Multi screen and graphics cards failed with radeon driver on debian stretch

[pbasnews-deb at yahoo.fr]

Without xorg.conf, I have only one half of screen. I can enable 2 screens of same graphics card with xrandr of course (and kde system setting too)

But unfortunaly I can not use xrand to enable my 3rd screen connected on my second graphic cars
DISPLAY=:1 /usr/bin/xrandr --output VGA-0 --mode 1280x1024
and
DISPLAY=:1 xrandr --setprovideroutputsource "ATI Radeon 3000 Graphics @ pci:0000:01:05.0"

return is: "Can't open display :1"
(I tested with and without "xhost +" command previously)

and xrandr --setprovideroutputsource 1 "ATI Radeon 3000 Graphics @ pci:0000:01:05.0" crash kde

What do you mean by "Oops, the DISPLAY=:1 wasn't supposed to be there." ?

ty

Pascal

--------------------------------------------
En date de : Ven 30.6.17, Michel Dänzer <michel at daenzer.net> a écrit :

...
 > If anyone could help me?
 
 You shouldn't need any
 xorg.conf file.
  
 > $ xrandr --listproviders
 > Providers: number : 2
 >
 Provider 0: id: 0x99 cap: 0xf, Source Output, Sink Output,
 Source Offload, Sink Offload crtcs: 2 outputs: 3 associated
 providers: 0 name:ATI Radeon X1550 64-bit @
 pci:0000:02:00.0
 > Provider 1: id: 0x53
 cap: 0xf, Source Output, Sink Output, Source Offload, Sink
 Offload crtcs: 2 outputs: 2 associated providers: 0 name:ATI
 Radeon 3000 Graphics @ pci:0000:01:05.0
 
 After starting X, run
 
  DISPLAY=:1 xrandr
 --setprovideroutputsource "ATI Radeon 3000 Graphics @
 pci:0000:01:05.0" "ATI Radeon X1550 64-bit @
 pci:0000:02:00.0"
 
 (in
 a terminal, or a session startup script such as
 ~/.xsession). Then
 you can configure the
 outputs of both GPUs with xrandr or any graphical
 monitor configuration tool using the RandR
 extension.