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

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=81476

--- Comment #20 from Alex Deucher <agd5f at yahoo.com> ---
(In reply to comment #19)
> 
> I'd believe it as a hardware limitation if inverting the first two monitors
> didn't fix it.  If I were willing to physically turn my first two monitors
> upside-down, it would work fine with the given hardware.  The hardware can
> display across my three monitors at 4800x1200 as long as two of the screens
> are inverted.  We just need to make the software put those screens into the
> hardware the other way up!

As I said before, it only works with rotation because it's scanning out of
different buffers when rotation is enabled.  When you enable rotation a shadow
buffer is created which is a rotated duplicate of the sub-region of the entire
desktop.  So for the rotated case, the display hw is scanning out of a
1600x1200 surface rather than the full 4800x1200 surface.  If you wanted to you
could do something similar for the non-rotated case in the xserver, but that
would incur an extra copy for every update of the desktop in the non-rotated
case.

If you want to test further, try and disable acceleration (Option "NoAccel"
"true" in the device section of your xorg.conf) or use the modesetting driver
(xf86-video-modesetting).  You won't be able to use rotation since that
requires acceleration do to the rotated blit, but that will take all
non-display hw factors (3D, etc.) out of the equation.

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