I'm forwarding this bug report from a Ubuntu user:
With a Iiyama monitor, xdpyinfo reports 300x230mm yet the actual physical size differs. Xorg.0.log lists the correct dimensions (no mention of 300x230mm in the log), and ddcprobe and xrandr also both report the correct dimensions.
Looking through the xdpyinfo code and referring to xlib's implementation of DisplayHeightMM() and DisplayWidthMM(), I didn't spot code that manipulates those values or sets them to default values, so my guess is the EDID from the monitor simply needs quirked? Does it need quirk_detailed_use_maximum_size() like the LPL monitors?
Iiyama ProLite E431S, ATI Radeon 9800 Pro, DVI, -ati
Manufacturer: IVM Model: 46ac Serial#: 16832
== Original Report ==
Binary package hint: xorg
It seems tools like xdpyinfo are reporting screen sizes as 300x230 mm in a lot of cases when they are not.
On my own Iiyama ProLite E431S with an ATI Radeon 9800 Pro connected over DVI using the "ati" driver, I get this from xdpyinfo:
dimensions: 1280x1024 pixels (300x230 millimeters)
resolution: 108x113 dots per inch
ddcprobe correctly detects the screen physical dimensions:
screensize: 34 27
In another report, https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/201032 the reporter shows that xdpyinfo quotes:
dimensions: 1280x800 pixels (300x230 millimeters)
resolution: 108x88 dots per inch
and ddcprobe reports:
screensize: 33 21
Notice that the screens are actually different dimensions, both in physical terms and pixel terms, yet xdpyinfo thinks they are 300x230 mm. Also, the other reporter is using the intel driver.
This did indeed used to work correctly in the early Hardy alphas, and previous releases of Ubuntu.
[Note that the bug the user references is about a LGPhilipsLCD LP154W01-TLD (LPL prod id 48128) monitor, which might be similar to the LGPhilipsLCD LP154W01-A5 from bug #10304, which is quirked with quirk_detailed_use_maximum_size().]
ddcprobe thinks: 34 x 27 (cm) (approx. 95.6 x 96.3 dpi) - Correct!
So radeon thinks 340 x 270 (mm) - yep... correct
xrandr thinks 340mm x 270mm - correct
xdpyinfo thinks 300 x 230 ((approx. 108.4 x 113.1 dpi) WRONG!
In case it wasn't obvious, this only became an issue fairly recently. In Ubuntu 7.10 which shipped Xorg 7.3 (I think), this seemed to work OK.
Xorg reports wrong screen size by design. See bug 23705
This bug is still present. xdpyinfo reports always 96x96 dpi resulting in wrong screen dimensions and incorrect scaling.
$ xdpyinfo | grep -B2 resolution
dimensions: 3840x2160 pixels (1016x571 millimeters)
resolution: 96x96 dots per inch
however, xrandr is right;
$ xrandr | grep DP1
DP1 connected primary 3840x2160+0+0 (normal left inverted right x axis y axis) 600mm x 340mm
I have something maybe related to this problem. Incorrect resolution detected:
dimensions: 5120x1024 pixels (1354x270 millimeters)
resolution: -2147483648x-2147483648 dots per inch
More on https://bugs.launchpad.net/xorg-server/+bug/1650192
xdpyinfo on XRandR 1.2+ screen really reports wrong DPI information. Last version of patch for xdpyinfo which fixes this problem is there: https://lists.x.org/archives/xorg-devel/2017-May/053743.html
Also this patch (v2) fixes reporing of negative values in resolution information.
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