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[Laércio de Sousa <lbsousajr@xxxxxxxxx>]

A possible workaround is preventing your seat-1 X server from
suspending. Include the following config to your lightdm.conf:

[Seat:seat-1]
xserver-command=/usr/bin/X -core -dpms -s 0

Anyway, if you swap your video cards bewteen seats (i.e. swap "seat0"
and "seat-1" occurences in your xorg.conf), do you see the same
behaviour?

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https://bugs.launchpad.net/bugs/1209008

Title:
  [SRU] X doesn't work for video cards with non-KMS drivers on non-seat0
  seats

Status in X.Org X server:
  Fix Released
Status in xorg-server package in Ubuntu:
  Fix Released
Status in xorg-server source package in Trusty:
  Fix Committed

Bug description:
  [Impact]

  * In Xorg-server versions older than 1.16, if Xorg is started as a
  non-seat0 X server (i.e. if it was started with "-seat XXXX" option
  where "XXXX" != "seat0"), it only probes platform bus for graphics
  devices, leaving devices with non-KMS drivers (like NVIDIA proprietary
  drivers and old ones like xf86-video-siliconmotion) unreachable.

  Patch "xfree86_allow_fallback_to_pci_bus_probe_for_non_seat0.patch"
  from lp:~ubuntu-multiseat/xorg-server/trusty-matchseat fixes this.

  * systemd-logind has no intrinsic mechanism to assign non-KMS graphics
  devices to seats. In this case, one should tag another seat device
  (keyboard, USB hub, etc.) in udev as "master-of-seat", and provide a
  suitable xorg.conf for that seat. In order to avoid that a given
  xorg.conf affects other seats, it should be passed to Xorg via
  "-config xorg.conf.custom" option, which requires X server command
  line customization support in DM in use (GDM, for example, has no such
  support).

  Patch "xfree86_add_matchseat_key_to_xorg_conf.patch" from lp:~ubuntu-
  multiseat/xorg-server/trusty-matchseat introduces a new MatchSeat key
  for xorg.conf sections "Device", "Screen", and "ServerLayout", which
  will target a given seat only. Example file
  /etc/X11/xorg.conf.d/90-seat-1.conf:

  Section "Device"
      Identifier "card0"
      Driver "nvidia"
      Option "NoLogo" "True"
      # This section will only apply for seat-1
      MatchSeat "seat-1"
  EndSection

  * systemd-logind can't handle any VT switching for non-seat0 seats,
  but Xorg-server older than 1.16 still opens VT even if it's started as
  a non-seat0 X server. This may lead to a race condition between X
  servers: if a non-seat0 one is started before a seat0 one, the former
  "steals" VTs from the later. If a seat0 X server can't open VTs,
  systemd-logind can't activate sessions for seat0, leaving user without
  sound or usb input support, for example.

  Patch "xfree86_keep_non_seat0_from_touching_vts.patch" from lp
  :~ubuntu-multiseat/xorg-server/trusty-matchseat fixes this.

  [Test Case]

  * Set up a multiseat system where your non-seat0 seats use non-KMS
  video drivers (you may need to tag another device in udev as "master-
  of-seat" in order to achieve it).

  * When LightDM/GDM tries to start a Xorg server for your non-seat0
  seats, it will end up with the following error: "(EE) No devices
  found".

  * After applying
  xfree86_allow_fallback_to_pci_bus_probe_for_non_seat0.patch, you can
  configure your seats in LightDM by setting appropriate "xserver-
  config" properties for each seat. However, you can't configure them in
  GDM.

  * After applying xfree86_add_matchseat_key_to_xorg_conf.patch,
  configure your seats properly and restart LightDM/GDM services (or
  reboot your computer) repeatedly. In some iterations, it may happen
  that logind can't activate graphical sessions for seat0. If it occurs,
  check output of command "ps -FC Xorg" and you'll see that a non-seat0
  Xorg server has a lower PID than seat0 one. Moreover, the non-seat0
  has opened a TTY, while the seat0 hasn't.

  [Regression Potential]

  * Very low. All these patches were well tested in xorg-server "trusty"
  package available in ppa:ubuntu-multiseat/ppa.

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