[LightDM] hotplug problems and questions

[Martin Carroll]

Hi all,

I am having hair-raising difficulties getting lightdm to detect a
hotplugged monitor on my gentoo machines.  I would much appreciate it if
one of you could help me -- I have been unable to find the necessary
info anywhere else.

On my ubuntu laptop, hotplug during lightdm works fine.  Specifically,
if I boot my laptop with just the built-in monitor, wait for lightdm to
present the login screen, and then plug in a second monitor, lightdm
automatically detects the second monitor and presents the login screen
on it.

On my gentoo machine, however, hotplugging during lightdm does not work
-- specifically, the second monitor stays black, and no amount of
poking, banging, and kicking gets it to display the login screen.

I have verified (using udevadm and also some printfs that I added to the
X server) that when I hotplug the second monitor on the gentoo machine,
a udev event *is* generated and *does* make it all the way to the X
server.  The uevent is a "change" event on the gpu device.  The X
server, however, explicitly ignores all "change" events coming from
drm.  So clearly that is not the intended path by which lightdm detects
the second monitor. 

So how is lightdm detecting the hotplugged monitor on the ubuntu
machine?  Is it polling something?  Is X polling something?  Or is my
udev event the wrong type?  Did ubuntu add some magic patch to some
package somewhere?

Before I start adding printfs to my graphics drivers, udev rules, X
server, and lightdm itself, can someone please explain to me the
intended path by which lightdm "sees" the hotplugged monitor and fires
up the login screen on it?  At the moment, I don't even know who the
culprit is on my gentoo machine.

Thanks,
Martin