Home / ubuntu / Questions / 697529
Accepted
user.dz
Asked: 2015-11-14 01:54:16 +0800 CST

LightDM & SDDM fight on greeter screen

  • 772

I had installed Kubuntu 15.04 sometime ago. I upgraded to 15.10, things were going fine till I added ubuntu-desktop to use Unity DE.

When booting with systemd, on startup or login, I get black or white screen for all Virtual Terminamls (TV's / TTY's) including TV7 graphical one. In some rare cases flickering screen between lightdm & sddm GUI, and that's how I got to know that both were running and may be the cause of this problem (black screen).

It works fine: when booting with upstart or switching DM back to sddm.

  • Flickering screen between KDE/sddm Unity/lightdm

    https://youtu.be/xI1fpTyklNo
    https://youtu.be/HpH309GqFcU

  • ps ax | grep dm

     1377 ?        SLsl   0:00 /usr/sbin/lightdm
 1459 tty7     Ssl+   1:10 /usr/bin/X -core :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
 1526 ?        Ssl    0:00 /usr/bin/sddm
 1626 ?        SLl    0:00 lightdm --session-child 12 19
 3843 pts/2    S+     0:00 grep --color=auto dm

  • systemctl status default.target

    ● graphical.target - Graphical Interface
   Loaded: loaded (/lib/systemd/system/graphical.target; static; vendor preset: enabled)
   Active: active since Fri 2015-11-13 09:46:21 CET; 50min ago
     Docs: man:systemd.special(7)

Nov 13 09:46:21 BlueSkies systemd[1]: Reached target Graphical Interface.

  • sudo dpkg-reconfigure lightdm

    Default display manager:
    gdm
  * lightdm
    sddm
    xdm

  • ls -l /etc/systemd/system/display-manager.service

    lrwxrwxrwx 1 root root 35 Nov 13 17:26 /etc/systemd/system/display-manager.service -> /lib/systemd/system/lightdm.service

I don't want to remove any DM, also I do want to keep LightDM as default. So, is there a canonical answer to How to set the default DM for this mixed release 15.10 (systemd/upstart)?

I could find many similar question without the answer I'm looking for:

upgrade

1 Answers

  1. Best Answer

    I end up adding a ExecStartPre line to check for default display manager on legacy settings /etc/X11/default-display-manager, now it's working well:

    /lib/systemd/system/sddm.service

    [Unit]
Description=Simple Desktop Display Manager
Documentation=man:sddm(1) man:sddm.conf(5)
After=systemd-user-sessions.service [email protected] plymouth-quit.service

[Service]
ExecStartPre=/bin/sh -c '[ "$(basename $(cat /etc/X11/default-display-manager 2>/dev/null))" = "sddm" ]'
ExecStart=/usr/bin/sddm
Restart=always
#PrivateTmp=yes

[Install]
Alias=display-manager.service
WantedBy=graphical.target

    Reference: Copied from /lib/systemd/system/lightdm.service

    I'm not familiar with systemd, so no idea why sddm was started on boot. I have gdm too, it does not start by itself.

    Update: I Could find a related bug report

    • 4