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Old 08-05-2015, 02:06 PM   #1
Registered: Jun 2010
Location: AZ
Distribution: Slackware
Posts: 106

Rep: Reputation: 1

Hey guys,

I am having some trouble with my DisplayLink IOGEAR External VGA USB adapter. This utilizes the udlfb kernel module. I have read a lot of guides which explain how to configure to use this with X, however since its attached to a server my primary concern is simply getting it to work with a console. I actually was able to get this to work yesterday via con2fb. The udlfb module creates a new framebuffer (/dev/fb1), and con2fb allows you to move an existing tty over to another framebuffer (so I did con2fb /dev/fb1 /dev/tty6).

The problem started after I rebooted my server. When I initially connected the device, I saw a green screen on the monitor attached to the displaylink adapter (as mentioned in udlfb's kernel documentation). However, after I rebooted I now have a black screen.

Upon further inspection, I discovered that udlfb now sets the resoltion to 1600x900--which is a supported mode by my monitor, but not by this adapter apparently.

My goal is to force udlfb to use 800x600. Thats where I'm having difficulty.

Here is udlfb's parameter info:

root@diegodev:~# modinfo udlfb
filename:       /lib/modules/3.18.11/kernel/drivers/video/fbdev/udlfb.ko
license:        GPL
description:    DisplayLink kernel framebuffer driver
author:         Roberto De Ioris <>, Jaya Kumar <>, Bernie Thompson <>
alias:          usb:v17E9p*d*dc*dsc*dp*icFFisc00ip00in*
depends:        fb_sys_fops,sysfillrect,syscopyarea,sysimgblt
intree:         Y
vermagic:       3.18.11 SMP mod_unload
parm:           console:Allow fbcon to open framebuffer (bool)
parm:           fb_defio:Page fault detection of mmap writes (bool)
parm:           shadow:Shadow vid mem. Disable to save mem but lose perf (bool)
parm:           pixel_limit:Force limit on max mode (in x*y pixels(int)
So, I attempt do the following:

modprobe udlfb pixel_limit=800*600
which results in:

modprobe: ERROR: could not insert 'udlfb': Invalid argument
I've also tried:

root@diegodev:~# history | grep "modprobe" | grep udlfb | grep pixel | uniq -u
  446  modprobe udlfb pixel_limit 800*600
  447  modprobe udlfb pixel_limit 800x600
  448  modprobe udlfb pixel_limit=800*600
  449  modprobe udlfb pixel_limit:800*600
  451  modprobe udlfb pixel_limit:800x600
  458  modprobe udlfb pixel_limit=800x600
  459  modprobe udlfb pixel_limit 800x600
  460  modprobe udlfb pixel_limi: 800x600
  482  modprobe udlfb pixel_limit=800*600
  488  modprobe udlfb pixel_limit=800*600
And apparently none of those are correct.

Does anyone have any ideas? Your assistance is greatly appreciated!

udlfb kernel documentation:


Last edited by slugman; 08-05-2015 at 02:07 PM. Reason: added sources
Old 08-05-2015, 02:53 PM   #2
Registered: Jun 2010
Location: AZ
Distribution: Slackware
Posts: 106

Original Poster
Rep: Reputation: 1
Hey guys, I found a solution, and it did not require passing a parameter to udlfb!

This can be considered a "low-tech," solution.

So, I was wondering why udlfb was automatically set to 800x600 when I initially loaded the device. When I was perusing through /var/log/messages, I noticed that when I initially connected the device, udlfb generated an error when it attempted to identify the attached monitor's edid (because there was no monitor connected).

In all of my recent trials, I had the monitor connected, which properly grabbed the edid, and auto set the resolution to 1600x900 (which unfortunately did not work).

Also, unloading the module, and reseating the device to another usb port produced the same results.

However, I decided to try connecting the Displaylink adapter without a monitor attached.

Upon looking at dmesg, I discovered udlfb was set to 800x600!

I connected the monitor's vga cable to the Displaylink adapter and I got the green screen! I was able to resume as normal using con2fb.



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