Created attachment 179111 [details] xrandr I attached several monitors to my laptop through a VGA connection successfully. I can't say that when I attach one to the minidisplay port though. The monitor just shows a black screen; the only method which actually allows me to see something on the screen is changing the resolution. Sometimes it works (on 640x480 everything seems ok), sometimes not. Although, the video output is scrambled with visible graphic glitches (a lot of horizontal green lines) and vsync problems. I tested this bug on several distros: Ubuntu 14.04, 14.10, 15.04 and my daily one, Gentoo.
Created attachment 179121 [details] xorg log
Created attachment 179131 [details] lspci -k | grep -i vga
Created attachment 179141 [details] lshw -c display
Created attachment 179151 [details] kernel config
Please add drm.debug=0xe to your kernel command line and attach the output of dmesg from boot. Also, a picture of the glitches you see might be helpful.
Created attachment 180561 [details] dmesg with drm.debug=0xe Here I've attached the external monitor ~30 seconds after the boot. I changed various resolutions (all of them ended up in a black screen) until I got a glitchy one (picture attached).
Created attachment 180571 [details] Bug picture (green stripes)
Your log got truncated. Could you add log_buf_len=1M to the kernel command line and try again? Also, I have seen similar glitches caused by a faulty cable/adapter. It would be good to rule out any cable issues first if possible.
I can confirm that was a cable issue. I bought an active one and now everything works perfectly. I'm sorry for having wasted your time.