keithp seems to be the only one on the interwebs to have written anything about the subject and he worked with 3.5_rc. I don't want to go above 3.4 at the moment for various stability reasons and am trying to see whether I can get this to work. Xorg 1.13 recognizes the display on connection, "udl" module is loaded, xorg-video-modesetting driver also loads, display lights up. So everything seems to be good.

I emerged xrandr-9999 (not many changes on top of 1.3.5):

$ xrandr --listproviders
Providers: number : 2
Provider 0: id: 69 cap: 0x0 crtcs: 2 outputs: 4 associated providers: 0 name:Intel
Provider 1: id: 338 cap: 0x0 crtcs: 1 outputs: 1 associated providers: 0 name:modesetting

But I can't get any further, just like this guy:

$ xrandr --setprovideroutputsource 338 69
X Error of failed request:  BadValue (integer parameter out of range for operation)
  Major opcode of failed request:  139 (RANDR)
  Minor opcode of failed request:  35 ()
  Value in failed request:  0x152
  Serial number of failed request:  11
  Current serial number in output stream:  12

$ xrandr --setprovideroutputsource 1 0
X Error of failed request:  148
  Major opcode of failed request:  139 (RANDR)
  Minor opcode of failed request:  35 ()
  Serial number of failed request:  11
  Current serial number in output stream:  12

Any thoughts?