Comment 18 for bug 201491

Although I'm not an Ubuntu user (I'm on Debian sid), I've a similar problem with the RadeonHD driver:

* Configuration

- On-Board Radeon chip; lspci info:
   01:05.0 VGA compatible controller: ATI Technologies Inc Radeon HD 3200 Graphics
- radeonhd driver 1.2.5
- xorg-server 1.6.1.901 (from experimental)

* Symptoms

- ddcprobe shows correct dimensions: "screensize: 51 29"
- Xorg seems to know proper size (from Xorg.0.log):

(II) RADEONHD(0): Max Image Size [cm]: horiz.: 51 vert.: 29

- But later somehow comes up with incorrect numbers, which
  propagate into xpdyinfo:

(II) RADEONHD(0): clock: 138.5 MHz Image Size: 477 x 268 mm

- xdpyinfo:

  dimensions: 1920x1080 pixels (477x268 millimeters)

- xrandr has the wrong numbers as well (this is different from the original reporter, who had xrandr being right):

  DVI-D_1 connected 1920x1080+0+0 (0x3d) normal (normal left inverted right x axis y axis) 477mm x 268mm

I'm sitting just in front of that monitor, and can help debugging this issue (assuming it's the same issue as the original reporter's). It seems to me that X is using two different ways to get physical screen size, and then chooses the
wrong one.