When I attach an external monitor to my laptop and put it next to the internal
one using xrandr, the internal monitor goes blank.
If I then disable the external monitor, the internal one starts working.
Sometimes (after a lot of switching), they both go blank. Weren't able to make
them both work at the same time.
Based on the logs, the system thinks that both outputs are working (the screen
size gets increased to fit both monitors, the taskbar remains on the primary
screen even if it is not working).
I'm using the free "radeon" driver. I don't think the version is important,
because I have the same issue with Fedora 20 and Fedora 22.
The system has a second discreet radeon GPU, but I don't use it.
00:01.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc.
[AMD/ATI] Trinity [Radeon HD 7660G] [1002:9900]
01:00.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc.
[AMD/ATI] Thames [Radeon HD 7500M/7600M Series] [1002:6840] (rev ff)
And from xrandr output it looks like all outputs are connected to the default
integrated GPU:
[skarap@roadrunner ~]$ xrandr --listproviders
Providers: number : 3
Provider 0: id: 0x8c cap: 0xf, Source Output, Sink Output, Source Offload, Sink
Offload crtcs: 4 outputs: 3 associated providers: 2 name:radeon
Provider 1: id: 0x55 cap: 0xf, Source Output, Sink Output, Source Offload, Sink
Offload crtcs: 6 outputs: 0 associated providers: 2 name:radeon
Provider 2: id: 0x55 cap: 0xf, Source Output, Sink Output, Source Offload, Sink
Offload crtcs: 6 outputs: 0 associated providers: 2 name:radeon