FBIOGET_VSCREENINFO does not work with alias

[Rainer]

At first my environment:

rainer@BN591-LINUX:~$ lsb_release -rd
Description: Ubuntu 11.10
Release: 11.10
rainer@BN591-LINUX:~$

today Oct. 31 2011 no updates outstanding

xserver-xorg-video-displaylink installed version 0.3-ubuntu9

I tried to get the displaylink running as a second screen. After a long expirience I get it working. The problem was that i got: ioctl FBIOGET_VSCREENINFO: Operation not permitted. I find an eays way to reproduce the error:

rainer@BN591-LINUX:~$ fbset -fb /dev/dl0
ioctl FBIOGET_VSCREENINFO: Operation not permitted
rainer@BN591-LINUX:~$

You should known that /dev/dl0 is an valid alias which i produced using /etc/udev/rules.d/10-rainder.rules. Those file contain:
SUBSYSTEM=="usb", ATTRS{idVendor}=="17e9", ATTRS{idProduct}=="0141", SYMLINK+="dl0"

here is the output of ls /dev :
rainer@BN591-LINUX:~$ ls /dev/dl*
/dev/dl0
rainer@BN591-LINUX:~$ ls /dev/fb*
/dev/fb0 /dev/fb1
rainer@BN591-LINUX:~$

/dev/dl0 is an alias of /dev/fb0

if i do fbset for the real displaylink adapter the output is:
rainer@BN591-LINUX:~$ fbset -fb /dev/fb0

mode "1440x900-60"
    # D: 106.508 MHz, H: 55.939 kHz, V: 59.892 Hz
    geometry 1440 900 1440 900 16
    timings 9389 232 80 25 3 152 6
    hsync high
    rgba 5/11,6/5,5/0,0/0
endmode

rainer@BN591-LINUX:~$

The ioctl FBIOGET_VSCREENINFO should work on the alias too. I need the alias because dependent on my attachment the displalinkadapter has different /dev/fbn. To prevent that i used the alias.