Well... Now I can tell the whole story:
1. Following http://www.freedesktop.org/wiki/Software/systemd/writing-
display-managers/ , LightDM gains automatic multiseat support in release
1.11.7. This support improves a bit in release 1.11.8, which now watches
on changes in property "CanGraphical" of logind seats. LightDM only
spawns a greeter on a seat that has property "CanGraphical=yes".
2. Reading systemd-logind source code, I've found that a requisite to a
logind seat gain property "CanGraphical=yes" is attaching a device
tagged in udev as "master-of-seat" to it.
3. systemd-logind ships a default set of udev rules that tags any
framebuffer devices (/dev/fb*) as "master-of-seat", which is OK for all
KMS-compliant video drivers like Intel, Nouveau (open-source driver for
NVIDIA graphics devices) and Radeon (open-source driver for AMD graphics
devices).
4. However, NVIDIA or AMD proprietary drivers don't expose kernel
framebuffer devices, so they need their own udev rule to tag another
kernel device they expose as "master-of-seat" (in @elfy example, we tag
kernel device /dev/dri/card0):
SUBSYSTEM=="drm", KERNEL=="card[0-9]*", ATTRS{vendor}=="0x10de", TAG
+="master-of-seat"
5. Without this custom udev rule, seat0 will change his property
"CanGraphical" from "no" to "yes", so LightDM fails to spawn a greeter
on it.
I don't believe that systemd developers will accept to ship this udev
rule in upstream releases, because they only support open-source
drivers.
The right thing to do right now is reporting a bug to NVIDIA proprietary
driver's Ubuntu package maintainers, asking them to ship a suitable udev
rule like the one above. There's nothing to do with lightdm in this
case.
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https://bugs.launchpad.net/bugs/1365336
Title:
Lightdm update=No desktop
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