[LightDM] hotplug problems and questions

[Robert Ancell]

Hi Martin,

LightDM is not involved in monitor detection / setup so it seems likely the
problem is with your X server and/or the greeter you are using.

Specifically what happens is:
1. LightDM starts an X server
2. When the X server is ready, LightDM starts a greeter

The greeter listens for XRANDR events (if it is capable, unity-greeter is,
I'm not sure about the other ones) and reconfigures the monitors
appropriately. The greeter resizes / changes layout to match the monitors
(again, depends on greeter). This is the same behaviour as inside any X
session (e.g GNOME, KDE).

If the session (greeter or user session) does not configure the monitors
using XRANDR X has a default behaviour which I believe is to mirror the
displays. However, some distributions patch this to be span by default.

You can check in /var/log/lightdm/lightdm.log to see how LightDM is
starting the X server and /var/log/lightdm/x-0.log to see what the X server
is reporting.

--Robert


On 6 July 2013 08:54, Martin Carroll <martin.carroll at alcatel-lucent.com>wrote:

> 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
>
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