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https://bugs.launchpad.net/bugs/316514

Title:
  MASTER:  Xinerama/Multihead with multiple video cards is not supported
  for most video drivers

Status in X.Org X server:
  Invalid
Status in “xorg-server” package in Ubuntu:
  Triaged

Bug description:
  Binary package hint: xorg

  [Problem]
  X does not support (or does not support very well) displaying a desktop 
across more than one video card.  When it is possible, a variety of bugs are 
experienced.

  [Discussion]
  Prior to the introduction of Xrandr it was (sometimes) possible to configure 
xorg.conf to display on multiple video cards.  One could then use Xinerama, 
etc. to stitch screens into a contiguous desktop.  Of course, any alterations 
to this setup usually required hand-tuning xorg.conf.

  The introduction of Xrandr made a number of things much easier,
  however it is not good at handling the case of multiple video cards.
  It can handle dual-head displays, where both monitors are connected to
  separate outputs on the same video card, but does not work as well
  with outputs on two different cards.

  The fundamental problem (as I understand it) is essentially that X can
  talk to only one physical "pool" of memory, and each card has its own
  physical pool.  Recent developments including GEM and several kernel
  changes promise to remedy this by enabling these pools to be
  aggregated and managed as a single virtual pool.  Once the X server
  and drivers are updated to utilize this new architecture, they should
  be able to support multi-card functionality with Xrandr as well as
  (and better than) the old Xinerama configuration, including
  reconfiguring the display without needing to edit xorg.conf or restart
  X.

  [Exceptions]
  As mentioned above, the problem is most notable with drivers that have 
switched from Xinerama to Xrandr.  If you're using an older driver that still 
uses the old Xinerama approach, you *might* find it works acceptably.

  In my own testing, I've found cases where I could get displays across
  multiple cards (separate screens per-card) when running xrandr, but I
  ran into so many different bugs (mouse not working properly, X
  crashing, display corruption, etc.) that it was essentially unusable.
  Even if it could be made to work, this configuration is quite non-
  standard and not well tested.  Resolving those issues will likely wait
  until the aforementioned architecture is fully in place.

  The closed source -nvidia driver still uses Xinerama (it never got
  around to implementing Xrandr), so you can achieve multiple head
  displays that way.  The nvidia configuration tool will be able to
  construct the xorg.conf settings for you to achieve Xinerama multi-
  head.

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