How to activate UDL DRM from xorg.conf?

[Böszörményi Zoltán]

Hi,

I need to setup a three monitor system on an Intel based
touchscreen POS machine using separate X screens so standalone
fullscreen apps can be shown on each screen.

Using the Intel driver with two Section "Device" with Driver "intel"
and different ZaphodHeads settings work so I can have DISPLAY=:0
and DISPLAY=:0.1 already.

What I can't seem to be able to achieve is using a 3rd
Section "Device" that would use the modesetting driver over
the udl / udldrmfb kernel driver. Setting

     Option "kmsdev" "/dev/dri/card1"

does not help. Also, /dev/fb1 does not appear after udldrmfb is
registered during boot. /dev/fb0 for i915 exists.

When Xorg attempts to use both the two device sections with the
Intel driver AND the third device section with the modesetting
driver to drive the Intel chip and the result is a crash in the server.

When I try with an empty configuration, Xorg picks up all
Intel outputs and it also sees the XRandr provider from UDL
but no output is activated for it.

The thing is that I need a fixed configuration with pre-set
resolutions for all monitors in xorg.conf so I can also
pre-calculate the touchscreen transformation matrix in advance.

So, how can I convince the modesetting driver to activate the
UDL KMS mode from xorg.conf?

I am using kernel 4.18.11 and Xorg 1.19.6.

Thanks in advance,
Zoltán Böszörményi