The plasma 5 systemsettings fontsettings panel has a checkbox for forcing a DPI value for fonts and a accompanying DPI value field. So far so good.

On a MacBook-pro with retina display this field is always set to 96 DPI, which is wrong. This setting forces me to adjust the value manually to 192 doubling the size to obtain a reasonable desktop. The correct physical DPI value is 227.

After some research for other non Qt based applications, I found that 96 is always reported by Xinerama on this machine, with all other values like size and so on computed to match 96 DPI. However XRandR reports the size in millimeter correctly. Based on this information and the desktops pixel size, the DPI can be computed correctly.

Reproducible: Always

Steps to Reproduce:
1. install KDE Plasma 5 on MacBookPro Retina
2. see the too small desktop windows rendering
3. go to systemsettings -> font settings -> force DPI box

Actual Results:  
see 96 DPI in the fonts force DPI value box

Expected Results:  
see the xrandr reported DPI value in the fonts force DPI value box

// Here some code showing the correct DPI values by using the XRandR API.
// Hope this helps.
// cc -Wall -g -pedantic XRandRfl.cxx -o XRandRfl -lX11 `pkg-config --libs xrandr`
#include <X11/extensions/Xrandr.h>
#include <stdio.h>

int main(int argc, char ** argv)
{
  Display * display = XOpenDisplay(":0.0");
  int screen = DefaultScreen( display );
  Window w = RootWindow(display, screen);
  XRRScreenResources * res = XRRGetScreenResources(display, w);
  for(int i=0; i < res->noutput; ++i)
  {
    XRROutputInfo * output_info = XRRGetOutputInfo( display, res, res->outputs[i]);
    if(output_info->crtc)
    {
      XRRCrtcInfo * crtc_info = XRRGetCrtcInfo( display, res,
                                                output_info->crtc );
      unsigned int pixel_width = crtc_info->width,
                   pixel_height = crtc_info->height;
      float xdpi = pixel_width * 25.4f / (float)output_info->mm_width,
            ydpi = pixel_height * 25.4f / (float)output_info->mm_height;
      printf( "[%d] %upx x %upx  Dimensions: %limm x %limm  DPI: %.02f x %.02f\n",
              i, pixel_width, pixel_height,
              output_info->mm_width, output_info->mm_height,
              xdpi, ydpi );
    }
    XRRFreeOutputInfo( output_info );
  }
  XRRFreeScreenResources(res);
  XCloseDisplay( display );

  return 0;
}