with resistive touchscreens, the touchscreen has to be calibrated.

At our yocto images, we usually use libinput as library for handling input.

===== Prerequisites =====
* Yocto packages ''weston'', ''weston-init'' and ''weston-examples'' have to be installed inside the image.
* The config file <tt>/etc/xdg/weston/weston.ini</tt> needs the following lines (to enable touchscreen calibration):
 [libinput]
 touchscreen_calibrator=true
After changing the configuration of weston, weston needs to be restarted:
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| <tt>systemctl restart weston</tt>
|}

===== Steps for calibration =====

'''1.''' Get the touchscreen device name with weston-touch-calibrator
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| <tt>XDG_RUNTIME_DIR=/var/run/user/$(id -u weston) weston-touch-calibrator</tt>
|}

Example result (bold part is needed for next steps):
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| <tt>device "'''/sys/devices/platform/soc/2100000.bus/21a8000.i2c/i2c-2/2-0049/input/input0/event0'''" - head "DPI-1"</tt>
|}

'''2.''' Run weston-touch-calibrator on the device for starting the calibration. With the example part of the last result, the command looks like this (bold part is from the last result):
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| <tt>XDG_RUNTIME_DIR=/var/run/user/$(id -u weston) weston-touch-calibrator -v '''/sys/devices/platform/soc/2100000.bus/21a8000.i2c/i2c-2/2-0049/input/input0/event0'''</tt>
|}

Now you press the highlighted points at the touchscreen to calibrate it. You will now get an output on the console like this (the important part is marked bold):
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| <tt>Calibration values: '''-0.001472 -1.081086 1.033880 -1.135099 0.000319 1.082698'''</tt>
|}

'''3.''' To make the calibration permanent, you need to add a udev rule. Create the file <tt>/etc/udev/rules.d/touchscreen-cal.rules</tt> and use the calibration values there. With the result of the previous example, the file looks like this (bold part is the result of the last example):
 SUBSYSTEM=="input", KERNEL=="event[0-9]*", ENV{ID_INPUT_TOUCHSCREEN}=="1", ENV{LIBINPUT_CALIBRATION_MATRIX}="'''-0.001472 -1.081086 1.033880 -1.135099 0.000319 1.082698'''"