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I've got a laptop with a DVI and VGA output on the back. With everything connected it will only allow me to run 2 of the 3 monitors e.g. laptop display + VGA or DVI+VGA but not all 3. xrandr says I have 2 CRTC's, 0 and 1. The internet says I should be able to share a CRTC if the modeline is the same, and my 2 external monitors are the exact same make and model. How do I convince the software to drive all this hardware?

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I think this means your driver can't handle the size resolution you want to stretch across those.

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xrandr says I can do 8192 x 8192, My monitors are 1920x1080 + 1920x1080 + 1280x800. – MDCore Aug 15 '10 at 10:17
how do you actually get that information about the maximum resolution that can be supported, from xrandr? – matt Oct 6 '15 at 19:04
up vote 1 down vote accepted

It turns out the 2 monitor outputs on the laptop were mutually exclusive. It wasn't a software fault after all.

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