I've have my Dell U2311H monitor working fine at 1920x1080x60hz using the DVI-D cable that came with it (18 pin, single-link).
When I try and attach a longer 5m cable which supports Dual-link (24 pins) the monitor doesn't work and displays the following message:
"The current input timing is not supported by the monitor display. Please change your input timing to 1920x1080@60Hz or any other monitor listed timing as per the monitor specifications."
Nothing else changes except the cable, rebooting doesn't help. As far as I understand, dual-link cables should be backwards compatible with single-link.
Weirdly, in nvidia-settings it says DFP-0 has "Connection link: Dual"
What could be causing this? Is there some way in xorg.conf to force it to single-link mode? There doesn't seem to be any option on the monitor other than selecting the DVI-D input.
Update: Since it seems to be the cable length causing the issue (see my answer below), is there a way to save the EDID info correctly returned with the short cable and force it to be used with the long cable?