[Libdlo] xinerama displaylink xorg config working (just)

Luke Kenneth Casson Leighton lkcl at lkcl.net
Mon Nov 16 14:56:24 PST 2009

by hook and by crook i've managed to get a displaylink xinerama
configuration working that i am totally dissatisfied with.  nothing to
do with displaylink - at all: it's xorg that is complete pants.

the working but otherwise undesirable configuration is here:


i'm using usbseat.sh from multiseat but have disabled gdm activation,
which means that the symlink /dev/usbseat/1/display is used in the

i've had to _completely_ hack down the "main" display (an intel 945
chipset with a 1280x800 24bpp laptop screen) to a pathetic (and very
odd-looking) vesa 1024x768 at 16 bpp, and make the displaylink screen
the primary and the laptop the secondary, because xorg cannot cope

* running 16bpp on the displaylink and 24bpp on the laptop screen

* running the "intel" xorg driver AT ALL at the same time as the
displaylink (xorg segfaults)

* placing the displaylink as the "second" display - at all.

* 1280x800 on the vesa xorg driver: xorg detects 1280x800 but cannot
cope with using it (i've never been able to get that to work, even
without the displaylink).

if you specify "Screen 0" and "Screen 1" as advised, again, xorg segfaults;

if you leave out "Screen 0" and "Screen 1" as in the above config, but
have the screens in the reverse order "DisplayLink Screen" rightof
"Default Screen", xorg completely fails to take note of the existence
of the DisplayLink Screen in any way, shape or form.

basically if you specify "Screen 0" as the main screen, by default or
by explicit config options, xorg segfaults.

if you allow auto-detection of parameters (remove ServerFlags Option
AutoEnableFlags and AutoAddDevices), xorg segfaults.

this unsatisfactory arrangement was literally the only configuration
that would work that did not result in a crash of xorg or in behaviour
inconsistent with the configuration file.

i think basically that the crux of the matter is that xorg was never
designed for xinerama use with different types of screens.

also i suspect that other people may have better luck with non-intel drivers.

i _did_ also manage to get fbdev working (rather than vesa) on the
"main" screen, but, again (and this is the linux kernel intelfb driver
this time... *sigh*), intelfb doesn't support 1200x800 screen sizes
(!) so i am forced to use 1024x768 @ 16bpp.

but, aside from all that: it _does_ actually work, which is a starting
point, at least.


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