Good news: I can now set various 3840x2160 modes without making nouveau halt or
crash (It may still hang after a heavy reconfiguration session though). The
only drawback about it is that my displays (AOC u2868pqu, Samsung flat-tv) will
stay dark (3840x2160 non-interlaced) or in a fact show a 2560x1600 mode when I
try to set a 3840x2160 interlaced mode (the monitor does not claim to support
interlaced modes by its EDID though). Tested with various modelines as well as
the modeline suggested by my xorg.conf. Would anyone have a look whether it
should be possible to achieve 3840x2160 with this hardware configuration
(Xorg.0.log is available; decoded edid will come shortly.)?