Does the kernel REALLY have displaylink support?

[oneukum]

Originally Posted by shaun

Hi

I have a Lenovo LT1421 USB external monitor, which ubuntu recognises, so in lsusb I have:

Bus 003 Device 015: ID 17e9:03e0 DisplayLink

and in dmesg:

[ 2462.124838] usb 3-4: new high-speed USB device number 15 using xhci_hcd
[ 2462.144951] usb 3-4: New USB device found, idVendor=17e9, idProduct=03e0
[ 2462.144960] usb 3-4: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 2462.144965] usb 3-4: Product: Lenovo LT1421 wide
[ 2462.144968] usb 3-4: Manufacturer: DisplayLink
[ 2462.144971] usb 3-4: SerialNumber: 6V9CAAB7
[ 2462.146656] [drm] vendor descriptor length:17 data:17 5f 01 00 15 05 00 01 03 00 04
[ 2462.226249] udl 3-4:1.0: fb1: udldrmfb frame buffer device
[ 2462.226258] [drm] Initialized udl 0.0.1 20120220 on minor 1

I do not see the display in xrandr (or in the displays panel). I have edited blacklist-framebuffer.conf to contain these two lines at the end:

# blacklist udl
blacklist udlfb

Ok. As soon as you see "udl 3-4:1.0: fb1: udldrmfb frame buffer device" you know that the kernel driver has bound. So far so good.

I cannot give you an explanation specific to ubuntu, but generically you need:

1. X with support for xrandr1.4
2. kernel 3.6 or newer
3. the "modesetting" X driver

As the kernel driver has bound you need the generic driver for framebuffers with kernel modesetting. That is the "modesetting" driver. Generally, if you've installed it, it will be loaded when you start X.

In addition you need to slave the second graphics device to the first. Before you do that xrandr will <b>not</b> show the attached monitors. Hence your trouble with xrandr.

For example:
xrandr --listproviders
Providers: number : 2
Provider 0: id: 0x5a cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 6 outputs: 5 associated providers: 0 name:radeon
Provider 1: id: 0x4ae cap: 0x2, Sink Output crtcs: 1 outputs: 1 associated providers: 0 name:modesetting

That lists the devices. In my case I need to slave provider 1 to provider 0:
xrandr --setprovideroutputsource 1 0

After that xrandr will list the display and you use it like a second monitor on a primary graphics device.