Thanks to all three of you
BTW, I've noticed something strange.
I use an LG 22MK430H-B monitor (HDMI, Freesync capable). I did manage to get VSYNC to work in the EDID 1280*720@50Hz mode but no way to get a VSYNCed 1920*1080@50 Hz mode (black screen). I tried tons of combinations of values, clock tweaking etc. Any idea by any chance ?