X on a 4k panel with AMD/ATI Radeon HD8890M on a Dell Precision 7510

Making the bottom 80 or so lines visible

There must be something special with the EDID stored in the 4k panel or the VESA implementation of this system because the standard VESA drivers for both Windows aand X.org exhibit the same problem : the bottom 80 or so lines are not visible. For windows the fix consists in installing the ATI driver. For X enabling kernel mode setting (KMS) did the trick : remove nomodeset from the boot parameters.