Bug#918457: xserver-xephyr: multi-screen setup with xinerama and RANDR extensions: extra screens not directly usable

[Michel Dänzer <michel@daenzer.net>]

On 2019-01-06 11:29 a.m., Ondřej Grover wrote:
> Package: xserver-xephyr
> Version: 2:1.19.2-1+deb9u5
> Severity: normal
> 
> Dear Maintainer,
> 
> I'm trying to use Xephyr to test my awesome (WM) multi-screen setup.
> As per instructions found on the Internet I spawned a Xephyr server with 2 screens and the Xinerama and RANDR extensions activated:
> 
> Xephyr -screen 960x540+0+0 -screen 960x540+960+0  +extension RANDR  +xinerama :1 &
> 
> This spawns 2 windows side-by-side (due to the +X+Y panning information, but also works without it and has the same issue), labeled :1.0 and :1.1
> However, when I ask xrandr about the screens, it sees only one screen/output
> 
> DISPLAY=:1 xrandr
> xrandr: Failed to get size of gamma for output default
> Screen 0: minimum 160 x 160, current 1920 x 540, maximum 1600 x 1200
> default connected 960x540+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
>    1600x1200      0.00  
>    1400x1050      0.00  
>    1280x960       0.00  
>    1280x1024      0.00  
>    1152x864       0.00  
>    1024x768       0.00  
>    832x624        0.00  
>    800x600        0.00  
>    720x400        0.00  
>    480x640        0.00  
>    640x480        0.00  
>    640x400        0.00  
>    320x240        0.00  
>    240x320        0.00  
>    160x160        0.00  
>    960x540        0.00*
> 
> So when I start awesome (WM) on this display, it detects only the first screen/output
> 
> DISPLAY=:1 awesome &
> 
> Now if I attempt to start an xterm on the first screen, it works correctly
> 
> DISPLAY=:1.0 xterm &
> 
> I can even move this terminal toward the right screen and it becomes visible on the second (:1.1) screen, so perhaps Xinerama is working.
> 
> However, If I attempt to start an xterm on the second screen directly, it fails (even without awesome)
> 
> DISPLAY=:1.1 xterm &
> xterm: Xt error: Can't open display: :1.1

That's the point of Xinerama. :) It combines the "physical" screens to a
single logical one. You can see the information about the "physical"
screens provided by the Xinerama extension via

 xdpyinfo -ext XINERAMA


> My feeling so far is that the second (possibly any extra) display is not properly advertised or registered. Perhaps the RANDR extension is not working properly?

Xinerama and RandR are generally incompatible. The Xorg server
completely disables RandR when Xinerama is enabled. I suspect Xephyr
leaves it enabled by accident, not intentionally.


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer