Bug 466504

Summary: External touchscreen monitor maps to wrong monitor (in wayland and X11)
Product: [Plasma] kwin Reporter: Kelvie Wong <kelvie>
Component: libinputAssignee: KWin default assignee <kwin-bugs-null>
Status: RESOLVED NOT A BUG    
Severity: normal    
Priority: NOR    
Version First Reported In: 5.27.1   
Target Milestone: ---   
Platform: Arch Linux   
OS: Linux   
Latest Commit: Version Fixed/Implemented In:
Sentry Crash Report:

Description Kelvie Wong 2023-02-27 04:34:38 UTC 
SUMMARY
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STEPS TO REPRODUCE
1. Connect an external monitor with touchscreen via USB-C / DP alt mode (or via USB for the input device, and HDMI for the display)
2. Extend the desktop to the external touchscreen monitor

OBSERVED RESULT

The touchscreen input gets mapped to the wrong monitor


EXPECTED RESULT

The touchscreen input device should be mapped to just the touchscreen display. Or at least this should be configurable, since it's basically impossible to detect what touchscreen belongs to what monitor (although there are heuristics, such as both plugged in at nearly the same time)


SOFTWARE/OS VERSIONS

Operating System: Arch Linux 
KDE Plasma Version: 5.27.1
KDE Frameworks Version: 5.103.0
Qt Version: 5.15.8
Kernel Version: 6.1.12-arch1-1 (64-bit)


ADDITIONAL INFORMATION

lsusb shows the touchscreen as:

Bus 005 Device 025: ID 222a:0001 ILI Technology Corp. Multi-Touch Screen

kscreen-doctor shows:

~ kscreen-doctor -o
Output: 1 eDP-1 enabled connected priority 2 Panel Modes: 0:2256x1504@60*! 1:2256x1504@48 2:1600x1200@60 3:1280x1024@60 4:1024x768@60 5:1920x1200@60 6:1280x800@60 7:1920x1080@60 8:1600x900@60 9:1368x768@60 10:1280x720@60 Geometry: 1920,0 1805x1203 Scale: 1.25 Rotation: 1 Overscan: 0 Vrr: incapable RgbRange: Automatic
Output: 2 DP-4 enabled connected priority 1 DisplayPort Modes: 0:3840x2160@60*! 1:3840x2160@50 2:3840x2160@30 3:3840x2160@30 4:3840x2160@30 5:3840x2160@25 6:3840x2160@24 7:3840x2160@24 8:2560x1440@60 9:1920x1080@60 10:1920x1080@60 11:1920x1080@60 12:1920x1080@60 13:1920x1080@60 14:1920x1080@50 15:1920x1080@30 16:1920x1080@30 17:1920x1080@25 18:1920x1080@24 19:1920x1080@24 20:1400x1050@60 21:1280x1024@60 22:1440x900@60 23:1280x960@60 24:1280x720@60 25:1280x720@60 26:1280x720@60 27:1280x720@50 28:1280x720@30 29:1280x720@30 30:1280x720@25 31:1280x720@24 32:1280x720@24 33:1024x768@60 34:800x600@60 35:720x480@60 36:720x480@60 37:720x480@60 38:720x480@60 39:640x480@60 40:640x480@60 41:640x480@60 42:1600x1200@60 43:1280x1024@60 44:1024x768@60 45:2560x1600@60 46:1920x1200@60 47:1280x800@60 48:3200x1800@60 49:2880x1620@60 50:2560x1440@60 51:1920x1080@60 52:1600x900@60 53:1368x768@60 54:1280x720@60 Geometry: 0,0 1920x1080 Scale: 2 Rotation: 1 Overscan: 0 Vrr: incapable RgbRange: Automatic

where DP-4 is the touchscreen monitor I want to map it to, but it always maps to eDP-1 (and on another system it mapped to DP-2 instead of HDMI-0 like it was supposed to)
Comment 1 Kelvie Wong 2023-02-27 04:51:41 UTC 
Just took a look, and it seems like the code that assigns this is in kwin: kwin/src/backens/libinput/connection.cpp, Connection::applyScreenToDevice

The touchscreen geometry doesn't seem to match the libinput device geometry for this touchscreen.

For the record, libinput list-devices has the touchscreen at 309x174mm, and xrandr has the display listed at 310x180mm (measured it and it's definitely closer to 309x174)
Comment 2 Kelvie Wong 2023-02-27 05:12:51 UTC 
Ultimately, I think we need a way for a user to choose and save the input -> output mapping (probably a job for kscreen?) but in the meantime, perhaps we can change the check here (https://invent.kde.org/plasma/kwin/-/blob/6745cb87ecb57669389df3925f8e1660b8a637ad/src/backends/libinput/connection.cpp#L612-617) :

        auto testScreenMatches = [device](const Output *output) {
            const auto &size = device->size();
            const auto &screenSize = output->physicalSize();
            return std::round(size.width()) == std::round(screenSize.width())
                && std::round(size.height()) == std::round(screenSize.height());
        };

To allow a 5% (or some other value) margin of error in the height and width?
Comment 3 Bug Janitor Service 2023-02-27 05:38:26 UTC 
A possibly relevant merge request was started @ https://invent.kde.org/plasma/kwin/-/merge_requests/3705
Comment 4 Kelvie Wong 2023-02-27 08:32:46 UTC 
As mentioned in the PR comments, apparently there was a toucscreen KCM module that allows this assignment, I just didn't think to look.