[Bug 81476] three monitors on two radeon cards works with some layouts not others

bugzilla-daemon at freedesktop.org bugzilla-daemon at freedesktop.org
Fri Jul 18 11:25:09 PDT 2014


--- Comment #7 from Alex Deucher <agd5f at yahoo.com> ---
(In reply to comment #4)
> (In reply to comment #3)
> > They way the core provider stuff in the xserver works the rendering
> > is done on one GPU and the respective updated regions are sent to the
> > other displays.
> If the above limits are true, why do I get a working 4800x1200 desktop when
> I invert two of the three screens?  Why can it show my 4800x1200 desktop if
> two screens are upside-down but not if they are right-side-up?  How is this
> possible if 4800x1200 can't be done? [*]

Rotation uses an additional buffer for each display.  The contents of the
respective rotated displays are copied out of the primary surface over to the
rotated shadow buffer.

If you want to check what coordinate limit is causing a problem, try
configuring the displays for a 3x1 configuration less than 4096 pixels, e.g.,
1600x1200+1600x1200+800x600.  If that works then try bumping the size of the
last monitor above the limit. E.g., 1600x1200+1600x1200+1024x768.

> > If you disable the OpenGL compositor, your desktop should work fine.
> I don't know enough about the graphics chain to know how to "disable the
> OpenGL compositor".  All I do is start Xorg, a window manager, and some
> clients.  There is nothing mentioned about "composit" actually being used
> anywhere in the Xorg.0.log.  The net searches I do all find stuff that's
> years old.  Please point me at some current documentation on how to disable
> this?

It depends on your desktop environment (gonme, kde, etc.) and your window
manager.  E.g., kwin or gnome shell, etc..

> > The OpenGL compositing manager you are running should check for the max
> > limits of the GL driver before using it for surfaces which may exceed that. 
> Yes, it should check instead of quietly screwing up.  Can it not query the
> GPU for the max limit?  (The max limit you say of 4096 is not obvious in the
> Xorg.0.log output anywhere.  Why does xrandr tell me "maximum 8192 x 8192"?)
> To whom should I submit this "failure to check limit" bug report?

8192x8192 is the max surface size the display hardware can handle.  It's
independent of the 3D engine.  The limits of the 3D engine are exposed via
OpenGL and any OpenGL app can query them.

Try starting X without a window manager or a simple one like twm.  It should
work correctly because no OpenGL operations would be used in that case so the
3D engine limits are not getting hit.

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