screen physical size, DPI, font size - how does it work?

[Alexej Davidov]

Hi,

up to now I've set the DPI value explicitely with --dpi 100. This gives
sensible font sizes. I've now tried to let X do it automatically, but
with no success. The detected physical screen size is wrong and I
didn't manage to convince X of the correct one. So I always get 96x96
dpi. Also I don't really understand How the DPI value given in Xorg.log
correlates to the actually set value.

I use the ati driver on a R300 (version 6.7.195) with xorg-core 1.4 on
an IBM Thinkpad.

I'll explain my struggles to get X to use the correct values. What I
want to know is, what the right way is to do this. Maybe I'm
completely misguided and it's all different now with xrandr-1.2.

The other thing is: if I set the resolution to the theoretically
correct value, which is 125x125 dpi in my case (1400x1025 at
285x215mm), all the fonts are far to big. 

1) not specifying anything in xorg.conf:
Xorg.0.log says:
...
(II) RADEON(0): Setting screen physical size to 370 x 277

The physical size is wrong. It is actually 285x215 mm. Also,
xdpyinfo gives 96x96 dpi. How does this relate to the entry in Xorg.log?

2) specifying the physical screen size in a monitor section and binding
the monitor to the LVDS output:

I've done the following in xorg.conf:

Section "Device"
  ...
  Option "Monitor-LVDS" "Thinkpad Display"
EndSection

Section "Monitor"
    Identifier  "Thinkpad Display"
    DisplaySize 285 215
EndSection

Now, in Xorg.log I get:
(II) RADEON(0): Output LVDS using monitor section Thinkpad Display
but I still get
(==) RADEON(0): DPI set to (75, 75)
Also the pysical size is still wrong:
(II) RADEON(0): Setting screen physical size to 370 x 277

3) specifying the monitor in the screen section:
In addition to what I've done in 2), I've also specified the monitor in
the screen section. Now this monitor is bound to both, VGA-0 and LVDS:
(II) RADEON(0): Output VGA-0 using monitor section Thinkpad Display
(II) RADEON(0): Output LVDS using monitor section Thinkpad Display

Now I get a different DPI entry in Xorg.log:
(**) RADEON(0): DPI set to (142, 151)
but still the wrong physical size:
(II) RADEON(0): Setting screen physical size to 370 x 277


No matter what I do, the physical size is always reported as 370 x 277.
xdpyinfo always gives 96x96 dpi. Also, in all 3 cases, Xorg.log
always reports the following:
(II) RADEON(0): Port2:
 Monitor   -- AUTO
 Connector -- Proprietary/LVDS
 DAC Type  -- None
 TMDS Type -- None
 DDC Type  -- None

Regards,
a.